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Relationships

TinyORM relationships are defined as methods on your TinyORM model classes. Since relationships also serve as powerful query builders, defining relationships as methods provides powerful method chaining and querying capabilities.

c++ orm

relationships

relations

tinyorm

import Link from '@docusaurus/Link'

TinyORM: Relationships

Introduction
Defining Relationships
Many To Many Relationships
- Retrieving Intermediate Table Columns
- Defining Custom Intermediate Table Models
Querying Relations
Eager Loading
- Constraining Eager Loads
- Lazy Eager Loading
Inserting & Updating Related Models
Touching Parent Timestamps

Introduction

__Stability: 1__ - Preview
High memory consumption during compilation using the `GCC` compiler if the number of TinyORM model instantiations with deep __Relationships__ is counting in hundreds or thousands (eg. like TinyORM functional tests). Generated executables are fine and are small. This is not a problem if only a few hundred instantiations are being done. Other compilers like `MSVC` or `Clang` are fine.

Database tables are often related to one another. For example, a blog post may have many comments or an order could be related to the user who placed it. TinyORM makes managing and working with these relationships easy, and supports basic relationships:

One To One
One To Many
Many To Many

Defining Relationships

TinyORM relationships are defined as methods on your TinyORM model classes. Since relationships also serve as powerful query builders, defining relationships as methods provides powerful method chaining and querying capabilities. For example, we may chain additional query constraints on this posts relationship:

user->posts()->whereEq("active", 1).get();

But, before diving too deep into using relationships, let's learn how to define each type of relationship supported by TinyORM.

Common Rules

Before you start defining relationship methods, you have to declare a model class, let's examine following model class with a "one" type relation:

#pragma once
#ifndef USER_HPP
#define USER_HPP

#include <orm/tiny/model.hpp>

#include "models/phone.hpp"

using Orm::Tiny::Model;

class User final : public Model<User, Phone>
{
    friend Model;
    using Model::Model;

public:
    /*! Get the phone associated with the user. */
    std::unique_ptr<HasOne<User, Phone>>
    phone()
    {
        return hasOne<Phone>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"phone", &User::phone); }},
    };
};

#endif // USER_HPP

First, you have to extend the Model<Derived, AllRelations...>, it is a common class for all models, the first template parameter is the type-id of the defined model itself, this pattern is called a Curiously recurring template pattern pattern.

However, the second parameter is more interesting, here you have to provide a type-id of all related models. The TinyORM needs these types to store relationships in the hash.

Next, you have to define the u_relations hash, which maps relation names to relationship methods. 🔥🚀🙌

:::tip You may omit the friend Model declaration and define all the private data and function members as public. :::

One To One

A one-to-one relationship is a very basic type of database relationship. For example, a User model might be associated with one Phone model. To define this relationship, we will place a phone method on the User model. The phone method should call the hasOne method and return its result. The hasOne<Related> method is available to your model via the model's Orm::Tiny::Model<Derived, AllRelations...> base class:

#pragma once
#ifndef USER_HPP
#define USER_HPP

#include <orm/tiny/model.hpp>

#include "models/phone.hpp"

using Orm::Tiny::Model;

class User final : public Model<User, Phone>
{
    friend Model;
    using Model::Model;

public:
    /*! Get the phone associated with the user. */
    std::unique_ptr<HasOne<User, Phone>>
    phone()
    {
        return hasOne<Phone>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"phone", [](auto &v) { v(&User::phone); }},
    };
};

#endif // USER_HPP

The Related template argument provided to the hasOne<Related> method is the type-id of the related model class. Once the relationship is defined, we may retrieve the related record using Model's getRelationValue<Related, Tag> method:

auto phone = User::find(1)->getRelationValue<Phone, Orm::One>("phone");

TinyORM determines the foreign key of the relationship based on the parent model name. In this case, the Phone model is automatically assumed to have a user_id foreign key. If you wish to override this convention, you may pass a first argument to the hasOne method:

return hasOne<Phone>("foreign_key");

Additionally, TinyORM assumes that the foreign key should have a value matching the primary key column of the parent. In other words, TinyORM will look for the value of the user's id column in the user_id column of the Phone record. If you would like the relationship to use a primary key value other than id or your model's u_primaryKey data member, you may pass a second argument to the hasOne method:

return hasOne<Phone>("foreign_key", "local_key");

Defining The Inverse Of The Relationship

So, we can access the Phone model from our User model. Next, let's define a relationship on the Phone model that will let us access the user that owns the phone. We can define the inverse of a hasOne relationship using the belongsTo<Related> method:

#pragma once
#ifndef PHONE_HPP
#define PHONE_HPP

#include <orm/tiny/model.hpp>

#include "models/user.hpp"

using Orm::Tiny::Model;

class Phone final : public Model<Phone, User>
{
    friend Model;
    using Model::Model;

public:
    /*! Get the user that owns the phone. */
    std::unique_ptr<BelongsTo<Phone, User>>
    user()
    {
        return belongsTo<User>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"user", [](auto &v) { v(&Phone::user); }},
    };
};

#endif // PHONE_HPP

When invoking the user method, TinyORM will attempt to find a User model that has an id which matches the user_id column on the Phone model.

TinyORM determines the foreign key name by examining the type-name of the Related template parameter and suffixing the type-name with _id. So, in this case, TinyORM assumes that the Phone model has a user_id column.

However, if the foreign key on the Phone model is not user_id, you may pass a custom key name as the first argument to the belongsTo method:

/*! Get the user that owns the phone. */
std::unique_ptr<BelongsTo<Phone, User>>
user()
{
    return belongsTo<User>("foreign_key");
}

If the parent model does not use id as its primary key, or you wish to find the associated model using a different column, you may pass a second argument to the belongsTo method specifying the parent table's custom key:

/*! Get the user that owns the phone. */
std::unique_ptr<BelongsTo<Phone, User>>
user()
{
    return belongsTo<User>("foreign_key", "owner_key");
}

The third belongsTo parameter is the relation name, if you pass it, the foreign key name will be determined from it. By convention, TinyORM will "snake_case" this relation name and suffix it with a _ followed by the name of the parent model's primary key column to generate foreign key, the __func__ predefined identifier is ideal for this. The relation name is also used in BelongsTo's associate and disassociate methods:

/*! Get the user that owns the phone. */
std::unique_ptr<BelongsTo<Phone, User>>
someUser()
{
    return belongsTo<User>({}, {}, QString::fromUtf8(__func__)); // the foreign key will be some_user_id
}

The relation name will be guessed from the type-id of the Related template parameter, TinyORM takes this name and changes the first character to lower case, so in the example above, the relation name will be user.

One To Many

A one-to-many relationship is used to define relationships where a single model is the parent to one or more child models. For example, a blog post may have an infinite number of comments. Like all other TinyORM relationships, one-to-many relationships are defined by defining a hasMany<Related> method on your TinyORM model:

#pragma once
#ifndef POST_HPP
#define POST_HPP

#include <orm/tiny/model.hpp>

#include "models/comment.hpp"

using Orm::Tiny::Model;

class Post final : public Model<Post, Comment>
{
    friend Model;
    using Model::Model;

public:
    /*! Get the comments for the blog post. */
    std::unique_ptr<HasMany<Post, Comment>>
    comments()
    {
        return hasMany<Comment>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"comments", [](auto &v) { v(&Post::comments); }},
    };
};

#endif // POST_HPP

Remember, TinyORM will automatically determine the proper foreign key column for the Comment model. By convention, TinyORM will take the "snake_case" name of the parent model and suffix it with _id. So, in this example, TinyORM will assume the foreign key column on the Comment model is post_id.

Once the relationship method has been defined, we can access the QList<Related *> of related comments by Model's getRelationValue<Related, Container = QList> method:

#include "models/post.hpp"

auto comments = Post::find(1)->getRelationValue<Comment>("comments");

for (auto *comment : comments) {
    //
}

Since all relationships also serve as query builders, you may add further constraints to the relationship query by calling the comments method and continuing to chain conditions onto the query, all the TinyBuilder methods which are related to building queries are proxied:

auto comment = Post::find(1)->comments()
                     ->whereEq("title", "foo")
                     .first();

Like the hasOne method, you may also override the foreign and local keys by passing additional arguments to the hasMany method:

return hasMany<Comment>("foreign_key");

return hasMany<Comment>("foreign_key", "local_key");

One To Many (Inverse) / Belongs To {#one-to-many-inverse}

Now that we can access all of a post's comments, let's define a relationship to allow a comment to access its parent post. To define the inverse of a hasMany relationship, define a relationship method on the child model which calls the belongsTo method:

#pragma once
#ifndef COMMENT_HPP
#define COMMENT_HPP

#include <orm/tiny/model.hpp>

#include "models/post.hpp"

using Orm::Tiny::Model;

class Comment final : public Model<Comment, Post>
{
    friend Model;
    using Model::Model;

public:
    /*! Get the post that owns the comment. */
    std::unique_ptr<BelongsTo<Comment, Post>>
    post()
    {
        return belongsTo<Post>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"post", [](auto &v) { v(&Comment::post); }},
    };
};

#endif // COMMENT_HPP

Once the relationship has been defined, we can retrieve a comment's parent post by Model's getRelationValue<Related, Tag> method:

#include "models/comment.hpp"

auto comment = Comment::find(1);

return comment->getRelationValue<Post, Orm::One>("post")->getAttribute<QString>("title");

In the example above, TinyORM will attempt to find a Post model that has an id which matches the post_id column on the Comment model.

TinyORM determines the foreign key name by examining the type-name of the Related template parameter and suffixing the type-name with a _ followed by the name of the parent model's primary key column. So, in this case, TinyORM assumes that the Post model's foreign key on the comments table is post_id.

However, if the foreign key for your relationship does not follow these conventions, you may pass a custom foreign key name as the first argument to the belongsTo method:

/*! Get the post that owns the comment. */
std::unique_ptr<BelongsTo<Comment, Post>>
post()
{
    return belongsTo<Post>("foreign_key");
}

If your parent model does not use id as its primary key, or you wish to find the associated model using a different column, you may pass a second argument to the belongsTo method specifying your parent table's custom key:

/*! Get the post that owns the comment. */
std::unique_ptr<BelongsTo<Comment, Post>>
post()
{
    return belongsTo<Post>("foreign_key", "owner_key");
}

The third belongsTo parameter is the relation name, if you pass it, the foreign key name will be determined from it. By convention, TinyORM will "snake_case" this relation name and suffix it with a _ followed by the name of the parent model's primary key column to generate foreign key, the __func__ predefined identifier is ideal for this. The relation name is also used in BelongsTo's associate and disassociate methods:

/*! Get the post that owns the comment. */
std::unique_ptr<BelongsTo<Comment, Post>>
somePost()
{
    return belongsTo<Post>({}, {}, QString::fromUtf8(__func__)); // the foreign key will be some_post_id
}

The relation name will be guessed from the type-id of the Related template parameter, TinyORM takes this name and changes the first character to lower case, so in the example above, the relation name will be user.

Default Models

The belongsTo, and hasOne relationships allow you to define a default model that will be returned if the given relationship is null. This pattern is often referred to as the Null Object pattern and can help remove conditional checks in your code. In the following example, the user relation will return an empty User model if no user is attached to the Post model:

/*! Get the author of the post. */
std::unique_ptr<BelongsTo<Post, User>>
user()
{
    // Ownership of a unique_ptr()
    auto relation = belongsTo<User>();

    relation->withDefault();

    return relation;
}

To populate the default model with attributes, you may pass the vector of attributes to the withDefault method:

/*! Get the author of the post. */
std::unique_ptr<BelongsTo<Post, User>>
user()
{
    // Ownership of a unique_ptr()
    auto relation = belongsTo<User>();

    relation->withDefault({{"name", "Guest Author"},
                           {"is_active", false}});

    return relation;
}

Many To Many Relationships {#many-to-many}

Many-to-many relations are slightly more complicated than hasOne and hasMany relationships. An example of a many-to-many relationship is a user that has many roles and those roles are also shared by other users in the application. For example, a user may be assigned the role of "Author" and "Editor"; however, those roles may also be assigned to other users as well. So, a user has many roles and a role has many users.

Table Structure

To define this relationship, three database tables are needed: users, roles, and role_user. The role_user table is derived from the alphabetical order of the related model names and contains user_id and role_id columns. This table is used as an intermediate table linking the users and roles.

Remember, since a role can belong to many users, we cannot simply place a user_id column on the roles table. This would mean that a role could only belong to a single user. In order to provide support for roles being assigned to multiple users, the role_user table is needed. We can summarize the relationship's table structure like so:

users
  id - integer
  name - string

roles
  id - integer
  name - string

role_user
  user_id - integer
  role_id - integer

Model Structure

Many-to-many relationships are defined by writing a method that returns the result of the belongsToMany method. The belongsToMany method is provided by the Orm::Tiny::Model<Derived, AllRelations...> base class that is used by all of your application's TinyORM models. For example, let's define a roles method on our User model. The first argument passed to this method is the name of the related model class:

#pragma once
#ifndef USER_HPP
#define USER_HPP

#include <orm/tiny/relations/pivot.hpp>

#include "models/role.hpp"

using Orm::Tiny::Model;
using Orm::Tiny::Relations::Pivot;

class User final : public Model<User, Role, Pivot>
{
    friend Model;
    using Model::Model;

public:
    /*! The roles that belong to the user. */
    std::unique_ptr<BelongsToMany<User, Role>>
    roles()
    {
        return belongsToMany<Role>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"roles", [](auto &v) { v(&User::roles); }},
    };
};

#endif // USER_HPP

Once the relationship is defined, you may access the user's roles as the QList<Related *> by Model's getRelationValue<Related, Container = QList> method:

#include <QDebug>

#include "models/user.hpp"

auto user = User::find(1);

for (auto *role : user->getRelationValue<Role>("roles"))
    qDebug() << role->getAttribute<quint64>("id");

Since all relationships also serve as query builders, you may add further constraints to the relationship query by calling the roles method and continuing to chain conditions onto the query:

auto roles = User::find(1)->roles()->orderBy("name").get();

To determine the table name of the relationship's intermediate table, TinyORM will join the two related model names in alphabetical order. However, you are free to override this convention. You may do so by passing a first argument to the belongsToMany method:

return belongsToMany<Role>("role_user");

In addition to customizing the name of the intermediate table, you may also customize the column names of the keys on the table by passing additional arguments to the belongsToMany method. The second argument is the foreign key name of the model on which you are defining the relationship, while the third argument is the foreign key name of the model that you are joining to:

return belongsToMany<Role>("role_user", "user_id", "role_id");

The fourth and fifth arguments are primary key names on models in the many-to-many relation and the sixth argument is the relation name.

The relation name is used during Touching Parent Timestamps and will be guessed from the type-id of the Related template parameter, TinyORM takes this name, changes the first character to lower case, and appends s character. So in the example above, the relation name will be roles.

Defining The Inverse Of The Relationship

To define the "inverse" of a many-to-many relationship, you should define a method on the related model which also returns the result of the belongsToMany method. To complete our user / role example, let's define the users method on the Role model:

#pragma once
#ifndef ROLE_HPP
#define ROLE_HPP

#include <orm/tiny/relations/pivot.hpp>

using Orm::Tiny::Model;
using Orm::Tiny::Relations::Pivot;

class User; // Forward declaration to avoid cyclic dependency

class Role final : public Model<Role, User, Pivot>
{
    friend Model;
    using Model::Model;

public:
    /*! The users that belong to the role. */
    std::unique_ptr<BelongsToMany<Role, User>>
    users()
    {
        return belongsToMany<User>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"users", [](auto &v) { v(&Role::users); }},
    };
};

#endif // ROLE_HPP

As you can see, the relationship is defined exactly the same as its User model counterpart with the exception of referencing the User model. Since we're reusing the belongsToMany method, all of the usual table and key customization options are available when defining the "inverse" of many-to-many relationships.

Retrieving Intermediate Table Columns

As you have already learned, working with many-to-many relations requires the presence of an intermediate table. TinyORM provides some very helpful ways of interacting with this table. For example, let's assume our User model has many Role models that it is related to. After accessing this relationship, we may access the intermediate table using the pivot attribute on the models:

#include <QDebug>

#include "models/user.hpp"

using Orm::Tiny::Relations::Pivot;

auto user = User::find(1);

for (auto *role : user->getRelationValue<Role>("roles"))
    qDebug() << role->getRelation<Pivot, Orm::One>("pivot")
                ->getAttribute("created_at");

Notice that each Role model we retrieve has automatically assigned a pivot relationship. This relation contains a model representing the intermediate table and it is an instance of the Orm::Tiny::Relations::Pivot model class.

By default, only the model keys will be present on the pivot model. If your intermediate table contains extra attributes, you must specify them when defining the relationship:

// Ownership of a unique_ptr()
auto relation = belongsToMany<Role>();

relation->withPivot({"active", "created_by"});

return relation;

If you would like your intermediate table to have created_at and updated_at timestamps that are automatically maintained by TinyORM, call the withTimestamps method when defining the relationship:

// Ownership of a unique_ptr()
auto relation = belongsToMany<Role>();

relation->withTimestamps();

return relation;

:::warning Intermediate tables that utilize TinyORM's automatically maintained timestamps are required to have both created_at and updated_at timestamp columns. :::

Customizing The `pivot` Relation Name

As noted previously, attributes from the intermediate table may be accessed on models via the pivot relation name. However, you are free to customize the name of this relation to better reflect its purpose within your application.

For example, if your application contains users that may subscribe to podcasts, you likely have a many-to-many relationship between users and podcasts. If this is the case, you may wish to rename your intermediate table relation name to subscription instead of pivot. This can be done using the as method when defining the relationship:

// Ownership of a unique_ptr()
auto relation = belongsToMany<Podcast>();

relation->as("subscription")
        .withTimestamps();

return relation;

Once the custom intermediate table relation name has been specified, you may access the intermediate table data using the customized name:

#include <QDebug>

#include "models/user.hpp"

using Orm::Tiny::Relations::Pivot;

auto users = User::with("podcasts")->get();

for (auto &user : users)
    for (auto *podcast : user.getRelation<Podcast>("podcasts"))
        qDebug() << podcast->getRelation<Pivot, Orm::One>("subscription")
                    ->getAttribute("created_at");

Defining Custom Intermediate Table Models

If you would like to define a custom model to represent the intermediate table of your many-to-many relationship, you may pass the custom pivot type as a second template argument to the belongsToMany<Related, PivotType = Pivot> method when defining the relationship. Custom pivot models give you the opportunity to define additional methods on the pivot model.

Custom many-to-many pivot models should extend the Orm::Tiny::Relations::BasePivot<PivotModel> class. For example, we may define a User model which uses a custom RoleUser pivot model:

#pragma once
#ifndef USER_HPP
#define USER_HPP

#include <orm/tiny/relations/pivot.hpp>

#include "models/role.hpp"

using Orm::Tiny::Model;
using Orm::Tiny::Relations::Pivot;

class User final : public Model<User, Role, Pivot>
{
    friend Model;
    using Model::Model;

public:
    /*! The roles that belong to the user. */
    std::unique_ptr<BelongsToMany<User, Role, RoleUser>>
    roles()
    {
        return belongsToMany<Role, RoleUser>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"roles", [](auto &v) { v(&User::roles); }},
    };
};

#endif // USER_HPP

When defining the RoleUser model, you should extend the Orm::Tiny::Relations::BasePivot<PivotModel> class:

#pragma once
#ifndef ROLEUSER_HPP
#define ROLEUSER_HPP

#include <orm/tiny/relations/basepivot.hpp>

using Orm::Tiny::Relations::BasePivot;

class RoleUser final : public BasePivot<RoleUser>
{
    friend Model;
    friend BasePivot;

    using BasePivot::BasePivot;
};

#endif // ROLEUSER_HPP

You have to pass a custom pivot type to the AllRelations template parameter pack on Model<Derived, AllRelations...> so that the Model knows how to generate a std::variant, which holds all the relations and also you have to add a new mapping from the relation name to the custom pivot model type-id, this is described in more detail in the Common Rules:

#pragma once
#ifndef ROLE_HPP
#define ROLE_HPP

#include <orm/tiny/model.hpp>

#include "models/roleuser.hpp"

using Orm::Tiny::Model;

class User; // Forward declaration to avoid cyclic dependency

class Role final : public Model<Role, User, RoleUser>
{
    friend Model;
    using Model::Model;

public:
    /*! The users that belong to the role. */
    std::unique_ptr<BelongsToMany<Role, User>>
    users()
    {
        return belongsToMany<User>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"users", [](auto &v) { v(&Role::users); }},
    };
};

#endif // ROLE_HPP

Once the custom pivot model RoleUser has been defined, getRelation or getRelationValue method returns proper pivot type:

#include <QDebug>

#include "models/user.hpp"

auto users = User::with("roles")->get();

for (auto &user : users)
    for (auto *role : user.getRelation<Role>("roles"))
        qDebug() << role->getRelation<RoleUser, Orm::One>("pivot")
                    ->getAttribute("created_at");

:::warning Custom Pivot models may not use the SoftDeletes base class. If you need to soft delete pivot records consider converting your pivot model to an actual TinyORM model. :::

Custom Pivot Models And Incrementing IDs

If you have defined a many-to-many relationship that uses a custom pivot model, and that pivot model has an auto-incrementing primary key, you should ensure your custom pivot model class defines an u_incrementing data member that is set to true.

/*! Indicates if the IDs are auto-incrementing. */
bool u_incrementing = true;

Closer Look At Defining Custom Intermediate Table Models

I can tell that defining a custom intermediate table models is the most confusing part of the TinyORM framework, let's look closer at it.

If you are defining a custom RoleUser intermediate table for the Role model like in the example above then you have to pass the RoleUser pivot type as the second template argument to the User::roles() belongsToMany relationship method and you have to pass the RoleUser pivot type to the AllRelations template parameter pack on the Role model!

Do you see the confusing part? In short, if defining the User::roles() relation with the custom UserRole pivot model then add the UserRole type to the AllRelations template parameter pack on the Role model.

The same is true for the basic Pivot model, if you are using a basic pivot model and not a custom pivot model you still need to add the Pivot type to the AllRelations template parameter pack on the Model class!

The reason for all of this is that the Model knows how to generate and work with the std::variant that holds the pivot model in the Model::m_relations data member map, then you can get the pivot model using the Model::getRelationValue or Model::getRelation methods.

User Data Members on Custom Intermediate Table Models

This is another nonstandard part of the custom pivot models. The u_connection and u_timestamps user data members and the CREATED_AT() and UPDATED_AT() static getter methods are ignored when obtaining pivot records from the database during the lazy or eager loading.

Let's describe how these data members are resolved:

u_connection - inferred from the parent model
u_timestamps - true if obtained pivot attributes contain both the CREATED_AT and UPDATED_AT attributes
CREATED_AT, UPDATED_AT - inferred from the parent model, can be overridden using the withTimestamps() method

All these data members are taken into account normally when you call the create, save, update, ... on the Custom Pivot models!

Querying Relations

Since all TinyORM relationships are defined via methods, you may call those methods to obtain an instance of the relationship without actually executing a query to load the related models. In addition, all types of TinyORM relationships also serve as query builders, allowing you to continue to chain constraints onto the relationship query before finally executing the SQL query against your database.

For example, imagine a blog application in which a User model has many associated Post models:

#include "models/post.hpp"

using Orm::Tiny::Model;

class User final : public Model<User, Post>
{
    friend Model;
    using Model::Model;

public:
    /*! Get all of the posts for the user. */
    std::unique_ptr<HasMany<User, Post>>
    posts()
    {
        return hasMany<Post>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"posts", [](auto &v) { v(&User::posts); }},
    };
};

You may query the posts relationship and add additional constraints to the relationship like so:

#include "models/user.hpp"

auto user = User::find(1);

user->posts()->whereEq("active", 1).get();

You are able to use any of the TinyORM query builder's methods on the relationship, so be sure to explore the query builder documentation to learn about all of the methods that are available to you.

:::note All the TinyBuilder methods which are related to building queries are proxied on the Relation base class. :::

Chaining `orWhere` Clauses After Relationships

As demonstrated in the example above, you are free to add additional constraints to relationships when querying them. However, be careful when chaining orWhere clauses onto a relationship, as the orWhere clauses will be logically grouped at the same level as the relationship constraint:

user->posts()
    ->whereEq("active", 1)
    .orWhere("votes", ">=", 100)
    .get();

The example above will generate the following SQL. As you can see, the or clause instructs the query to return any user with greater than 100 votes. The query is no longer constrained to a specific user:

select *
from posts
where user_id = ? and active = 1 or votes >= 100

In most situations, you should use logical groups to group the conditional checks between parentheses:

user->posts()
    ->where([](auto &query)
    {
        query.whereEq("active", 1)
             .orWhere("votes", ">=", 100);
    })
    .get();

The example above will produce the following SQL. Note that the logical grouping has properly grouped the constraints and the query remains constrained to a specific user:

select *
from posts
where user_id = ? and (active = 1 or votes >= 100)

Relationship Methods

If you do not need to add additional constraints to the TinyORM relationship query, you may access the relationship directly. For example, continuing to use our User and Post example models, we may access all of a user's posts like so:

#include "models/user.hpp"

auto user = User::find(1);

for (auto *post : user->getRelationValue<Post>("posts")) {
    //
}

The getRelationValue<Related> method performs "lazy loading", meaning they will only load their relationship data when you actually access them. Because of this, developers often use eager loading to pre-load relationships they know will be accessed after loading the model. Eager loading provides a significant reduction in SQL queries that must be executed to load a model's relations.

To access eager loaded relationship use Model's getRelation<Related> method:

auto user = User::find(1);

for (auto *post : user->getRelation<Post>("posts")) {
    //
}

As described above TinyORM offers two methods to access relationships; getRelation and getRelationValue.

The getRelation method is for "eager loaded" relations, when the relationship is not loaded, it throws the exception RelationNotLoadedError. The getRelationValue is for "lazy loading", when the relationship is not loaded, it will load it.

Both methods have two overloads, the getRelation<Related, Container = QList> overload is for obtaining many type relationships:

auto posts = User::find(1)->getRelation<Post>("posts");

The getRelation<Related, Tag> overload is for obtaining "one" type relationships:

auto user = Post::find(1)->getRelation<User, Orm::One>("user");

The same is true for the getRelationValue method.

Querying Relationship Existence

When retrieving model records, you may wish to limit your results based on the existence of a relationship. For example, imagine you want to retrieve all blog posts that have at least one comment. To do so, you may pass the name of the relationship to the has and orHas methods:

#include "models/post.hpp"

// Retrieve all posts that have at least one comment...
auto posts = Post::has("comments")->get();

You may also specify an operator and count value to further customize the query:

// Retrieve all posts that have three or more comments...
auto posts = Post::has("comments", ">=", 3)->get();

Nested has statements may be constructed using "dot" notation. For example, you may retrieve all posts that have at least one comment that has at least one image:

// Retrieve posts that have at least one comment with images...
auto posts = Post::has<Image>("comments.images")->get();

If you need even more power, you may use the whereHas and orWhereHas methods to define additional query constraints on your has queries, such as inspecting the content of a comment:

// Retrieve posts with at least one comment containing words like code%...
auto posts = Post::whereHas("comments", [](auto &query)
{
    query.where("content", LIKE, "code%");
})->get();

// Retrieve posts with at least ten comments containing words like code%...
auto posts = Post::whereHas("comments", [](auto &query)
{
    query.where("content", LIKE, "code%");
}, ">=", 10)->get();

:::note TinyORM does not currently support querying for relationship existence across databases. The relationships must exist within the same database. :::

Related template parameter

All the has-related methods are templated by the Related template parameter, it looks something like the following has<Related>(..., const std::function<void(CallbackType<Related> &)> &callback = nullptr), you can pass a query callback to this methods and on the base of the Related template argument will be decided whether the Orm::QueryBuilder or Orm::TinyBuilder<Related> will be passed to the callback. As you can see this Related parameter exists because the Orm::TinyBuilder<Related> needs it.

The rule of thumbs are:

if you don't pass the Related template parameter or you pass void then the Orm::QueryBuilder & will be passed to the callback
if you pass it, then the Orm::TinyBuilder<Related> & will be passed to the callback
Related has to be of the same type as a relation name passed to the has-related method (a real type of the relation eg. type of the posts relation name is Post)
you have to always pass the Related template parameter for nested relations, you can not use nested relations with Related = void
in nested relations, where you can pass more relation names using "dot" notation, Related has to be of the same type as the last relation name passed to the has-related method like you can see in the nested example above or below

Querying Relationship Absence

When retrieving model records, you may wish to limit your results based on the absence of a relationship. For example, imagine you want to retrieve all blog posts that don't have any comments. To do so, you may pass the name of the relationship to the doesntHave and orDoesntHave methods:

#include "models/post.hpp"

auto posts = Post::doesntHave("comments")->get();

If you need even more power, you may use the whereDoesntHave and orWhereDoesntHave methods to add additional query constraints to your doesntHave queries, such as inspecting the content of a comment:

auto posts = Post::whereDoesntHave("comments", [](auto &query)
{
    query.where("content", LIKE, "code%");
})->get();

You may use "dot" notation to execute a query against a nested relationship. For example, the following query will retrieve all posts that have comments from authors that are not banned:

auto posts = Post::whereDoesntHave<Author>("comments.author",
                                           [](auto &query)
{
    query.where("banned", false);
})->get();

Eager Loading

When accessing TinyORM relationships by Model's getRelationValue method, the related models are "lazy loaded". This means the relationship data is not actually loaded until you first access them. However, TinyORM can "eager load" relationships at the time you query the parent model. Eager loading alleviates the "N + 1" query problem. To illustrate the N + 1 query problem, consider a Book model that "belongs to" to an Author model:

using Orm::Tiny::Model;

class Book final : public Model<Book, Author>
{
    friend Model;
    using Model::Model;

public:
    /*! Get the author that wrote the book. */
    std::unique_ptr<BelongsTo<Book, Author>>
    author()
    {
        return belongsTo<Author>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"author", [](auto &v) { v(&Book::author); }},
    };
};

Now, let's retrieve all books and their authors:

#include <QDebug>

#include "models/book.hpp"

auto books = Book::all();

for (auto &book : books)
    qDebug() << book.getRelationValue<Author, Orm::One>("author")
                ->getAttribute<QString>("name");

This loop will execute one query to retrieve all of the books within the database table, then another query for each book in order to retrieve the book's author. So, if we have 25 books, the code above would run 26 queries: one for the original book, and 25 additional queries to retrieve the author of each book.

Thankfully, we can use eager loading to reduce this operation to just two queries. When building a query, you may specify which relationships should be eager loaded using the with method:

auto books = Book::with("author")->get();

for (auto &book : books)
    qDebug() << book.getRelation<Author, Orm::One>("author")
                ->getAttribute<QString>("name");

For this operation, only two queries will be executed - one query to retrieve all of the books and one query to retrieve all of the authors for all of the books:

select * from books

select * from authors where id in (1, 2, 3, 4, 5, ...)

Eager Loading Multiple Relationships

Sometimes you may need to eager load several different relationships. To do so, just pass a QList<Orm::WithItem> of relationships to the with method:

auto books = Book::with({"author", "publisher"})->get();

Nested Eager Loading

To eager a relationship's relationships, you may use "dot" syntax. For example, let's eager load all of the book's authors and all of the author's personal contacts:

auto books = Book::with("author.contacts")->get();

Eager Loading Specific Columns

You may not always need every column from the relationships you are retrieving. For this reason, TinyORM allows you to specify which columns of the relationship you would like to retrieve:

auto books = Book::with("author:id,name,book_id")->get();

:::warning When using this feature, you should always include the id column and any relevant foreign key columns in the list of columns you wish to retrieve, otherwise relations will not be loaded correctly. :::

Eager Loading By Default

Sometimes you might want to always load some relationships when retrieving a model. To accomplish this, you may define a u_with data member on the model:

using Orm::Tiny::Model;

class Book final : public Model<Book, Author>
{
    friend Model;
    using Model::Model;

public:
    /*! Get the author that wrote the book. */
    std::unique_ptr<BelongsTo<Book, Author>>
    author()
    {
        return belongsTo<Author>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"author", [](auto &v) { v(&Book::author); }},
    };

    /*! The relationships that should always be loaded. */
    QList<QString> u_with {
        "author",
    };
};

If you would like to remove an item from the u_with data member for a single query, you may use the without method:

auto books = Book::without("author")->get();

If you would like to override all items within the u_with data member for a single query, you may use the withOnly method:

auto books = Book::withOnly("genre")->get();

Constraining Eager Loads

Sometimes you may wish to eager load a relationship but also specify additional query conditions for the eager loading query. You can accomplish this by passing a QList<Orm::WithItem> of relationships to the with method where the name data member of Orm::WithItem struct is a relationship name and the constraints data member expects a lambda expression that adds additional constraints to the eager loading query. The first argument passed to the constraints lambda expression is an underlying Orm::QueryBuilder for a related model:

#include "models/user.hpp"

auto users = User::with({{"posts", [](auto &query)
{
    query.where("title", "like", "%code%");
}}})->get();

In this example, TinyORM will only eager load posts where the post's title column contains the word code. You may call other query builder methods to further customize the eager loading operation:

auto users = User::with({{"posts", [](auto &query)
{
    query.orderBy("created_at", "desc");
}}})->get();

:::note The limit and take query builder methods may not be used when constraining eager loads. :::

Lazy Eager Loading

Sometimes you may need to eager load a relationship after the parent model has already been retrieved. For example, this may be useful if you need to dynamically decide whether to load related models:

#include "models/book.hpp"

auto book = Book::find(1);

if (someCondition)
    book->load("author");

You may load more relationships at once, to do so, just pass a QList<Orm::WithItem> of relationships to the load method:

Book::find(1)->load({"author", "publisher"});

:::note So far, this only works on models, not on containers returned from Model's get or all methods. :::

If you need to set additional query constraints on the eager loading query, you may pass a QList<Orm::WithItem> of relationships to the load method where the name data member of Orm::WithItem struct is a relationship name and the constraints data member expects a lambda expression that adds additional constraints to the eager loading query. The first argument passed to the constraints lambda expression is an underlying Orm::QueryBuilder for a related model:

author->load({{"books", [](auto &query)
{
    query.orderBy("published_date", "asc");
}}});

:::tip You can also use eager constraining in the Model's fresh method. :::

Inserting & Updating Related Models {#inserting-and-updating-related-models}

The `save` Method

TinyORM provides convenient methods for adding new models to relationships. For example, perhaps you need to add a new comment to a post. Instead of manually setting the post_id attribute on the Comment model you may insert the comment using the relationship's save method:

#include "models/comment.hpp"
#include "models/post.hpp"

Comment comment({{"message", "A new comment."}});

auto post = Post::find(1);

post->comments()->save(comment);

Note that we did not access the comments relationship with the getRelation or getRelationValue method. Instead, we called the comments method to obtain an instance of the relationship. The save method will automatically add the appropriate post_id value to the new Comment model.

If you need to save multiple related models, you may use the saveMany method:

auto post = Post::find(1);

post->comments()->saveMany({
    {{"message", "A new comment."}},
    {{"message", "Another new comment."}},
});

The save and saveMany methods will not add the new models to any in-memory relationships that are already loaded onto the parent model. If you plan on accessing the relationship after using the save or saveMany methods, you may wish to use the refresh method to reload the model and its relationships:

post->comments()->save(comment);

post->refresh();

// All comments, including the newly saved comment...
post->getRelation<Comment>("comments");

The many-to-many relationship also supports the save and saveMany methods. In addition, you may pass the pivot attributes as a second argument and select if you want to touch parent timestamps as a third argument:

auto user = User::find(2);

Role role {{"name", "admin"}};

user->roles()->save(role, {{"active", true}});

Role role1 {{"name", "edit"}};
Role role2 {{"name", "view"}};

user->roles()->saveMany({role1, role2}, {{{"active", true}},
                                         {{"active", false}}});

// No pivot attributes for role1
user->roles()->saveMany({role1, role2}, {{}, {{"active", false}}});

Recursively Saving Models & Relationships

If you would like to save your model and all of its associated relationships, you may use the push method. In this example, the Post model will be saved as well as its comments and the comment's authors:

auto post = Post::find(1);

post->getRelationValue<Comment>("comments").at(0)->setAttribute("message", "Message");

post->getRelationValue<Comment>("comments").first()
    ->getRelationValue<User, Orm::One>("author")->setAttribute("name", "Author Name");

post->push();

The `create` Method

In addition to the save and saveMany methods, you may also use the create method, which accepts a vector of attributes, creates a model, and inserts it into the database. The difference between save and create is that save accepts a full TinyORM model instance while create accepts a QList<Orm::AttributeItem>. The newly created model will be returned by the create method:

#include "models/post.hpp"

auto post = Post::find(1);

auto comment = post->comments()->create({
    {"message", "A new comment."},
});

You may use the createMany method to create multiple related models:

auto post = Post::find(1);

auto comments = post->comments()->createMany({
    {{"message", "A new comment."}, {"is_published", true}},
    {{"message", "Another new comment."}, {"is_published", false}},
});

The many-to-many relationship also supports the create and createMany methods. In addition, you may pass the pivot attributes as a second argument and select if you want to touch parent timestamps as a third argument:

auto user = User::find(2);

user->roles()->create({{"name", "admin"}}, {{"active", true}});

user->roles()->createMany({
    {{"name", "edit"}},
    {{"name", "view"}},
}, {
    {{"active", true}},
    {{"active", false}},
});

// No pivot attributes for the first role
user->roles()->createMany({
    {{"name", "edit"}},
    {{"name", "view"}},
}, {
    {},
    {{"active", false}},
});

You may also use the findOrNew, firstOrNew, firstOrCreate, and updateOrCreate methods to create and update models on relationships.

:::tip Before using the create method, be sure to review the mass assignment documentation. :::

Belongs To Relationships {#updating-belongs-to-relationships}

If you would like to assign a child model to a new parent model, you may use the associate method. In this example, the User model defines a belongsTo relationship to the Account model. The associate method will set the foreign key on the child model:

#include "models/user.hpp"

User user {{"name", "Mike"}};

auto account = Account::find(10);

user.account()->associate(*account);

user.save();

To remove a parent model from a child model, you may use the dissociate method. This method will set the relationship's foreign key to null:

user.account()->dissociate();

user.save();

Many To Many Relationships {#updating-many-to-many-relationships}

Attaching / Detaching

TinyORM also provides methods to make working with many-to-many relationships more convenient. For example, let's imagine a user can have many roles and a role can have many users. You may use the attach method to attach a role to a user by inserting a record in the relationship's intermediate table:

#include "models/user.hpp"

auto user = User::find(1);

user->roles()->attach(roleId);

When attaching a relationship to a model, you may also pass a vector of additional data to be inserted into the intermediate table:

const auto expires = true;

user->roles()->attach(roleId, {{"expires", expires}});

Sometimes it may be necessary to remove a role from a user. To remove a many-to-many relationship record, use the detach method. The detach method will delete the appropriate record out of the intermediate table; however, both models will remain in the database:

// Detach a single role from the user...
user->roles()->detach(roleId);

// Detach all roles from the user...
user->roles()->detachAll();

For convenience, attach and detach also accept vectors of IDs or Model instances as input:

auto user = User::find(1);

user->roles()->detach({1, 2, 3});

Role role1({{"name", "Role 1"}});
role1.save();
Role role2({{"name", "Role 2"}});
role2.save();

user->roles()->attach({{role1}, {role2}});

The attach method also accepts std::map as input, so you can pass different attributes for each model you are attaching:

user->roles()->attach({
    {1, {{"expires", true},  {"is_active", false}}},
    {2, {{"expires", false}, {"is_active", true}}},
});

Syncing Associations

You may also use the sync method to construct many-to-many associations. The sync method accepts a vector of IDs to place on the intermediate table. Any IDs that are not in the given vector will be removed from the intermediate table. So, after this operation is complete, only the IDs in the given vector will exist in the intermediate table:

user->roles()->sync({1, 2, 3});

You may also pass additional intermediate table values with the IDs:

user->roles()->sync({
    {1, {{"expires", true}}},
    {2, {}},
    {3, {}},
});

If you do not want to detach existing IDs that are missing from the given vector, you may use the syncWithoutDetaching method:

user->roles()->syncWithoutDetaching({1, 2, 3});

Updating A Record On The Intermediate Table

If you need to update an existing row in your relationship's intermediate table, you may use the updateExistingPivot method. This method accepts the intermediate record foreign key and the vector of attributes to update:

auto user = User::find(1);

user->roles()->updateExistingPivot(roleId, {
    {"active", false},
});

Touching Parent Timestamps

When a model defines a belongsTo relationship to another model, such as a Comment which belongs to a Post, it is sometimes helpful to update the parent's timestamp when the child model is updated.

For example, when a Comment model is updated, you may want to automatically "touch" the updated_at timestamp of the owning Post so that it is set to the current date and time. To accomplish this, you may add a u_touches data member to your child model containing the names of the relationships that should have their updated_at timestamps updated when the child model is updated:

using Orm::Tiny::Model;

class Comment final : public Model<Comment, Post>
{
    friend Model;
    using Model::Model;

public:
    /*! Get the post that owns the comment. */
    std::unique_ptr<BelongsTo<Comment, Post>>
    post()
    {
        return belongsTo<Post>();
    }

private:
    /*! Map of relation names to methods. */
    QHash<QString, RelationVisitor> u_relations {
        {"post", [](auto &v) { v(&Comment::post); }},
    };

    /*! All of the relationships to be touched. */
    QStringList u_touches {"post"};
};

:::note Parent model timestamps will only be updated if the child model is updated using TinyORM's save, push, or remove method. :::

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TinyORM: Relationships

Introduction

Defining Relationships

Common Rules

One To One

Defining The Inverse Of The Relationship

One To Many

One To Many (Inverse) / Belongs To {#one-to-many-inverse}

Default Models

Many To Many Relationships {#many-to-many}

Table Structure

Model Structure

Defining The Inverse Of The Relationship

Retrieving Intermediate Table Columns

Customizing The pivot Relation Name

Defining Custom Intermediate Table Models

Custom Pivot Models And Incrementing IDs

Closer Look At Defining Custom Intermediate Table Models

User Data Members on Custom Intermediate Table Models

Querying Relations

Chaining orWhere Clauses After Relationships

Relationship Methods

Querying Relationship Existence

Related template parameter

Querying Relationship Absence

Eager Loading

Eager Loading Multiple Relationships

Nested Eager Loading

Eager Loading Specific Columns

Eager Loading By Default

Constraining Eager Loads

Lazy Eager Loading

Inserting & Updating Related Models {#inserting-and-updating-related-models}

The save Method

Recursively Saving Models & Relationships

The create Method

Belongs To Relationships {#updating-belongs-to-relationships}

Many To Many Relationships {#updating-many-to-many-relationships}

Attaching / Detaching

Syncing Associations

Updating A Record On The Intermediate Table

Touching Parent Timestamps

Customizing The `pivot` Relation Name

Chaining `orWhere` Clauses After Relationships

The `save` Method

The `create` Method