lesson6

Unit 1, Lesson 6 - Querying Fundamentals in C#

Querying is a large part of what a database does, and RavenDB doesn't disappoint.

As you learned in the lesson 2, querying with RavenDB is really easy. In this lesson, you will learn the fundamentals of querying using RavenDB in C#.

LINQ Support via the Query Session Method

RavenDB takes full advantage of LINQ support in C#. This allows you to express very natural queries on top of RavenDB in a strongly typed and safe manner.

Queries allow you to load documents that match a particular predicate.

You get access to LINQ support via the Query method from the session object.

Like documents loaded via the Load call, documents that were loaded via Query are managed by the session (unless you are doing a projection).

Queries in RavenDB don't behave like queries in relational databases. RavenDB does not allow computation during queries and it doesn't have problems with table scans because all queries are indexed -- even if you didn't create any indexes!

Exercise: Querying Orders of a Company

This time, you will write an application which requests a company ID. Then you will list the orders made by this company.

I think you got the basics from the previous exercises so I will not repeat details that you already know.

Step 1: Create a new project and install the latest RavenDB.Client package

Step 2: Create the DocumentStoreHolder

Step 3: Add Northwind model classes to your project

Step 4: Request a company ID

Back to Program.cs, let's create a minimal user interface which requests a company ID.

using System;
using System.Linq;
using static System.Console;
using Raven.Client.Documents;
using NorthwindModels;

namespace OrdersExplorer
{
    class Program
    {
        static void Main(string[] args)
        {
            while (true)
            {
                WriteLine("Please, enter a company id (0 to exit): ");

                if (!int.TryParse(ReadLine(), out var companyId))
                {
                    WriteLine("Company # is invalid.");
                    continue;
                }

                if (companyId == 0) break;

                QueryCompanyOrders(companyId);
            }

            WriteLine("Goodbye!");
        }

        private static void QueryCompanyOrders(int companyId)
        {
        }
    }
}

Step 5: Query the orders for the specified company

With an ID, you can now query the orders.

private static void QueryCompanyOrders(int companyId)
{
    string companyReference = $"companies/{companyId}-A";

    using (var session = DocumentStoreHolder.Store.OpenSession())
    {
        var orders = (
            from order in session.Query<Order>()
                                    .Include(o => o.Company)
            where order.Company == companyReference
            select order
            ).ToList();

        var company = session.Load<Company>(companyReference);

        if (company == null)
        {
            WriteLine("Company not found.");
            return;
        }

        WriteLine($"Orders for {company.Name}");

        foreach (var order in orders)
        {
            WriteLine($"{order.Id} - {order.OrderedAt}");
        }
    }
}

Why Not Use RQL?

Using LINQ is natural for C# developers, but what if you want to discover the power of RQL?

private static void QueryCompanyOrders(int companyId)
{
    string companyReference = $"companies/{companyId}-A";

    using (var session = DocumentStoreHolder.Store.OpenSession())
    {
        var orders = session.Advanced.RawQuery<Order>(
            "from Orders " +
            "where Company== $companyId " +
            "include Company"
        ).AddParameter("companyId", companyReference)
        .ToList();

        var company = session.Load<Company>(companyReference);

        if (company == null)
        {
            WriteLine("Company not found.");
            return;
        }

        WriteLine($"Orders for {company.Name}");

        foreach (var order in orders)
        {
            WriteLine($"{order.Id} - {order.OrderedAt}");
        }
    }
}

Great Job!

Let's move on to Lesson 7 and learn how to create, change and delete documents.