Many To Many Relationships

##Many To Many Relationship Configuration

JsonApi does not provide any specific notation for expressing manyToMany relationships. A JsonApi resource can have one or more oneToMany relationships but if another JsonApi resource also has a relationship to the same resource then this is still represented as a oneToMany relation in each resource. This is more like a document database would represent the relationships between documents.

A relational representation however requires an implementation specific joining table so that multiple tables can refer to each other. This is a relational implementation specific requirement. js-data represents data in a way that closely represents a relational system. It uses foreign keys for example for oneToMany relationships. For a manyToMany relationship this would require multiple localKey's (which is possible in js-data).

We have chosen to represent manyToMany relationships via a joining table which most people who have dealt with relational databases would be familiar with.

In order for the adapter to detect that a relationship should be represented as a manyToMany and not the default oneToMany additional meta data was added to the js-data resource configuration which describes the manyToMany relationship.

 meta: {
       // Tells adapter that this is a joining table to person used for many to many relations
        article_person : { type: 'person', joinType: 'article_to_person' }
}

This additional configuration is required at each end of the manyToMany so that behind the scenes this adapter can transparently create and populate this joining data. This joining table should be considered an implementation detail in much the same way that a database ORM tool such as NHibernate might. With an ORM if we configure a relationship as a manyToMany the ORM will automatically create the relational database structure to represent this relationship behind the scenes which would typically include a joining table.

Below is shown the relationships between the various pieces of js-data configuration and a JsonApi document containing a 'manyToMany' relationship.

See the examples folder for the json and js-data configuration below and the test/examples.js file for related tests.

TODO Image to be posted soon