Identity attribute relationships

[ManueGE]

If your JSON has the relationships by referencing the identity attribute instead of by nesting JSONs, you can take advantage of the identityAttributeRelated attribute. For example, if your JSON NSDictionary is:

NSDictionary *batman = @{
      @"id": @"1",
      @"name": @"Batman",
      @"publisher": @"1"
      }

you may not want Groot to serialize this dictionary by setting 1 to the Publisher relationship, but by assigning it with the Publisher object which has 1 as its identityAttribute value. You accomplish that by associating the identityAttribute key with true (or any other positive boolean value) in the entity user info dictionary:

This way, Groot will search in the publisher entity for a entry with the given identity attribute. If it is found, it will assign this entry as the Publisher object of the Character; if it doesn't Groot will create a placeholder Publisher entry with this value for the identity attribute. This placeholder object would be filled eventually if the new data is provided.

Note: To make this feature works, you must always to use the merge methods instead of the insert ones.

[ManueGE]

Includes test cases in the GRTIdentityRelatedSerializationTests class.

[gonzalezreal]

This is a very interesting feature. Groot is currently being reimplemented using Swift. Let me know if you'd be interested in providing this feature in that branch as well.

[ManueGE]

Sure, I'll implement it when I have some time.

[ManueGE]

I've done it.

Any comments are welcome.

[gonzalezreal]

Thanks! I will look into it as soon as I have some spare time.

[gonzalezreal]

There has been a lot of internal changes for milestone 1.0, so this PR is kind of updated. In any case your contribution was very useful and effectively has been the seed for #26. Thanks a lot!