Sparse field sets don't work with inherited types

[olokobayusuf]

• Suppose I have a base schema A, and another schema B : A
• Suppose we have two fields on A, someField and anotherField
• When performing a GET on B with sparse field sets, the field sets are ignored (likely because they are not defined directly on B):

api.somewhere.com/B?fields[B]=someField,anotherField

• Expected behaviour is that field sets are inherited, since A's fields are B's fields

[ethanresnick]

Under the JSON:API spec, the value in the square brackets for ?fields[TYPE] must match the value serialized in the resource object's type key for the sparse fieldset list to take effect.

Subtypes are serialized with their parent type in the type key (an intentional decision discussed #149), so the library's current behavior is actually correct per the JSON:API spec.

If you want to filter the subtype's fields, then, you actually have to put the supertype in the query string:

api.somewhere.com/B?fields[A]=someField,anotherField