has_many supports polymorphic associations

[jonah-williams]

See issue #366.

Previously has_many associations ignored the polymorphic: true option.

Now polymorphic has_many associations behave similarly to has_one associations. For included associations objects are grouped under root keys derived from their types rather than the association name.

For compatibility if a root key is specified on a embedded included polymorphic has_many association then all objects in that association will grouped under the specified root key instead of a key for each object type.

polymorphic has_many

has_many :edibles, polymorphic: true

{
  edibles: [
    {
      type: :orange,
      orange: {
        ...
      }
    }
    {
      type: :banana,
      banana: {
        ...
      }
    }
  ]
}

embedded polymorphic has_many

has_many :edibles, embed: :ids, polymorphic: true

{
  edibles: [
    {
      type: :orange,
      id: 1
    }
    {
      type: :banana,
      id: 1
    }
  ]
}

included embedded polymorphic has_many

has_many :edibles, embed: :ids, include: true, polymorphic: true

{
  meals: [{
    edibles: [
      {
        type: :orange,
        id: 1
      }
      {
        type: :banana,
        id: 1
      }
    ]
  }],
  oranges: [{
    id: 1,
    ...
  }],
  bananas: [{
    id: 1,
    ...
  }]
}

[coveralls]

Coverage increased (+0%) when pulling 1038ae1 on carbonfive:has_many_support_polymorphic_associations into 8ac4bf9 on rails-api:master.

[spastorino]

Polymorphic associations are not any longer on master and we would need to reimplement them before releasing 0.9. Support for has_many should be added also. If some of you want to take a look at that, please go ahead.
I'm closing this for now.