Make test attributes explicit

- Organize test poros with associations and by serializer
- Freeze derived attributes/associations against mutation
- Cleanup PORO fixtures

test/fixtures/active_record.rb

@@ -47,16 +47,6 @@ module ARModels
     has_many :comments
     belongs_to :author
   end
-
-  class Comment < ActiveRecord::Base
-    belongs_to :post
-    belongs_to :author
-  end
-
-  class Author < ActiveRecord::Base
-    has_many :posts
-  end
-
   class PostSerializer < ActiveModel::Serializer
     attributes :id, :title, :body
 
@@ -64,15 +54,60 @@ module ARModels
     belongs_to :author
   end
 
+  class Comment < ActiveRecord::Base
+    belongs_to :post
+    belongs_to :author
+  end
   class CommentSerializer < ActiveModel::Serializer
     attributes :id, :contents
 
     belongs_to :author
   end
 
+  class Author < ActiveRecord::Base
+    has_many :posts
+  end
   class AuthorSerializer < ActiveModel::Serializer
     attributes :id, :name
 
     has_many :posts
   end
 end
+
+class Employee < ActiveRecord::Base
+  has_many :pictures, as: :imageable
+  has_many :object_tags, as: :taggable
+end
+
+class PolymorphicSimpleSerializer < ActiveModel::Serializer
+  attributes :id
+end
+
+class ObjectTag < ActiveRecord::Base
+  belongs_to :poly_tag
+  belongs_to :taggable, polymorphic: true
+end
+class PolymorphicObjectTagSerializer < ActiveModel::Serializer
+  attributes :id
+  has_many :taggable, serializer: PolymorphicSimpleSerializer, polymorphic: true
+end
+
+class PolyTag < ActiveRecord::Base
+  has_many :object_tags
+end
+class PolymorphicTagSerializer < ActiveModel::Serializer
+  attributes :id, :phrase
+  has_many :object_tags, serializer: PolymorphicObjectTagSerializer
+end
+
+class Picture < ActiveRecord::Base
+  belongs_to :imageable, polymorphic: true
+  has_many :object_tags, as: :taggable
+end
+class PolymorphicHasManySerializer < ActiveModel::Serializer
+  attributes :id, :name
+end
+class PolymorphicBelongsToSerializer < ActiveModel::Serializer
+  attributes :id, :title
+  has_one :imageable, serializer: PolymorphicHasManySerializer, polymorphic: true
+end