Consider evaluating association in serializer context

For discussion:

Consider evaluating association in serializer context

That way, associations are really just anything that
can be conditionally included.  They no longer
have to actually be methods on the object or serializer.

e.g.

```diff
has_many :comments do
- last(1)
+ Comment.active.for_serialization(object).last(1)
end
```

docs/general/serializers.md

@@ -56,7 +56,7 @@ has_many :comments, key: :reviews
 has_many :comments, serializer: CommentPreviewSerializer
 has_many :reviews, virtual_value: [{ id: 1 }, { id: 2 }]
 has_many :comments, key: :last_comments do
-  last(1)
+  object.comments.last(1)
 end
 ```
 

lib/active_model/serializer/reflection.rb

@@ -8,14 +8,14 @@ class Serializer
     #     has_one :author, serializer: AuthorSerializer
     #     has_many :comments
     #     has_many :comments, key: :last_comments do
-    #       last(1)
+    #       object.comments.last(1)
     #     end
     #  end
     #
-    #  Notice that the association block is evaluated in the context of the association.
+    #  Notice that the association block is evaluated in the context of the serializer.
     #  Specifically, the association 'comments' is evaluated two different ways:
     #  1) as 'comments' and named 'comments'.
-    #  2) as 'comments.last(1)' and named 'last_comments'.
+    #  2) as 'object.comments.last(1)' and named 'last_comments'.
     #
     #  PostSerializer._reflections #=>
     #    # [
@@ -29,7 +29,7 @@ class Serializer
       # @api private
       def value(instance)
         if block
-          instance.read_attribute_for_serialization(name).instance_eval(&block)
+          instance.instance_eval(&block)
         else
           instance.read_attribute_for_serialization(name)
         end

test/fixtures/poro.rb

@@ -33,6 +33,7 @@ class Profile < Model
 class ProfileSerializer < ActiveModel::Serializer
   attributes :name, :description
 
+  # TODO: is this used anywhere?
   def arguments_passed_in?
     instance_options[:my_options] == :accessible
   end
@@ -75,6 +76,7 @@ def blog
     Blog.new(id: 999, name: 'Custom blog')
   end
 
+  # TODO: is this used anywhere?
   def custom_options
     instance_options
   end

test/serializers/associations_test.rb

@@ -129,7 +129,7 @@ def test_associations_custom_keys
       class InlineAssociationTestPostSerializer < ActiveModel::Serializer
         has_many :comments
         has_many :comments, key: :last_comments do
-          last(1)
+          object.comments.last(1)
         end
       end