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
```

CHANGELOG.md

@@ -16,7 +16,22 @@ Breaking changes:
 
 Features:
 
-- [#1336](https://github.com/rails-api/active_model_serializers/pull/1336) Added support for Grape >= 0.13, < 1.0
+- [#1378](https://github.com/rails-api/active_model_serializers/pull/1378) Change association blocks
+  to be evaluated in *serializer* scope, rather than *association* scope. (@bf4)
+    * Syntax changes from e.g.
+      `has_many :titles do customers.pluck(:title) end` (in #1356) to
+      `has_many :titles do object.customers.pluck(:title) end`
+- [#1356](https://github.com/rails-api/active_model_serializers/pull/1356) Add inline syntax for
+  attributes and associations (@bf4 @beauby @noahsilas)
+  * Allows defining attributes so that they don't conflict with existing methods. e.g. `attribute
+      :title do 'Mr. Topum Hat' end`
+  * Allows defining associations so that they don't conflict with existing methods. e.g. `has_many
+      :titles do customers.pluck(:title) end`
+    * Allows dynamic associations, as compared to compare to using
+        [`virtual_value`](https://github.com/rails-api/active_model_serializers/pull/1356#discussion_r47146466).
+        e.g. `has_many :reviews, virtual_value: [{ id: 1 }, { id: 2 }]`
+  * Removes dynamically defined methods on the serializer
+- [#1336](https://github.com/rails-api/active_model_serializers/pull/1336) Added support for Grape >= 0.13, < 1.0 (@johnhamelink)
 - [#1291](https://github.com/rails-api/active_model_serializers/pull/1291) Add logging (@maurogeorge)
 - [#1225](https://github.com/rails-api/active_model_serializers/pull/1225) Better serializer lookup, use nested serializer when it exists (@beauby)
 - [#1172](https://github.com/rails-api/active_model_serializers/pull/1172) Better serializer registration, get more than just the first module (@bf4)

lib/active_model/serializer/reflection.rb

@@ -8,14 +8,14 @@ module ActiveModel
     #     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 @@ module ActiveModel
       # @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 @@ end
 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 @@ PostSerializer = Class.new(ActiveModel::Serializer) do
     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 @@ module ActiveModel
       class InlineAssociationTestPostSerializer < ActiveModel::Serializer
         has_many :comments
         has_many :comments, key: :last_comments do
-          last(1)
+          object.comments.last(1)
         end
       end