Prefer explicitly yielding the serializer, per groyoh

CHANGELOG.md

@@ -3,6 +3,7 @@
 Breaking changes:
 
 Features:
+- [#1633](https://github.com/rails-api/active_model_serializers/pull/1633) Yield 'serializer' to serializer association blocks. (@bf4)
 - [#1616](https://github.com/rails-api/active_model_serializers/pull/1616) SerializableResource handles no serializer like controller. (@bf4)
 - [#1618](https://github.com/rails-api/active_model_serializers/issues/1618) Get collection root key for
   empty collection from explicit serializer option, when possible. (@bf4)

docs/general/serializers.md

@@ -48,6 +48,18 @@ has_one :blog, key: :site
 has_one :maker, virtual_value: { id: 1 }
 ```
 
+``ruby
+has_one :blog do |serializer|
+  serializer.cached_blog
+end
+
+def cached_blog
+  cache_store.fetch("cached_blog:#{object.updated_at}") do
+    Blog.find(object.blog_id)
+  end
+end
+```
+
 #### ::has_many
 
 e.g.

lib/active_model/serializer/reflection.rb

@@ -56,14 +56,25 @@ module ActiveModel
         :nil
       end
 
+      # @param serializer [ActiveModel::Serializer]
+      # @yield [ActiveModel::Serializer]
+      # @return [:nil, associated resource or resource collection]
+      # @example
+      #   has_one :blog do |serializer|
+      #     serializer.cached_blog
+      #   end
+      #
+      #   def cached_blog
+      #     cache_store.fetch("cached_blog:#{object.updated_at}") do
+      #       Blog.find(object.blog_id)
+      #     end
+      #   end
       def value(serializer)
         @object = serializer.object
         @scope = serializer.scope
-        # Add '@serializer' to binding for use in association block as 'serializer'
-        @serializer = serializer
 
         if block
-          block_value = instance_eval(&block)
+          block_value = instance_exec(serializer, &block)
           if block_value == :nil
             serializer.read_attribute_for_serialization(name)
           else
@@ -119,7 +130,7 @@ module ActiveModel
 
       protected
 
-      attr_accessor :object, :scope, :serializer
+      attr_accessor :object, :scope
 
       private
 

test/adapter/json_api/relationships_test.rb

@@ -38,7 +38,7 @@ module ActiveModel
               end
             end
 
-            has_many :roles do
+            has_many :roles do |serializer|
               meta count: object.posts.count
               serializer.cached_roles
             end