Add serializer to association block context

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

@@ -56,12 +56,25 @@ def include_data(value = true)
         :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
 
         if block
-          block_value = instance_eval(&block)
+          block_value = instance_exec(serializer, &block)
           if block_value == :nil
             serializer.read_attribute_for_serialization(name)
           else

test/adapter/json_api/relationships_test.rb

@@ -38,8 +38,9 @@ class RelationshipAuthorSerializer < ActiveModel::Serializer
               end
             end
 
-            has_many :roles do
+            has_many :roles do |serializer|
               meta count: object.posts.count
+              serializer.cached_roles
             end
 
             has_one :blog do
@@ -60,14 +61,20 @@ class RelationshipAuthorSerializer < ActiveModel::Serializer
               end
               meta liked: object.likes.any?
             end
+
+            def cached_roles
+              [
+                Role.new(id: 'from-serializer-method')
+              ]
+            end
           end
 
           def setup
             @post = Post.new(id: 1337, comments: [], author: nil)
             @blog = Blog.new(id: 1337, name: 'extra')
             @bio = Bio.new(id: 1337)
             @like = Like.new(id: 1337)
-            @role = Role.new(id: 1337)
+            @role = Role.new(id: 'from-record')
             @profile = Profile.new(id: 1337)
             @location = Location.new(id: 1337)
             @reviewer = Author.new(id: 1337)
@@ -144,7 +151,7 @@ def test_relationship_block_link_meta
 
           def test_relationship_meta
             expected = {
-              data: [{ id: '1337', type: 'roles' }],
+              data: [{ id: 'from-serializer-method', type: 'roles' }],
               meta: { count: 1 }
             }
             assert_relationship(:roles, expected)