Merge branch 'beauby-resource-level-meta'

Followup concerns:
- https://github.com/rails-api/active_model_serializers/pull/1340/files#r47451387
- https://github.com/rails-api/active_model_serializers/pull/1340/files#r48116963
- https://github.com/rails-api/active_model_serializers/pull/1340#discussion_r47451387
- https://github.com/rails-api/active_model_serializers/pull/1340#issuecomment-164306882
- https://github.com/rails-api/active_model_serializers/pull/1340#issuecomment-166202978
- https://github.com/rails-api/active_model_serializers/pull/1340#issuecomment-173028896

lib/active_model/serializer/adapter/json_api.rb

@@ -8,6 +8,7 @@ module ActiveModel
         autoload :Link
         autoload :Association
         autoload :ResourceIdentifier
+        autoload :Meta
         autoload :Deserialization
 
         # TODO: if we like this abstraction and other API objects to it,
@@ -150,6 +151,9 @@ module ActiveModel
           links = links_for(serializer)
           resource_object[:links] = links if links.any?
 
+          meta = meta_for(serializer)
+          resource_object[:meta] = meta unless meta.nil?
+
           resource_object
         end
 
@@ -174,6 +178,10 @@ module ActiveModel
         def pagination_links_for(serializer, options)
           JsonApi::PaginationLinks.new(serializer.object, options[:serialization_context]).serializable_hash(options)
         end
+
+        def meta_for(serializer)
+          Meta.new(serializer).as_json
+        end
       end
     end
   end

lib/active_model/serializer/adapter/json_api/meta.rb

@@ -0,0 +1,29 @@
+module ActiveModel
+  class Serializer
+    module Adapter
+      class JsonApi
+        class Meta
+          def initialize(serializer)
+            @object = serializer.object
+            @scope = serializer.scope
+
+            # Use the return value of the block unless it is nil.
+            if serializer._meta.respond_to?(:call)
+              @value = instance_eval(&serializer._meta)
+            else
+              @value = serializer._meta
+            end
+          end
+
+          def as_json
+            @value
+          end
+
+          protected
+
+          attr_reader :object, :scope
+        end
+      end
+    end
+  end
+end

lib/active_model/serializer/meta.rb

@@ -0,0 +1,29 @@
+module ActiveModel
+  class Serializer
+    module Meta
+      extend ActiveSupport::Concern
+
+      included do
+        with_options instance_writer: false, instance_reader: true do |serializer|
+          serializer.class_attribute :_meta # @api private
+        end
+
+        extend ActiveSupport::Autoload
+      end
+
+      module ClassMethods
+        # Set the JSON API meta attribute of a serializer.
+        # @example
+        #   class AdminAuthorSerializer < ActiveModel::Serializer
+        #     meta { stuff: 'value' }
+        # @example
+        #     meta do
+        #       { comment_count: object.comments.count }
+        #     end
+        def meta(value = nil, &block)
+          self._meta = block || value
+        end
+      end
+    end
+  end
+end