Clean up meta handling.

lib/active_model/serializer.rb

@@ -9,6 +9,7 @@ require 'active_model/serializer/configuration'
 require 'active_model/serializer/fieldset'
 require 'active_model/serializer/lint'
 require 'active_model/serializer/links'
+require 'active_model/serializer/meta'
 require 'active_model/serializer/type'
 
 # ActiveModel::Serializer is an abstract class that is
@@ -20,26 +21,10 @@ module ActiveModel
     include Attributes
     include Caching
     include Links
+    include Meta
     include Type
     require 'active_model/serializer/adapter'
 
-    with_options instance_writer: false, instance_reader: false do |serializer|
-      serializer.class_attribute :_meta # @api private : meta definition, @see Serializer#meta
-    end
-
-    # Register a meta attribute for the corresponding resource.
-    #
-    # @param [Hash] hash Optional hash
-    # @param [Block] block Optional block
-    def self.meta(hash = nil, &block)
-      self._meta =
-        if !block.nil?
-          block
-        else
-          hash
-        end
-    end
-
     # @param resource [ActiveRecord::Base, ActiveModelSerializers::Model]
     # @return [ActiveModel::Serializer]
     #   Preferentially returns
@@ -145,14 +130,6 @@ module ActiveModel
       end
     end
 
-    def meta
-      if self.class._meta.respond_to?(:call)
-        instance_eval(&self.class._meta)
-      else
-        self.class._meta
-      end
-    end
-
     protected
 
     attr_accessor :instance_options

lib/active_model/serializer/adapter/json_api.rb

@@ -6,6 +6,7 @@ module ActiveModel
         autoload :PaginationLinks
         autoload :FragmentCache
         autoload :Link
+        autoload :Meta
         autoload :Deserialization
 
         # TODO: if we like this abstraction and other API objects to it,
@@ -157,7 +158,7 @@ module ActiveModel
           links = links_for(serializer)
           resource_object[:links] = links if links.any?
 
-          meta = serializer.meta
+          meta = meta_for(serializer)
           resource_object[:meta] = meta unless meta.nil?
 
           resource_object
@@ -220,6 +221,10 @@ 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