Add support for resource-level meta.

lib/active_model/serializer.rb

@@ -23,6 +23,23 @@ module ActiveModel
     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
@@ -128,6 +145,14 @@ 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

@@ -157,6 +157,9 @@ module ActiveModel
           links = links_for(serializer)
           resource_object[:links] = links if links.any?
 
+          meta = serializer.meta
+          resource_object[:meta] = meta unless meta.nil?
+
           resource_object
         end