Add DSL for assocations

lib/active_model/serializer.rb

@@ -2,16 +2,18 @@ module ActiveModel
   class Serializer
     class << self
       attr_accessor :_attributes
+      attr_accessor :_associations
     end
 
     def self.inherited(base)
       base._attributes = []
+      base._associations = {}
     end
 
     def self.attributes(*attrs)
       @_attributes.concat attrs
 
-      
+
       attrs.each do |attr|
         define_method attr do
           object.read_attribute_for_serialization(attr)
@@ -19,6 +21,41 @@ module ActiveModel
       end
     end
 
+    # Defines an association in the object should be rendered.
+    #
+    # The serializer object should implement the association name
+    # as a method which should return an array when invoked. If a method
+    # with the association name does not exist, the association name is
+    # dispatched to the serialized object.
+    def self.has_many(*attrs)
+      associate(:has_many, attrs)
+    end
+
+    # Defines an association in the object should be rendered.
+    #
+    # The serializer object should implement the association name
+    # as a method which should return an object when invoked. If a method
+    # with the association name does not exist, the association name is
+    # dispatched to the serialized object.
+    def self.belongs_to(*attrs)
+      associate(:belongs_to, attrs)
+    end
+
+    def self.associate(type, attrs) #:nodoc:
+      options = attrs.extract_options!
+      self._associations = _associations.dup
+
+      attrs.each do |attr|
+        unless method_defined?(attr)
+          define_method attr do
+            object.send attr
+          end
+        end
+
+        self._associations[attr] = {type: type, options: options}
+      end
+    end
+
     if RUBY_VERSION >= '2.0'
       def self.serializer_for(resource)
         if resource.respond_to?(:to_ary)