Use method instead of asssociation_ids if method exists. Fixes #267

lib/active_model/serializer/associations.rb

@@ -133,7 +133,7 @@ module ActiveModel
 
         def serialize_ids
           ids_key = "#{@name.to_s.singularize}_ids".to_sym
-          if !option(:embed_key) && source_serializer.object.respond_to?(ids_key)
+          if !option(:embed_key) && !source_serializer.respond_to?(@name.to_s) && source_serializer.object.respond_to?(ids_key)
             source_serializer.object.read_attribute_for_serialization(ids_key)
           else
             associated_object.map do |item|
@@ -219,7 +219,7 @@ module ActiveModel
             else
               nil
             end
-          elsif !option(:embed_key) && source_serializer.object.respond_to?(id_key)
+          elsif !option(:embed_key) && !source_serializer.respond_to?(@name.to_s) && source_serializer.object.respond_to?(id_key)
             source_serializer.object.read_attribute_for_serialization(id_key)
           elsif associated_object
             associated_object.read_attribute_for_serialization(embed_key)

test/serializer_test.rb

@@ -406,6 +406,33 @@ class SerializerTest < ActiveModel::TestCase
     }, serializer.as_json)
   end
 
+  def test_methods_take_priority_over_associations
+    post_serializer = Class.new(ActiveModel::Serializer) do
+      attributes :title
+      has_many :comments
+      embed :ids
+
+      def comments
+        object.comments[0,1]
+      end
+    end
+
+    post = Post.new(title: "My Post")
+    comments = [Comment.new(:title => "Comment1", :id => 1), Comment.new(:title => "Comment2", :id => 2)]
+    post.comments = comments
+
+    post.class_eval do
+      define_method :comment_ids, lambda {
+        self.comments.map { |c| c.read_attribute_for_serialization(:id) }
+      }
+    end
+    json = post_serializer.new(post).as_json
+    assert_equal({
+      title: "My Post",
+      comment_ids: [1]
+    }, json)
+  end
+
   def test_embed_objects
     serializer = post_serializer