use read_attribute_for_serialization to serialize ids

* in order to allow some optimisation in simple cases, we call `read_attribute_for_serialization` on parent with association_id name

lib/active_model/serializer/associations.rb

@@ -124,12 +124,9 @@ module ActiveModel
         end
 
         def serialize_ids
-          # Use pluck or select_columns if available
-          # return collection.ids if collection.respond_to?(:ids)
-          ids_key = "#{key.to_s.singularize}_ids"
-
-          if !option(:include) && !option(:embed_key) && source_serializer.object.respond_to?(ids_key)
-            source_serializer.object.send(ids_key)
+          ids_key = "#{@name.to_s.singularize}_ids".to_sym
+          if !option(:embed_key) && source_serializer.object.respond_to?(ids_key)
+            source_serializer.object.read_attribute_for_serialization(ids_key)
           else
             associated_object.map do |item|
               item.read_attribute_for_serialization(embed_key)
@@ -203,6 +200,8 @@ module ActiveModel
         end
 
         def serialize_ids
+          id_key = "#{@name}_id".to_sym
+
           if polymorphic?
             if associated_object
               {
@@ -212,8 +211,8 @@ module ActiveModel
             else
               nil
             end
-          elsif !option(:embed_key) && source_serializer.object.respond_to?("#{name}_id")
-            source_serializer.object.send("#{name}_id")
+          elsif !option(:embed_key) && 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)
           else

test/association_test.rb

@@ -379,6 +379,56 @@ class AssociationTest < ActiveModel::TestCase
 
       assert_equal 1, serialized_times
     end
+
+    def test_include_with_read_association_id_for_serialization_hook
+      @post_serializer_class.class_eval do
+        has_one :comment, :embed => :ids, :include => true
+      end
+
+      association_name = nil
+      @post.class_eval do
+        define_method :read_attribute_for_serialization, lambda { |name|
+          association_name = name
+          send(name)
+        }
+        define_method :comment_id, lambda {
+          @attributes[:comment].id
+        }
+      end
+
+      include_bare! :comment
+
+      assert_equal :comment_id, association_name
+
+      assert_equal({
+        :comment_id => 1
+      }, @hash)
+    end
+
+    def test_include_with_read_association_ids_for_serialization_hook
+      @post_serializer_class.class_eval do
+        has_many :comments, :embed => :ids, :include => false
+      end
+
+      association_name = nil
+      @post.class_eval do
+        define_method :read_attribute_for_serialization, lambda { |name|
+          association_name = name
+          send(name)
+        }
+        define_method :comment_ids, lambda {
+          @attributes[:comments].map(&:id)
+        }
+      end
+
+      include_bare! :comments
+
+      assert_equal :comment_ids, association_name
+
+      assert_equal({
+        :comment_ids => [1]
+      }, @hash)
+    end
   end
 
   class InclusionTest < AssociationTest