diff --git a/lib/fast_jsonapi/object_serializer.rb b/lib/fast_jsonapi/object_serializer.rb
index 7f740c8..d6015c5 100644
--- a/lib/fast_jsonapi/object_serializer.rb
+++ b/lib/fast_jsonapi/object_serializer.rb
@@ -232,7 +232,11 @@ module FastJsonapi
         Relationship.new(
           key: options[:key] || run_key_transform(base_key),
           name: name,
-          id_method_name: options[:id_method_name] || "#{base_serialization_key}#{id_postfix}".to_sym,
+          id_method_name: compute_id_method_name(
+            options[:id_method_name],
+            "#{base_serialization_key}#{id_postfix}".to_sym,
+            block
+          ),
           record_type: options[:record_type] || run_key_transform(base_key_sym),
           object_method_name: options[:object_method_name] || name,
           object_block: block,
@@ -244,6 +248,14 @@ module FastJsonapi
         )
       end
 
+      def compute_id_method_name(custom_id_method_name, id_method_name_from_relationship, block)
+        if block.present?
+          custom_id_method_name || :id
+        else
+          custom_id_method_name || id_method_name_from_relationship
+        end
+      end
+
       def compute_serializer_name(serializer_key)
         return serializer_key unless serializer_key.is_a? Symbol
         namespace = self.name.gsub(/()?\w+Serializer$/, '')
diff --git a/lib/fast_jsonapi/relationship.rb b/lib/fast_jsonapi/relationship.rb
index 0b3a101..7924969 100644
--- a/lib/fast_jsonapi/relationship.rb
+++ b/lib/fast_jsonapi/relationship.rb
@@ -86,13 +86,11 @@ module FastJsonapi
     end
 
     def fetch_id(record, params)
-      unless object_block.nil?
+      if object_block.present?
         object = object_block.call(record, params)
-
-        return object.map(&:id) if object.respond_to? :map
-        return object.try(:id)
+        return object.map { |item| item.public_send(id_method_name) } if object.respond_to? :map
+        return object.try(id_method_name)
       end
-
       record.public_send(id_method_name)
     end
   end
diff --git a/spec/lib/object_serializer_class_methods_spec.rb b/spec/lib/object_serializer_class_methods_spec.rb
index 3c477c3..33138d5 100644
--- a/spec/lib/object_serializer_class_methods_spec.rb
+++ b/spec/lib/object_serializer_class_methods_spec.rb
@@ -87,6 +87,31 @@ describe FastJsonapi::ObjectSerializer do
     end
   end
 
+  describe '#has_many with block and id_method_name' do
+    before do
+      MovieSerializer.has_many(:awards, id_method_name: :imdb_award_id) do |movie|
+        movie.actors.map(&:awards).flatten
+      end
+    end
+
+    after do
+      MovieSerializer.relationships_to_serialize.delete(:awards)
+    end
+
+    context 'awards is not included' do
+      subject(:hash) { MovieSerializer.new(movie).serializable_hash }
+
+      it 'returns correct hash where id is obtained from the method specified via `id_method_name`' do
+        expected_award_data = movie.actors.map(&:awards).flatten.map do |actor|
+          { id: actor.imdb_award_id.to_s, type: actor.class.name.downcase.to_sym }
+        end
+        serialized_award_data = hash[:data][:relationships][:awards][:data]
+
+        expect(serialized_award_data).to eq(expected_award_data)
+      end
+    end
+  end
+
   describe '#belongs_to' do
     subject(:relationship) { MovieSerializer.relationships_to_serialize[:area] }
 
diff --git a/spec/shared/contexts/movie_context.rb b/spec/shared/contexts/movie_context.rb
index b92d3d5..f2c7697 100644
--- a/spec/shared/contexts/movie_context.rb
+++ b/spec/shared/contexts/movie_context.rb
@@ -80,6 +80,7 @@ RSpec.shared_context 'movie class' do
             a.id = i
             a.title = "Test Award #{i}"
             a.actor_id = id
+            a.imdb_award_id = i * 10
             a.year = 1990 + i
           end
         end
@@ -111,7 +112,7 @@ RSpec.shared_context 'movie class' do
     end
 
     class Award
-      attr_accessor :id, :title, :actor_id, :year
+      attr_accessor :id, :title, :actor_id, :year, :imdb_award_id
     end
 
     class State