Evaluate ids via the specified ‘id_method_name’ when relationships are evaluated via a block

lib/fast_jsonapi/object_serializer.rb

@@ -240,10 +240,15 @@ module FastJsonapi
           relationship_type: relationship_type,
           cached: options[:cached],
           polymorphic: fetch_polymorphic_option(options),
-          conditional_proc: options[:if]
+          conditional_proc: options[:if],
+          id_method_name_for_inferred_objects: compute_object_method_name_for_inferred_objects(options[:id_method_name], block)
         )
       end
 
+      def compute_object_method_name_for_inferred_objects(id_method_name, block)
+        (id_method_name.present? && block.present?) ? id_method_name : :id
+      end
+
       def compute_serializer_name(serializer_key)
         return serializer_key unless serializer_key.is_a? Symbol
         namespace = self.name.gsub(/()?\w+Serializer$/, '')

lib/fast_jsonapi/relationship.rb

@@ -1,6 +1,6 @@
 module FastJsonapi
   class Relationship
-    attr_reader :key, :name, :id_method_name, :record_type, :object_method_name, :object_block, :serializer, :relationship_type, :cached, :polymorphic, :conditional_proc
+    attr_reader :key, :name, :id_method_name, :record_type, :object_method_name, :object_block, :serializer, :relationship_type, :cached, :polymorphic, :conditional_proc, :id_method_name_for_inferred_objects
 
     def initialize(
       key:,
@@ -13,7 +13,8 @@ module FastJsonapi
       relationship_type:,
       cached: false,
       polymorphic:,
-      conditional_proc:
+      conditional_proc:,
+      id_method_name_for_inferred_objects:
     )
       @key = key
       @name = name
@@ -26,6 +27,7 @@ module FastJsonapi
       @cached = cached
       @polymorphic = polymorphic
       @conditional_proc = conditional_proc
+      @id_method_name_for_inferred_objects = id_method_name_for_inferred_objects
     end
 
     def serialize(record, serialization_params, output_hash)
@@ -86,13 +88,11 @@ 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 { |item| item.public_send(id_method_name_for_inferred_objects) } if object.respond_to? :map
-        return object.map(&:id) if object.respond_to? :map
+        return object.try(id_method_name_for_inferred_objects)
-        return object.try(:id)
       end
-
       record.public_send(id_method_name)
     end
   end

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] }
 

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
           end
         end
       end
@@ -110,7 +111,7 @@ RSpec.shared_context 'movie class' do
     end
 
     class Award
-      attr_accessor :id, :title, :actor_id
+      attr_accessor :id, :title, :actor_id, :imdb_award_id
     end
 
     class State