diff --git a/lib/fast_jsonapi/object_serializer.rb b/lib/fast_jsonapi/object_serializer.rb
index 2d83d3b..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,
@@ -240,13 +244,16 @@ module FastJsonapi
           relationship_type: relationship_type,
           cached: options[:cached],
           polymorphic: fetch_polymorphic_option(options),
-          conditional_proc: options[:if],
-          id_method_name_for_inferred_objects: compute_object_method_name_for_inferred_objects(options[:id_method_name], block)
+          conditional_proc: options[:if]
         )
       end
 
-      def compute_object_method_name_for_inferred_objects(id_method_name, block)
-        (id_method_name.present? && block.present?) ? id_method_name : :id
+      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)
diff --git a/lib/fast_jsonapi/relationship.rb b/lib/fast_jsonapi/relationship.rb
index 797e489..7924969 100644
--- a/lib/fast_jsonapi/relationship.rb
+++ b/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, :id_method_name_for_inferred_objects
+    attr_reader :key, :name, :id_method_name, :record_type, :object_method_name, :object_block, :serializer, :relationship_type, :cached, :polymorphic, :conditional_proc
 
     def initialize(
       key:,
@@ -13,8 +13,7 @@ module FastJsonapi
       relationship_type:,
       cached: false,
       polymorphic:,
-      conditional_proc:,
-      id_method_name_for_inferred_objects:
+      conditional_proc:
     )
       @key = key
       @name = name
@@ -27,7 +26,6 @@ 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)
@@ -90,8 +88,8 @@ module FastJsonapi
     def fetch_id(record, params)
       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.try(id_method_name_for_inferred_objects)
+        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