diff --git a/.gitignore b/.gitignore
index 42303d4bb..92698dd8f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,3 +33,6 @@ tags
 Icon?
 ehthumbs.db
 Thumbs.db
+
+# sdk
+.idea
\ No newline at end of file
diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index b5b95736c..2ecfd48ca 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -343,6 +343,7 @@ def associations(include_directive = ActiveModelSerializers.default_include_dire
       Enumerator.new do |y|
         self.class._reflections.each do |key, reflection|
           next if reflection.excluded?(self)
+          next if reflection.options[:optional] && ( instance_options[:included].nil? || !instance_options[:included].include?(key.to_s) )
           next unless include_directive.key?(key)
 
           association = reflection.build_association(self, instance_options, include_slice)