After upgrading mongoid-rspec it also upgrades mongoid from 6.2.1 to 6.4.0 and mongo from 2.4.3 to 2.5.1.

-    mongo (2.4.3)
-      bson (>= 4.2.1, < 5.0.0)
-    mongoid (6.2.1)
-      activemodel (~> 5.1)
-      mongo (>= 2.4.1, < 3.0.0)
+    mongo (2.5.1)
+      bson (>= 4.3.0, < 5.0.0)
+    mongoid (6.4.0)
+      activemodel (>= 5.1, < 6.0.0)
+      mongo (>= 2.5.1, < 3.0.0)
+    mongoid-compatibility (0.5.1)
+      activesupport
+      mongoid (>= 2.0)

After this tests of my rails app failed with the following error:

undefined method `retry_writes?' for true:TrueClass

It happens when I am trying to update the image attributes.

def edit
  load_image
  @image.update_attributes shown_at: DateTime.now
   render layout: 'plain'
end

Tracing back it shows me that the code really breaks is here: https://github.com/mongodb/mongo-ruby-driver/blob/master/lib/mongo/retryable.rb#L125

def retry_write_allowed?(session, write_concern)
   session && session.retry_writes? &&
       (write_concern.nil? || write_concern.acknowledged?)
end

So session is here only a boolean and not a session object somehow. I could not find any infos concerning upgrade changes so i consider this as a bug?