Upgrade to Mongoid 6.3.0
Mongoid 6.2 is incompatible with MongoDB 3.6. It seems as though they dropped $pushAll support which Mongoid 6.2 relied on. Mongoid 6.3.0 fixes this and is compatible with MongoDB 3.6. There is a minor configuration change that may not be obvious because Mongoid generates the wrong config parameters.
When you run Mongoid 6.3 with “localhost” in mongoid.yml it throws
Address family not supported by protocol family error and tends to lock up. Regenerating “mongoid.yml” still insists on using “localhost”. It needs to be changed to 127.0.0.1.
Make that simple change and it works fine.