Starting with version 7.1.0, MongoID models can no longer utilize Rails' delegate method. Attempting to call the method results in an ArgumentError.

I inspected this issue a little further and it turned out that delegate no longer refers to the Rails' method but to Ruby's built-in Forwardable module.

Rails' Module#delegate and Ruby's Forwardable#delegate have different semantics and cannot be used interchangeably.

If I'm not mistaken, this is due to https://github.com/mongodb/mongoid/commit/3cfabfd2e41c8e3738aa6e381b9113d89275f2f5 which added the Forwardable module to MongoID.