https://jira.mongodb.org/browse/MONGOID-5330 changed operator methods to always be additive to the existing query instead of replacing the previous conditions on the top level. The old behavior is still available via a feature flag.

We could implement merge strategies over all operators, including the ones affected by https://jira.mongodb.org/browse/MONGOID-5330, to provide a more convenient way of obtaining the previous replacement behavior.

However, as described in https://jira.mongodb.org/browse/MONGOID-5350, merge strategies do not currently work when conditions are added under $and, and this probably needs to be resolved first.