During an investigation into a bugfix for MONGOID-4525, Oleg Pudeyev found that the current Mongoid::Persistable::Settable#set behavior doesn't match MongoDB's $set operator.

Text from issue pasted below:

One problem with this implementation is it merges leaf hash values instead of writing them as is (which is what mongo shell does).

I.e. if I have {'a' => {'b' => 'c'}} and I set 'a' => {'d' => 'e'}, the result should be {'a' => {'d' => 'e'}} but it currently is {'a' => {'b' => 'c', 'd' => 'e'}}. (To achieve the merge on the leaf level the operation should be to set 'a.d' => 'e'.)