I'm not sure if this is intentional with Mongoid (it's not on http://mongoid.org/docs/validation.html if it is) or not but it's different behaviour from AR.

Basicly: take two records (Artist) with a uniqueness constraint on :name field. Both persisted to db with the same name. Using AR:

>> Artist.first.valid?
false

Mongoid:

>> Artist.first.valid?
true

I discovered this with validates_uniqueness_of. Not sure if it happens with other types of validations.

Mongoid 2.4.8 - ruby 1.9.3p125