Modify does not return an error on duplicate key

[Kozaky]

I have noticed that modify does not produce a duplicate key error when I try to update the field 'email' for an already in-use email. However, I see that the same query does produce an error in MongoDB if I do it manually.

Is this behaviour expected?

[VictorDenisov]

This driver uses whatever logic is built into OPS_UPDATE operation. There is a chance that operations implemented through OPS_MSG have the behavior that you describe.