[dspillett]

Unfortunately not.

An ORM may allow you to pass through queries or other directives that have DBMS specific behaviours so if you use any features like that the ORM can't protect you at all. Further more the ORM might change the way it talks to the underlying DB depending on what it is, to make use of efficiency enhancing features that one DB has but others may not, again you are not testing like-for-like in this case. The ORM itself may have bugs that are only apparent when exposed to a given DB.

An ORM often protects you from needing to know the specifics of the storage engine underneath, but you still need to test against the same storage engine(s) as you expect to see in production.