Does Cuba have a built-in way to avoid update problems?

We have been having problems with updating entities in edit screens because of primary keys. For instance, we want to update a rate with a new amount, but we cannot update it because the rate already exists in the database. We want to update the rate, not insert a new one. Something obvious is hiding in plain sight.

What type of primary key does your “rate” entity have? Can you provide a sample project showing the problem?