Composite DB keys with Entity Framework 4.0

Question

The re-design for a large database at our company makes extensive use of composite primary keys on the database.

Forgetting performance impacts, will this cause any

Answer 1

No, EF4 supports composite keys just fine.

The problem is a table with a surrogate key and composite keys. You can only set a single key on each model; that key can have multiple fields, but you can only have one from the designer standpoint. Not sure about manually editing xml or code only mapping.

You can set a field as an Identity and not a key if you need a composite and surrogate key on the same table. The Identity ( Id ) field won't be used by the ObjectContext or ObjectStateTracker but will increment and be queryable just fine though.

Answer 2

I have had problems with EF4 and composite keys. It doesn't support columns being used as components in more than one key in a join table.

See my previous question Mapping composite foreign keys in a many-many relationship in Entity Framework for more details. The nuts of it is that when you have a join table (describing a many-many relationship) where both of the relationships use a common key, you'll get an error like

Error 3021: Problem in mapping fragments...: Each of the following columns in table PageView is mapped to multiple conceptual side properties: PageView.Version is mapped to (PageView_Association.View.Version, PageView_Association.Page.Version)

The only way around it was to duplicate the column which defeats the purpose of having it there at all.

Good luck!