ADO.NET Entity Data Model - Duplicates

How c开发者_高级运维an I add a Unqiue index to a column like in SQL, so it wont allow duplicates in the column?

1. Double click your EDMX file. the entity model designer gui comes up.
2. Add one or more Entities if you don't have any already.
3. Select a field in an entity, and double-click it. The field property window should open up.
4. Select the property "ENTITY KEY" and set it to True.

As far as I understood, this makes the field unique.

Cheers, G.