 <p>It's still a regular many-to-many relationship, and it should get mapped correctly by default. Specific business (validation) rules do not change its nature.</p> <p>You can implement <a href="http://msdn.microsoft.com/en-us/library/system.componentmodel.dataannotations.ivalidatableobject.aspx" rel="nofollow">IValidatableObject</a> if you want to enforce specific cardinalities when saving.</p>