How to associate both a collection and one item of this collection to an entity

Question

I'm working on a project in which I have an entity, we may call Users and another entity
Address.
I want to define the entity User in a way that an user would have a collection of addresses (typically a One-To-Many relationship), but I also need an attribute of user, let it be
main_address that would target a single value of the previous collection.
I would like to know if there is a conventional of doing this. The only solution I have now is to create another One-to-One relationship between
Users . main_address and Address but this does not insure that the
main_address will actually be part of the User . addresses collection.
I don't know if it could be important, because it's a rather theoretical question, but I'm using Doctrine 2.

Answer 1

Add a field main_address_id in the User entity or table that holds the ID of the Address record that is the main one. Make that a one to one relationship, if you like.
The only solution I have now is to create another One-to-One relationship between
Users.main_address and Address but this does not insure that the
main_address will actually be part of the User.addresses collection.
Quite right. You need another mechanism that insures that. You can either validate that in your business logic, or use a stored procedure or trigger on the RDBMS during creation or editing of
User that enforces this restriction.
Doctrine might have a "business rules" module that you can use. The point is that the additional restriction you are imposing on
main_address_id doesn't have anything to do with your database schema, per se. It's a business rule, basically.