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Adding a foreign key relation in Entity Framework 4 (EF4)

by 8. July 2010 15:52

The EF4 bits are excellent but one of the pains i have had was how to use the EF designer and add a foreign key mapping to an entity. So you have a User entity and an Address entity and the Address entity has a UserId foreign key that links to the User Id property.

I kept running into an issue that said the Address entity wasn't mapped and specifically the foreign key. I had no idea what to do to solve this. I checked the Xml behind the EDMX and found that not storage model had been created to allow that mapping. This seems odd to me. I would think it would create it when you are working in the designer and make the update upon commit!?

Anyway, the resolution to this is to right-click the model and say Generate Database from Model. This creates the data scripts and once executed it creates the tables in your database and at the same time updates you entity model to allow the mapping to happen. All works after that.

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Tags: ef, designer, sql

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Comments

7/8/2010 4:13:15 PM #

Do you really want the foreign key showing up on your Entity though. Surely foreign keys are database constructs and shouldn't have anything to do with your domain model. To me this:

int userId = currentAddress.User.Id;

...just seems more semantic, elegant and and more 'object orienty', then having a foreign key mapping in your model where it doesn't belong.

Though I don't know about EF, but most modern ORMs (such as NHibernate) won't incur the additional database hit with addressObj.User.Id because it knows you're doing the equivalent of accessing the foreign key ID.

Dominic Pettifer | Reply

7/8/2010 4:16:50 PM #

It is a bit of an argument around that but can be useful in constructing instances where you don't want to create every entity to do so. Just use an id - esp when it is being passed to you anyway.

In any case you still need the FK on that entity even if you don't intend to expose it - simply to link the data at the storage layer (EF allows you to hide it if you wish, but you still need the relationship of course).

Steven | Reply

7/8/2010 5:16:44 PM #

Yes, you're right, the FK still needs to be there but it should be hidden by the ORM (which EF and NHibernate allow).

As for the constructing entities thing, a lot of devs are adopting DDD (Domain Driven Design) principles in conjunction with an ORM which advocates always passing the real entities around over Int32 identifiers, it's more objecty that way For instance, your User entity would be the Aggregate Root of an Aggregate that consists of your Address entity as well. So to construct an Address entity, you'd first get a reference to a User, then add an Address to the Addresses collection on User (making sure the User parent association is mapped on the other side with the Address entity), then save the User entity.

It's a more object-oriented design driven approach thinking in OO design principles, rather that coming at it from a database table relational design approach. Though DDD is pretty hardcore approach, maybe only suitable for massive enterprisey systems

Dominic Pettifer | Reply

7/8/2010 5:36:28 PM #

I know - is the way i do it just now. Overkill in many cases when you maybe already passed the id's in the querystring.

I'm 50/50 on it - in Xml i often used ID and IDREF rather than passing the entire Xml fragment. I'll probably use a mix of both.

Steven | Reply

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