I got pretty experienced with testing controllers, my question here is though, aren't we supposed to test the data context as well, and how ? I mean, there are a lot of relationships and constraints coming from the DB that simply testing controllers does not cover.
On the other hand, testing against the DB is not considered a good practice - what then ? Simply testing without db.SubmitChanges() or what ?
Yes, you should do integration testing of your data context to ensure that any "code" that you put in the database itself works -- uniqueness constraints, triggers, etc. This doesn't imply that you should do your unit testing against the database, however. Having said that, any code you put in your model classes should be unit tested. Usually, you can do this without having to test against the database directly. For example, any validation code should be able to run without requiring that you actually insert or update the DB.
When it comes to testing your repositories, the typical approach is to create an in-memory database that can be torn down and rebuilt for each time you run your tests. By using this approach, you will always know what the data will look like so you can more easily make assertions against it. In addition, you won't be touching your real data, which is always a positive. Sqlite is the most popular one out there in the .NET space for this.
IMHO you should not test the DataContext. Hopefully Microsoft have already done this, so testing that SubmitChanges will persist data into the DB is pointless for me. You should do integration or web testing where you would define a specific scenarios and verify the output from the application.