Entity Framework Code First : how to annotate a foreign key for a “Default” value?

Question

I have 2 classes: Client and Survey.

Each Client can have many surveys - but only one default survey.

I have defined the classes like this:

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Answer 1

Entity Framework does exactly what it's told to do. What you've told it is that there is both a one-to-many and a one-to-one relationship between Clients and Surveys. It generated both FKs in the Survey table in order to map both of the relationships that you've requested. It has no idea that you're trying to connect the two relationships together, nor do I think does it have the ability to deal with that.

As an alternative you might want to consider adding a IsDefaultSurvey field on the Survey object so that you can query for the default survey through the Surveys collection that you have on the Client object. You could even go one step further and put it in as a NotMapped property on the Client object so that you could still use Client.DefaultSurvey to get the correct survey, and not have to change any of your other code, as follows:

[NotMapped]
public Survey DefaultSurvey
{
  get { return this.Surveys.First(s => s.IsDefaultSurvey); }
}