EF Code First Navigation Property to same table

Question

I'm new to EF and struggling to implement the following scenario. I have an entity I'd like to have a navigation property to another of the same entity. E.g.

public class Stage {
    public int ID { get; set; }
    public int? NextStageID { get; set; }
    public string Name { get; set; }

    public virtual Stage NextStage { get; set;}
}

The only example I've found so far was where the entity had a parent / child relationship, i.e. the navigation property was an ICollection of the same entity. I tried adapting this but couldn't get it to work in my instance. Also, I only need it to be one way, i.e. the entity doesn't have a 'PreviousStage' property, just a 'NextStage' one. I'm configuring using Fluent API. Could someone advise if / how this can be achieved?

I am getting this error:

Unable to determine the principal end of an association between the types 'namespace.Stage' and 'namespace.Stage'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations

Edit Just realised in my slightly simplified example, I didn't show that NextStageID is optional (int?).

Answer 1

You can explicitly define the relation as follows:

public class Stage {
    public int ID { get; set; }
    public int NextStageID { get; set; }
    public string Name { get; set; }

    [ForeignKey("NextStageID ")]
    public virtual Stage NextStage { get; set;}
}