Unable to determine the principal end of an association - Entity Framework Model First

Question

I have created Entity Data Model in Visual Studio. Now I have file with SQL queries and C# classes generated from Model.

Question:

Classes are generated without annotations or code behind (Fluent API). Is it OK? I tried to run my application but exception was thrown:

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

I read that I can not use Fluent API with "Model First". So what can I do?

Code:

User

public partial class User
{
    public User()
    {
        this.Events = new HashSet<Event>();
        this.CreatedEvents = new HashSet<Event>();
    }

    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Photo { get; set; }
    public int EventId { get; set; }
    public string Nickname { get; set; }
    public OwnerType OwnerType { get; set; }
    public NetworkPlaceType PlaceType { get; set; }

    public virtual ICollection<Event> Events { get; set; }
    public virtual Address Address { get; set; }
    public virtual ICollection<Event> CreatedEvents { get; set; }
    public virtual Owner Owner { get; set; }
}

Address

public partial class Address
{
    public int Id { get; set; }
    public string Street { get; set; }
    public string StreetNumber { get; set; }
    public string City { get; set; }
    public string ZipCode { get; set; }
    public string Country { get; set; }

    public virtual User User { get; set; }
}

Context

//Model First does not use this method

protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Address>().HasRequired(address => address.User)
                                   .WithRequiredDependent();
        modelBuilder.Entity<User>().HasRequired(user => user.Address)
                                   .WithRequiredPrincipal();

        base.OnModelCreating(modelBuilder);
    }

Answer 1

You have to specify the principal in a one-to-one relationship.

public partial class Address
{
    [Key, ForeignKey("User")]
    public int Id { get; set; }
    public string Street { get; set; }
    public string StreetNumber { get; set; }
    public string City { get; set; }
    public string ZipCode { get; set; }
    public string Country { get; set; }

    public virtual User User { get; set; }
}

By specifying a FK constraint, EF knows the User must exists first (the principal) and the Address follows.

Further reading at MSDN.

Also, see this SO answer.

Updated from comments

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In the designer, select the association (line between Users & Address). On the properties window, hit the button with the [...] on Referential Constraint (or double click the line). Set the Principal as User.

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