如何与Entity Framework建立一对一或零关系，并在子实体上公开外键？

Question

I have a working configuration for a one-to-one relationship that looks like this:

public class ParentEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; } 
}

public class ChildEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; }

    [Required]
    public Guid ParentId { get; set; }

    [ForeignKey("ParentId")]
    public ParentEntity Parent { get; set; }
}

This solution is in production and works perfectly. Now, I need to be able to reference the child entity from the parent entity, so I tried doing this:

public class ParentEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; }

    public virtual ChildEntity Child { get; set; }
}

And I received this error:

System.InvalidOperationExceptionUnable to determine the principal end of an association between the types 'ChildEntity' and 'ParentEntity'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.

I've come across multiple solutions for how to configure the principle with the Fluent API, but nothing has allowed me to add the ChildEntity reference on ParentEntity and maintain the ParentId on the ChildEntity .

Answer 1

In the first solution EF is creating an unidirectional one to many relationship. To help you understand better what is going on, the configuration of that relationship using Fluent Api would be:

modelBuilder.Entity<ChildEntity>()
            .HasRequired(ce=>ce.Parent)
            .WithMany()
            .HasForeignKey(ce=>ce.ParentId);

Now, when you add the ChildEntity navigation property, EF understands you want to create a one-to one relationship. In an one to one relationship EF requires that the PK of the dependent entity also be the FK of the relationship:

public class ParentEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; } 
    public virtual ChildEntity Child { get; set; }
}

public class ChildEntity
{
    [Key, ForeignKey("Parent")]
    public Guid Id { get; set; }

    public ParentEntity Parent { get; set; }
}

But I don't think this is the model you are looking for because one ParentEntity could be parent of more than one ChildEntity , so in my opinion you need to configure an one to many relationship:

public class ParentEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; } 

    public virtual ICollection<ChildEntity> Children { get; set; }
}

public class ChildEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; }

    [ForeignKey("Parent")]
    public Guid ParentId { get; set; }

    public ParentEntity Parent { get; set; }
}

Update

If you want to create an one to one relationship and both entities have their owns PKs, then you can't define a FK property in the dependent entity due to the restriction I explain at the beginning of my answer about the one to one relationships. A solution for what you need could be using a model like this:

public class ParentEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; } 
    public virtual ChildEntity Child { get; set; }
}

public class ChildEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; }

    public ParentEntity Parent { get; set; }
}

Add this Fluent Api configuration to your context to specify who is the principal and the dependent in your relationship and also specify the name of the FK that you already have on your DB (you can't have a property on your entity with that name):

modelBuilder.Entity<ChildEntity>()
            .HasRequired(sr => sr.Parent)
            .WithOptional(s => s.Child)
            .Map(c=>c.MapKey("Parent_Id"));