EF Core ForeignKey Attribute

The ForeignKey attribute is used to specify which property is the foreign key in a relationship.

In the following example, the AuthorFK property in the Book entity does not follow Entity Framework Core's convention for foreign key names. Left as it is, Entity Framework Core will create an AuthorFK field and an AuthorId field which it will configure as a foreign key:

public class Author
{
    public int AuthorId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public ICollection<Book> Books { get; set; }
}

public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
    public Author Author { get; set; }
    public int AuthorFK { get; set; }
}

The ForeignKey attribute can be used in the following ways to configure the relationship correctly, preventing the generation of an AuthorId column in the Books table.

1. On the foreign key property in the dependent class, passing in the name of the navigation property:

   language-csharp

   Explain code | Copy code Copied!

   public class Book
   {
       public int BookId { get; set; }
       public string Title { get; set; }
       public Author Author { get; set; }
       [ForeignKey("Author")]
       public int AuthorFK { get; set; }
   }
   

2. On the navigation property in the dependent class, passing in the name of the foreign key property:

   language-csharp

   Explain code | Copy code Copied!

   public class Book
   {
       public int BookId { get; set; }
       public string Title { get; set; }
       [ForeignKey("AuthorFK")]
       public Author Author { get; set; }
       public int AuthorFK { get; set; }
   }
   

3. On the navigational property in the principal class, passing in the name of the foreign key property in the dependent class:

   language-csharp

   Explain code | Copy code Copied!

   public class Author
   {
       public int AuthorId { get; set; }
       public string FirstName { get; set; }
       public string LastName { get; set; }
       [ForeignKey("AuthorFK")]
       public ICollection<Book> Books { get; set; }
   }
   

If you place the ForeignKey attribute on a navigation property that has multiple foreign keys, separate each one with a comma.

Fluent API

The HasForeignKey method is used to specify foreign keys when using the Fluent API.