Caution

This documentation is for EF Core. For EF6.x and earlier release see http://msdn.com/data/ef.

Alternate Keys

An alternate key serves as a alternate unique identifier for each entity instance in addition to the primary key. When using a relational database this maps to the concept of a unique index/constraint. In EF, alternate keys provide greater functionality than unique Indexes because they can be used as the target of a foreign key.

Alternate keys are typically introduced for you when needed and you do not need to manually configure them. See Conventions for more details.

In this article:

Conventions

By convention, an alternate key is introduced for you when you identify a property, that is not the primary key, as the target of a relationship.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
    class MyContext : DbContext
    {
        public DbSet<Blog> Blogs { get; set; }
        public DbSet<Post> Posts { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Post>()
                .HasOne(p => p.Blog)
                .WithMany(b => b.Posts)
                .HasForeignKey(p => p.BlogUrl)
                .HasPrincipalKey(b => b.Url);
        }
    }

    public class Blog
    {
        public int BlogId { get; set; }
        public string Url { get; set; }

        public List<Post> Posts { get; set; }
    }

    public class Post
    {
        public int PostId { get; set; }
        public string Title { get; set; }
        public string Content { get; set; }

        public string BlogUrl { get; set; }
        public Blog Blog { get; set; }
    }

Data Annotations

Alternate keys can not be configured using Data Annotations.

Fluent API

You can use the Fluent API to configure a single property to be an alternate key.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
    class MyContext : DbContext
    {
        public DbSet<Car> Cars { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Car>()
                .HasAlternateKey(c => c.LicensePlate);
        }
    }

    class Car
    {
        public int CarId { get; set; }
        public string LicensePlate { get; set; }
        public string Make { get; set; }
        public string Model { get; set; }
    }

You can also use the Fluent API to configure multiple properties to be an alternate key (known as a composite alternate key).

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
    class MyContext : DbContext
    {
        public DbSet<Car> Cars { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Car>()
                .HasAlternateKey(c => new { c.State, c.LicensePlate });
        }
    }

    class Car
    {
        public int CarId { get; set; }
        public string State { get; set; }
        public string LicensePlate { get; set; }
        public string Make { get; set; }
        public string Model { get; set; }
    }