EF Core HasKey

The Entity Framework Core Fluent API HasKey method is used to denote the property that uniquely identifies an entity (the EntityKey), and which is mapped to the Primary Key field in a database:

public class SampleContext : DbContext
{
    public DbSet<Order> Orders { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Order>()
            .HasKey(o => o.OrderNumber);
    }
}

public class Order
{
    public int OrderNumber { get; set; }
    public DateTime DateCreated { get; set; }
    public Customer Customer { get; set; }
    ...
)

In the example above, the name of the key field in the Order class does not meet the Entity Framework Core's naming convention for entity keys, so the HasKey method is used to configure the OrderNumber property as the key field.

Composite Keys

Entity Framework Core supports composite keys - primary key values generated from two or more fields in the database. Composite keys are not covered by conventions or data annotation attributes. The only way to configure composite keys is to use the HasKey method. You specify the properties that form the composite key by passing them in as properties of an anonymous type to the HasKey method. In the following example, the composite key comprises an abbreviated version of the customer name and an integer e.g. ALFKI0001:

public class SampleContext : DbContext
{
    public DbSet<Order> Orders { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Order>()
            .HasKey(o => new { o.CustomerAbbreviation, o.OrderNumber });
    }
}

public class Order
{
    public string CustomerAbbreviation { get; set; }
    public int OrderNumber { get; set; }
    public DateTime DateCreated { get; set; }
    public Customer Customer { get; set; }
    ...
)

Data Annotation Attributes

The Data Annotation attribute alternative to the HasKey method is the Key attribute.