in entity framework code first, how to use KeyAttribute on multiple columns

Question

I'm creating a POCO model to use with entity framework code first CTP5. I'm using the <key()> decoration to make a property map to a PK column. But how can I define a PK on more then one column, and specifically, how can I control order of the columns in the index? Is it a result of the order of properties in the class?

Edit: look at @kara's answer for an updated solution.

Thanks!

Answer 1

NOTE: As of 2019 this answer became non-valid for later EntityFramework versions.

You can specify the column order in the attributes, for instance:

public class MyEntity
{
    [Key, Column(Order=0)]
    public int MyFirstKeyProperty { get; set; }

    [Key, Column(Order=1)]
    public int MySecondKeyProperty { get; set; }

    [Key, Column(Order=2)]
    public string MyThirdKeyProperty { get; set; }

    // other properties
}

If you are using the Find method of a DbSet you must take this order for the key parameters into account.

Answer 2

To complete the correct answer submitted by Slauma, you can use the HasKey method to specify an order for composite primary keys as well:

public class User
{        
    public int UserId { get; set; }       
    public string Username { get; set; }        
}        

public class Ctp5Context : DbContext
{
    public DbSet<User> Users { get; set; }        

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>().HasKey(u => new 
        { 
            u.UserId, 
            u.Username 
        });
    }
}

Answer 3

If, like me, you prefer to use a configuration file you can do that in this way (based on Manavi's example):

public class User
{
    public int UserId { get; set; }
    public string Username { get; set; }
}  

public class UserConfiguration : EntityTypeConfiguration<User>
{
    public UserConfiguration()
    {
        ToTable("Users");
        HasKey(x => new {x.UserId, x.Username});
    }
}

Obviously you have to add the configuration file to your context:

public class Ctp5Context : DbContext
{
    public DbSet<User> Users { get; set; }        

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
         modelBuilder.Configurations.Add(new UserConfiguration());
    }
}

Answer 4

In EF Core 7.0 a new attribute for classes [PrimaryKey] was introduced.

Example:

[PrimaryKey(nameof(FirstName),nameof(LastName))]
public class Person
{
    public string FirstName { get; set; }

    public string LastName { get; set; }

    public string SomeProperty { get; set; }
}

This way, you don't have to use the fluent-api again.

Answer 5

Use as a anonymous object:

modelBuilder.Entity<UserExamAttemptQuestion>().ToTable("Users").HasKey(o => new { o.UserId, o.Username });