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To define a composite primary key using EFCore Code First Migrations, you need to follow these steps:

1. Define your entity model with the properties you want to include in the composite primary key.

public class MyEntity
{
    public int KeyPart1 { get; set; }
    public int KeyPart2 { get; set; }
    public string OtherProperty { get; set; }
}

1. In the DbContext class, use the HasKey method to define the composite primary key.

public class MyDbContext : DbContext
{
    public DbSet<MyEntity> MyEntities { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<MyEntity>()
            .HasKey(e => new { e.KeyPart1, e.KeyPart2 });
    }
}

1. Generate a migration using the Add-Migration command in Package Manager Console.

2. Update the database using the Update-Database command.

Note that if your DbContext already has an existing primary key, you will need to remove it before adding the composite primary key. You can do this by using the HasAlternateKey method to define a unique constraint on the existing primary key property, and then using the HasKey method to define the composite primary key.

Here's an example:

public class MyEntity
{
    public int Id { get; set; }
    public int KeyPart1 { get; set; }
    public int KeyPart2 { get; set; }
    public string OtherProperty { get; set; }
}

public class MyDbContext : DbContext
{
    public DbSet<MyEntity> MyEntities { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<MyEntity>()
            .HasAlternateKey(e => e.Id)
            .HasName("AlternateKey_Id");

        modelBuilder.Entity<MyEntity>()
            .HasKey(e => new { e.KeyPart1, e.KeyPart2 });
    }
}

In this example, the HasAlternateKey method is used to define a unique constraint on the Id property, and the HasName method is used to give the constraint a specific name. This allows the primary key to be removed without losing the unique constraint.