How to query the database table with no primary key

The answer is [Column(Order = 0), Key]

[Table("Completion")]
public class CompletionsModel
{
    [Column(Order = 0), Key]
    public int UserId { get; set; }

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

    public DateTime CompletionDate { get; set; }

    public virtual CourseModel PRD { get; set; }
    public virtual StudentModel UserProfile { get; set; }
}