Unable to determine composite primary key ordering for type

Question

{"Unable to determine composite primary key ordering for type 'Conference_Project.Models.Login'. Use the ColumnAttribute (see http://go.microsoft.com/fwlink/?LinkId=386388) or the HasKey method (see http://go.microsoft.com/fwlink/?LinkId=386387) to specify an order for composite primary keys."}

 [Table("ConferenceLogin")]
public class Login
{

    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public long confid { get; set; }
    [Key]
    public string emailID { get; set; }       
    [Key]
    public string registration { get; set; }
    [Key]
    public long regNo { get; set; }        
}

Version Used : EntityFramework.6.1.3  And MVC5

I want unique value for this (emailID , registration , regNo ) for that set all as primary key then EntityFramework showed error

how to use multiple primary key with EntityFramework?

how to use multiple primary key with EntityFramework? Click and read the links in the exception message. — Ivan Stoev
Have you want to create n:n (many-to-many) relationship between tables with Code First (show all tables related with it)? Usually HasKey before CreateModel() used like this: builder.Entity<ForeignTablePropertyName>().HasKey(p => new { p.emailID, p.registration, p.regNo });. — Tetsuya Yamamoto
solution already available in my error link its only need to set [Column(Order = 1)] order number — User
possible duplicate: stackoverflow.com/questions/11910962/… — Kapila Perera

Patrick R. Conner Patrick R. Conner · Accepted Answer · 2017-12-04T20:32:45

Although I'm not sure as to the underlying logic why this error occurs, I did have the same issue with a project I was working on for class. What worked for me was to add a Column(order) decorator to the Key tag turning

[Key]

into

[Key, Column(Order = n)]

where n is the 0-based index of the key in question.

In this light, your class should look like the following:

[Table("ConferenceLogin")]
public class Login
{
    [Key, Column(Order = 0), DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public long confid { get; set; }
    [Key, Column(Order = 1)]
    public string emailID { get; set; }       
    [Key, Column(Order = 2)]
    public string registration { get; set; }
    [Key, Column(Order = 3)]
    public long regNo { get; set; }        
}

Hope this works for you as well as it did for me!

Unable to determine composite primary key ordering for type

2 Answers