How to handle an association table linking the same 2 tables

Question

I am using Entity Framework 4.3 code first using an already existing database.

There is a Users table that has the following columns:

- UserID (int)
- FirstName (string)
- Surname (string)

I have an association table called Impersonations. The Impersonations table is an association table between a user table and the same user table. A user can have a list of users. Here is the Impersonations table structure:

- ImpersonateID (int primary key)
- UserID (int FK to Users table's UserID column)
- ImpersonatedUserID (int FK to Users table's UserID column)

I have a User class with properties that I want mapped to these columns:

public class User : IEntity
{
     public int Id { get; set; }
     public string FirstName { get; set; }
     public string LastName { get; set; }
     public string EmployeeNumber { get; set; }
     public virtual ICollection<User> ImpersonatedUsers { get; set; }
     public virtual ICollection<User> Users { get; set; }
}

In my db context class I have the following:

public DbSet<User> Users { get; set; }

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

User configuration:

class UserConfiguration : EntityTypeConfiguration<User>
{
     internal UserConfiguration()
     {
          this.Property(x => x.Id).HasColumnName("UserID");
          this.Property(x => x.LastName).HasColumnName("Surname");
          this.Property(x => x.EmployeeNumber).HasColumnName("StaffNumber");

          this.HasMany(i => i.Users)
               .WithMany(c => c.ImpersonatedUsers)
               .Map(mc =>
               {
                    mc.MapLeftKey("UserID");
                    mc.MapRightKey("ImpersonatedUserID");
                    mc.ToTable("Impersonations");
               });
     }
}

Did I link this up correctly using Entity Framework? If I pass through a user id then a list of users needs to be returned that was marked as inpersonated users. How I do this? I have this but both User and ImpersonatedUsers are empty:

return DbContext.Users
     .Include("ImpersonatedUsers")
     .SingleOrDefault(x => x.Id == userId);

Lets say I have the following data in my Users table (id, first name, last name, employee number):

7 Craig Matthews 123456
8 Susan Renshaw 234567
9 Michelle du Toit 34567

And the following data in my Impersonations table (impersonated id, user id, impersonated user id):

1 7 8
2 7 9

So if I pass in a user id of 7 then it need to return records for users Susan Renshaw and Michelle du Toit.

Just for clarity if you are still confused...

I have a similar situation. I have a Products table and a Specifications table. These 2 are linked up by the assication table called ProductSpecifications. I am trying to achieve the same here, but the 2 tables would be the Users table and the association table is Impersonations.

Answer 1

If I well understand, I think I have quite the same: a user is also a group and so can have members.

Here is an extract of my code:

public partial class User : AuthorizableBaseObject, IHasUID {
    public User() {
        Members = new List<User>();
    }

    public Guid UID { get; set; }
    public DateTime EmailValidationDate { get; set; }

    public String EMail { get; set; }

    //public Int64 Id { get; set; }
    public String Login { get; set; }
    public String Password { get; set; }
    public String Description { get; set; }
    public DateTime LastConnectionDate { get; set; }

    public Boolean CanConnect { get; set; }
    public Boolean IsDisabled { get; set; }

    public virtual List<User> Members { get; set; }
}

with the following configuration :

public class UserConfiguration : EntityTypeConfiguration<VDE.User> {
    public UserConfiguration()
        : base() {
        ToTable("Users", "dbo");

        Property(u => u.EMail).IsRequired().HasColumnType("nvarchar").HasMaxLength(100);
        Property(u => u.Login).IsRequired().HasColumnType("nvarchar").HasMaxLength(20);
        Property(u => u.Password).IsRequired().HasColumnType("nvarchar").HasMaxLength(50);
        Property(u => u.Description).HasColumnType("nvarchar").HasMaxLength(200);
        Property(u => u.LastConnectionDate).HasColumnType("datetime2");

        HasMany(u => u.Members).WithMany().Map(m => m.MapLeftKey("UserId").MapRightKey("MemberId").ToTable("UserMembers"));
    }
}

From here to get the members of a group the query is easy:

context.Users.Where(u => u.Id == someId).Select(u => u.Members).FirstOrDefault()

This will give an IEnumerable

Answer 2

Try taking a look at this page on inheritance. Your explanation isn't great, but I think this is what you're trying to achieve.

Answer 3

You must create a class for Impersonation and add it as a DBSet to the dbcontext. EF may make you put a [Key] attribute on the foreign key fields, along with [Column(Order = 0 or 1)].

Then you can lazy load the property:

    public virtual List<Impersonation> UserImpersonations
    {
        get
        {
            if(_userImpersonations == null)
            {
            // Set _userImpersonations = 
            // lazy load using a linq to sql query where 
            // UserImpersonations.UserId = this.UserId
            }

            return __userImpersonations;
        }
    }