Entity framework 6 code first - one way many to many via annotations

Question

Is it possible to create one-way many-to-many association in entity framework 6 with code first and annotations? Example:

class Currency
{
    public int id          


        

Answer 1

You can do this by specifying the relationship explicitly using the Fluent API. Override the OnModelCreating() method of your DbContext class, and in your override specify the details of the mapping table like this:

class MyContext : DbContext
{
    public DbSet<Currency> Currencies { get; set; }
    public DbSet<Country> Countries { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Country>()
            .HasMany(c => c.Currencies)
            .WithMany()                 // Note the empty WithMany()
            .Map(x =>
            {
                x.MapLeftKey("CountryId");
                x.MapRightKey("CurrencyId");
                x.ToTable("CountryCurrencyMapping");
            });

        base.OnModelCreating(modelBuilder);
    }
}

Note that - in my quick test anyway - you will have to Include() the Currencies property when loading the EF object to have the list populated:

            var us = db.Countries
                        .Where(x => x.Name == "United States")
                        .Include(x=>x.Currencies)
                        .First();

EDIT

If you really want to do everything with Data Annotations, and not use Fluent at all, then you can model the join table explicitly as pointed out elsewhere. There are various usability disadvantages of this approach, though, so it seems the Fluent method is the best approach.

class Country
{
    public int Id { get; set; }
    public virtual ICollection<CountryCurrency> CountryCurrencies { get; set; }
}

class Currency
{
    public int Id { get; set; }
}

class CountryCurrency
{
    [Key, Column(Order=0)]
    public virtual int CountryId { get; set; }
    [Key, Column(Order=1)]
    public virtual int CurrencyId { get; set; }

    public virtual Country Country { get; set; }
    public virtual Currency Currency { get; set; }
}

Answer 2

I think you wanna learn how to separate relations from EF code first entities. I have started a topic in here about this question. I wanna separate the relation objects from entity, and I used partial classes. In my question I wanna learn how to separate partial classes by class bilrary. But couldn't.

While I was using NHibernate I was using XML mapping and creating relations here, In java platform is same thing. But I think Entity Framework is not ready yet.

Answer 3

You can do this in code first quite easily in EF 6.

    public class Country
{
   public int ID {get;set;}

   public virtual ICollection<Currency> Currencys {get;set;}//don't worry about the name,     pluralisation etc


}

public class Currency
{

   public int ID {get;set;}

   public virtual ICollection<Country> Countrys {get;set;}//same as above - 

}

Compile it, run it and hey presto - magic join table in the background. Depends if the naming conventions bother you. I personally think if you are doing code first, you should do it all in the code. Some people prefer annotation, some prefer fluent API - use whichever you prefer.