Many-to-many relation and unique constraint. How do I do that in EF Core?

Question

I have two entities Country and Business.

public class Country
{
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Business> Businesses { get; set; }

    public Country()
    {
        Businesses = new List<Business>();
    }
}

public class Business
    {
        public int Id { get; set; }
        public string Name { get; set; }

        public ICollection<Country> Countries { get; set; }

        public Business()
        {
            Countries = new List<Country>();
        }
    }

Country can have many Businesses inside it and Businesses can be present in many Countries. Business should be unique inside a country.

How can I make Business unique for the country in the many-to-many relation?

Answer 1

You can accomplish this with a composite key.

First you need a third entity to represent the association:

public class CountryBusiness {
    public int CountryId {get; set;}
    public Country Country {get; set;}

    public int BusinessId {get; set;}
    public Business Business {get; set;}
}

Then, you need to give it a composite key in your DbContext class:

protected override void OnModelCreating(ModelBuilder builder)
{
    base.OnModelCreating(builder);
    builder.Entity<CountryBusiness>().HasKey(cb => new {cb.CountryId, cb.BusinessId});
}