ENTITY FRAMEWORK Fluent API how to prevent creating a record if it already exists?

Question

C++ / C#

There are 2 tables with many to many relationship

public class Movie
    {
        [Key]
        public int MovieId{ get; set; }
        [MaxLength(100)]
        public string Title{ get; set; }
        public List<Country> Countrys{ get; set; }
    }
    public class Country
    {
        [Key]
        public int CountryId { get; set; }
        [MaxLength(10)]
        public string Key { get; set; }
        public List<Movie> Movies { get; set; }
    }

Tethered them to each other via Fluent API like this

modelBuilder.Entity<Cinema_Country>()
                .HasMany(c => c.Movies)
                .WithMany(c => c.Countrys)
                .Map(c =>
                {
                    c.ToTable("Cinema_CountryMovie");
                    c.MapRightKey("MovieId");
                    c.MapLeftKey("CountryId");
                });

But when you add the Country table to the Movie table and save the . Another entry is created in Country. How to forbid to add records to this table if record already exists?

Answer 1

This is because the EF change tracker does not have Country information at the time of saving.
You need to "attach" existing countries to the context before saving. Here is an example in the official docs