Question

How can I validate or block user to do Product.Ingredients.Add(new Ingredient()) ?
When user do it, ever a new ingredient is added into ingredient table. I want it add only existing ingredients.

Answer 1

You can check that by overriding the save changes method in your DBContext.

public class YourDbContext:DBContext
{
//......

   public override int SaveChanges(){

      foreach(var entry in ChangeTracker.Entries<Ingredient>())
      {
         var entity = entry.Entity;
        if (entry.State == EntityState.Added)
        {
           this.Detach(entity );
        }
       }
     base.SaveChanges();
  }

}