Question

I have two classes defined as such:

public class Questionnaire
    {
        public int QuestionnaireID { get; set; }
        public string Title { get; set; }
        public bool Active { get; set; }
        public virtual ICollection<Question> Questions { get; set; }
        public virtual ICollection<Vendor> Vendors { get; set; }
    }

public class Vendor
    {
        public int VendorID { get; set; }

        public string VendorName { get; set; }

        public virtual ICollection<Questionnaire> OpenQuestionnaires { get; set; }

        public virtual ICollection<Questionnaire> SubmittedQuestionnaires { get; set; }

        public virtual ICollection<QuestionnaireUser> QuestionnaireUsers { get; set; }
    }

I beleive this is the correct way to establish a many-to-many relationship between these classes, and when the project is built, I would expect three tables to be created.

However, when I attempt to to relate one Questionnaire to two different Vendors, I receive the following error when attempting to save the changes (context.SaveChanges()):

*Multiplicity constraint violated. The role ‘Vendor_OpenQuestionnaires_Source’ of the relationship ‘QuestionnaireApp.Models.Vendor_OpenQuestionnaires’ has multiplicity 1 or 0..1.*

If I assign a Questionnaire to only one Vendor, save the changes and then assign it to another and again save changes I no longer get the error; however the Questionaire is then related only to the last Vendor to which it was assigned, indicating that (at best) there is a one-to-many relationship being created.

I’m hoping that there is something wrong with the way I’m declaring the many-to-many relationship between these classes, or perhaps there is something I need to add to the context class to “encourage” the relationsip, but perhaps many-to-many relationships like this are not supported, or cannot be created using “Code First”?

Thank you for your time,

Jason

Answer 1

Go figure, after an hour or so of searching, I go and find the exact answer 30 seconds after I post my question.

The solution was to add the following to the context class:

modelBuilder.Entity<Vendor>()
                .HasMany<Questionnaire>(x => x.OpenQuestionnaires)
                .WithMany(x => x.Vendors)
                .Map(x =>
                    {
                        x.MapLeftKey("vID");
                        x.MapRightKey("qID");
                        x.ToTable("VendorQuestionnaires");
                    });

I found the answer by reading this Stack Overflow post: EF Code First Many-to-Many not working