Question

No matter which way I mix it, it gives me errors. I have a feeling I’m missing something obvious as I keep getting these errors.

One or more validation errors were detected during model generation:

System.Data.Edm.EdmAssociationType: : Multiplicity conflicts with the referential constraint in Role ‘Venue_Courses_Source’ in relationship ‘Venue_Courses’. Because all of the properties in the Dependent Role are non-nullable, multiplicity of the Principal Role must be ‘1’.

System.Data.Edm.EdmAssociationEnd: : Multiplicity is not valid in Role ‘Venue_Courses_Target’ in relationship ‘Venue_Courses’. Because the Dependent Role refers to the key properties, the upper bound of the multiplicity of the Dependent Role must be 1.

A Course can only have one venue, venues can be used by many Courses

public class Course
{
    [Key]
    public virtual int Id { get; set; }
    public string Title { get; set; }
    public DateTime StartDate { get; set; }
    public int VenueId { get; set; }
    
    public virtual Venue Venue { get; set; }
}

public class Venue
{
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
    
    public virtual ICollection<Course> Courses { get; set; }
}


protected override void OnModelCreating(DbModelBuilder modelBuilder)
{

    #region Courses
    //Table Alias
    modelBuilder.Entity<Course>().ToTable("DBSCHEMA.TR_COURSES");
    //Keys
    modelBuilder.Entity<Course>().HasKey(c => c.Id);
    //Joins
    //Join to Venues
    modelBuilder.Entity<Course>().HasOptional(c => c.Venue);
        
    //Fields
    modelBuilder.Entity<Course>().Property(c => c.Id).HasColumnName("COURSE_ID");
    modelBuilder.Entity<Course>().Property(c => c.Title).HasColumnName("CR_TITLE");
    modelBuilder.Entity<Course>().Property(c => c.StartDate).HasColumnName("START_DATE");
    modelBuilder.Entity<Course>().Property(c => c.VenueId).HasColumnName("VENUE_ID");
    #endregion


    #region Venues
    //Table Alias
    modelBuilder.Entity<Venue>().ToTable("DBSCHEMA.VENUES");
    //Keys
    modelBuilder.Entity<Venue>().HasKey(v => v.Id);
    //Joins
    modelBuilder.Entity<Venue>().HasMany(venue => venue.Courses);
    //Fields
    modelBuilder.Entity<Venue>().Property(v => v.Id).HasColumnName("VENUE_ID");
    modelBuilder.Entity<Venue>().Property(v => v.Name).HasColumnName("VENUE_NAME");
    #endregion
            
}

Answer 1

Hope this is still on time to help you. I was also having the exact same problem and was troubling with it for almost an hour until I could spot my mistake.

The problem is that Course.Venue relationship is optional (as declared on the fluent API), but the Id declaration of Course.VenueId is mandatory, so you can either make VenueId optional by changing it to

public int? VenueId { get; set;}

or change the relationship to mandatory on the fluent API, and the OnModelCreating should run fine once you changed that.