Question

I’m new to .NET MVC and I’m struggling with using Code First with an existing database in which a table has a one to none-or-one (1 -> 0..1) relationship.

I have a report, which can have many sections, and each section can have many questions. Now here’s the bit where I think I’m running into trouble… each question may have one answer or none.

I’m getting the following error:

System.Data.Edm.EdmAssociationEnd: : Multiplicity is not valid in Role
‘QuestionAnswer_Question_Source’ in relationship
‘QuestionAnswer_Question’. Because the Dependent Role properties are
not the key properties, the upper bound of the multiplicity of the
Dependent Role must be �*�.

Here are my model classes:

ModeratorReport.cs

public class ModeratorReport
{
    [Key, Column(Order = 0)]
    public int ModeratorReportID { get; set; }

    [Key, Column(Order = 1)]
    public string Status { get; set; }

    public string FileYear { get; set; }
    public string SessionCode { get; set; }
    public string CentreNumber { get; set; }
    public string SubjectNumber { get; set; }
    public string PaperNumber { get; set; }
    public string ModeratorNumber { get; set; }
    public DateTime? DateModified { get; set; }

    public virtual ICollection<AuditItem> Audit { get; set; }
}

Section.cs

public class Section
{
    [Key]
    public int SectionID { get; set; }
    public string SectionEnglish { get; set; }
    public string SectionWelsh { get; set; }

    public virtual ICollection<Question> Questions { get; set; }
}

Question.cs

public class Question
{
    [Key]
    public int QuestionID { get; set; }

    [ForeignKey("Section")]
    public int SectionID { get; set; }

    public string QuestionEnglish { get; set; }
    public string QuestionWelsh { get; set; }
    public string Type { get; set; }

    public virtual Section Section { get; set; }
    public virtual QuestionAnswer QuestionAnswer { get; set; }
}

QuestionAnswer.cs

public class QuestionAnswer
{
    [Key]
    public int AnswerID { get; set; }

    [ForeignKey("ModeratorReport"), Column(Order = 0)]
    public int ModeratorReportID { get; set; }
    [ForeignKey("ModeratorReport"), Column(Order = 1)]
    public string Status { get; set; }

    [ForeignKey("Section")]
    public int SectionID { get; set; }

    [ForeignKey("Question")]
    public int QuestionID { get; set; }

    public string Answer { get; set; }

    public virtual ModeratorReport ModeratorReport { get; set; }
    public virtual Section Section { get; set; }
    public virtual Question Question { get; set; }
}

I also have a one-to-many relationship with ModeratorReport and Audit but I don’t think that is what is causing the error.

Any help would be appreciated.

Thanks.

Answer 1

The EF is complaining because it sounds like you are using an FK Association – which means that the QuestionID is a property of the Entity and there is a Question reference too – and you can’t do this with FK Associations.
if you remove the QuestionID from QuestionAnswer it should work

public class QuestionAnswer
{
    [Key,ForeignKey("Question")]]
    public int AnswerID { get; set; }

    [ForeignKey("ModeratorReport"), Column(Order = 0)]
    public int ModeratorReportID { get; set; }
    [ForeignKey("ModeratorReport"), Column(Order = 1)]
    public string Status { get; set; }

    [ForeignKey("Section")]
    public int SectionID { get; set; }


    public string Answer { get; set; }

    public virtual ModeratorReport ModeratorReport { get; set; }
    public virtual Section Section { get; set; }
    public virtual Question Question { get; set; }
}

I would suggest you to use fluent mapping which is more clear:

modelBuilder.Entity<Question>()
            .HasOptional(q => q.QuestionAnswer)

modelBuilder.Entity<QuestionAnswer>()
                .HasRequired(qa => qa.Question)