Multiplicity constraint violated. The role 'Person_Projects_Source' of the relationship 'Repository.Person_Projects' has multiplicity 1 or 0..1

Question

Hello had this problem before with another data type. For more details you can see here. Multiplicity constraint violated Entity framework 5

But now that solution doesn't work or any other found on the net.

My classes are:

namespace Prometheus.Models
{
    [Table("People")]
    public class Person : IPerson, INamedEntity
    {
        [Key]
        [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
        public virtual int Id { get; set; }
        [DisplayName("Prenume")]
        public virtual string FirstName { get; set; }
        [DisplayName("Nume")]
        public virtual string LastName { get; set; }

        public string Name
        {
            get
            {
                return FirstName + " " + LastName;
            }
            set { }
        }
        [DisplayName("Email")]
        public virtual string Email { get; set; }
        public DateTime? LastModified { get; set; }
        public virtual ICollection<Result> Results { get; set; }
        public virtual ICollection<Project> Projects { get; set; }
    }
}

And

public class Project :INamedEntity
{

    public int Id { get; set; }
    public DateTime? LastModified { get; set; }
    [DisplayName("Titlu Proiect")]
    [Required]
    public string Name { get; set; }//Title
    [Required]
    [DisplayName("Tip proiect")]
    public virtual ProjectType ProjectType{ get; set; }
    [DisplayName("Status proiect")]
    public virtual ProjectStatus ProjectStatus { get; set; }
    [Required]
    [DisplayName("Tip program")]
    public virtual ProgramType ProgramType { get; set; }
    [Required]
    [DisplayName("Numar luni")]
    public int NumberOfMonths { get; set; }
    [Required]
    [DisplayName("Project title")]
    public string NameEn { get; set; }
    [Required]
    [DisplayName("Acronim")]
    public string Acronym { get; set; }
    [Required]
    [DisplayName("Numar contract")]
    public string ContractNo { get; set; }
    [Required]
    [DisplayName("Domeniu de activitate")]
    public virtual ActivityField ActivityField {get;set;}
    [Required]
    [DisplayName("Summary")]
    public string SummaryEn { get; set; }
    [Required]
    [DisplayName("Rezumat")]
    public string SumarryRo { get; set; }
    [Required]
    [DisplayName("Data inceput")]
    public virtual DateTime StartDate { get; set; }
    [Required]
    [DisplayName("Data sfarsit")]
    public virtual DateTime EndDate { get; set; }
    [Required]
    [DisplayName("Institutie coordonatoare")]
    public virtual string CoordInstitution { get; set; }
    [DisplayName("Valoare totala proiect")]
    public decimal TotalValue { get; set; }
    [DisplayName("Valuta")]
    public virtual Currency Currency { get; set; }
    [DisplayName("Rol in proiect")]
    public virtual RoleInProject RoleInProject { get; set; }//coord proiect/partener/in afara inst
    [DisplayName("Echipa proiect")]
    public virtual ICollection<Person> People { get; set; }
    [DisplayName("Director proiect")]
    public virtual Person ProjectManager { get; set; }
    public virtual ICollection<Institution> Partners { get;set;}
    public virtual Person PersonInCharge { get; set; }

    [DisplayName("Functie/Rol Expert In Proiect")]
    public string UserFunctionInProject { get; set; }
    [DisplayName("Pagina proiectului")]
    [Required]
    public string Website {get;set;}
    public virtual ICollection<Result> Results { get; set; }
}

When i had the previous problem it was with a class Result and the same Person and was told that if i want a many-to-many relationship i have to add a Collection to Person and a Collection to Result and that did the trick.

But now as you can see i already have

public virtual ICollection<Person> People { get; set; } in Project

and

public virtual ICollection<Project> Projects { get; set; } in Person

And every time i try to add more than one User to the project i get the error in that title when calling the Save() method from DbContext

I have no idea what to do. Thanks.

Accidentally posted the wrong class, it was the result instead of the Person, now it's correct

Update 1

public void AddProjectForUsers(IEnumerable<int> userIds, Project project)
{
    foreach (var id in userIds)
    {
        AddProjectForUser(id,project);
    }
}

public void AddProjectForUser(int userId, Project project)
{
    var user = _ctx.Users.SingleOrDefault(u => u.Id == userId);
    if (user != null)
    {
        if (!user.Projects.Contains(project))
        {
            user.Projects.Add(project);
        }
    }
}

public bool Save()
{
    return _ctx.SaveChanges() > 0; // the error is thrown here
}

The 3 are from my repository class

And the call to them is

Repo.AddProjectForUsers(team, project);
Repo.Save();

Where team is an IEnumerable<int> and project is of type Project

Answer 1

It looks like you may need the InverseProperty data annotation attribute to do this:

namespace Prometheus.Models
{
    [Table("People")]
    public class Person : IPerson, INamedEntity
    {
        ...
        [InverseProperty("People")]
        public virtual ICollection<Project> Projects { get; set; }
    }
}

public class Project :INamedEntity
{
    ...
    [DisplayName("Echipa proiect")]
    [InverseProperty("Projects")]
    public virtual ICollection<Person> People { get; set; }
}

The exception message you are receiving is very telling. Since EF is complaining that the relation is expected to be a one-to-zero-or-one, this leads me to believe that EF is confusing the People - Projects association with either the PersonInCharge - Project or ProjectManager - Project association.

One way to solve this would be to expose foreign key properties on your Project entity, which you should be doing anyway - you can get weird transient errors when you don't. Another way would be to use the fluent API (a.k.a. model builder) instead of data annotations.