Entity Framework Core One to Many

Question

I am trying to build a phone list with Entity Framework Core. Whenever I try to run the commend: Add-Migration "Initial"

I get the following error: Unable to determine the relationship represented by navigation property 'Member.MemberTeam' of type 'Team'. Either manually configure the relationship, or ignore this property from the model. Unable to determine the relationship represented by navigation property 'Member.MemberTeam' of type 'Team'. Either manually configure the relationship, or ignore this property from the model.

Here are my models:

public class Member {
        public int Id { get; set; }
        public string MemberName { get; set; }
        public string MemberTitle { get; set; }
        public Member MemberSupervisor { get; set; }
        public Team MemberTeam { get; set; }
    }

public class Team {
    public int Id { get; set; }
    public string TeamName { get; set; }
    public Member TeamSupervisor { get; set; }
    public ICollection<Member> TeamMembers { get; set; }
}

Any advice?

Answer 1

There are several relationships between Team and Member and from Member to Member . If you only had this model ...

public class Member
{
    public int Id { get; set; }
    public string MemberName { get; set; }
    public string MemberTitle { get; set; }
    public Team MemberTeam { get; set; }
}
public class Team
{
    public int Id { get; set; }
    public string TeamName { get; set; }
    public ICollection<Member> TeamMembers { get; set; }
}

... EF wouldn't need any help. It would understand that Team.TeamMembers and Member.MemberTeam are two ends of one association.

Now if you only add ...

public Member TeamSupervisor { get; set; }

... EF has to choose if this is the reverse end of a one-to-one association with MemberTeam , or a new association besides the one-to-many association. However, EF doesn't choose for you, you have to instruct it unambiguously, by mapping the associations explicitly. For example (in your DbContext subclass):

protected override void OnModelCreating(ModelBuilder mb)
{
    mb.Entity<Team>().Property(a => a.Id).UseSqlServerIdentityColumn();
    mb.Entity<Team>().HasMany(t => t.TeamMembers).WithOne(m => m.MemberTeam);
    mb.Entity<Team>().HasOne(t => t.TeamSupervisor).WithMany()
                     .HasForeignKey("SupervisorId");

    mb.Entity<Member>().Property(a => a.Id).UseSqlServerIdentityColumn();
    mb.Entity<Member>().HasOne(m => m.MemberSupervisor).WithMany()
                       .HasForeignKey("SupervisorId");
}

Answer 2

I know this question is quite old but if you're here this could help you out.

In order to solve these relationship issues you can use Has/With pattern .

You have two options to face this problem using fluent api.

Option 1: 1 to N

modelBuilder.Entity<Team>()
  .HasMany(t => t.TeamMembers)
  .WithOne(t => t.MemberTeam);

Option 2: Setting backwards.

modelBuilder.Entity<Member>()
  .HasOne(t => t.MemberTeam)
  .WithMany(t => t.TeamMembers);

Note: By default the relationship is optional but you can set .IsRequired() if you need it to be present.