How to create multiple relations to another class as different properties?

Question

I want to create class that contains multiple objects as a relation to another table.

For example, if I wanted to implement a team of three players using different objects instead of a collection, my code would've looked like this:

public class Team
{
    [Key]
    public int TeamId { get; set; }

    public string Name { get; set; }

    [ForeignKey("Goalkeeper")]
    [Required]
    public int GoalkeeperId { get; set; }
    public virtual Player Goalkeeper { get; set; }

    [ForeignKey("Defender")]
    [Required]
    public int DefenderId { get; set; }
    public virtual Player Defender { get; set; }

    [ForeignKey("Striker")]
    [Required]
    public int StrikerId { get; set; }
    public virtual Player Striker { get; set; }
}

public class Player
{
    [Key]
    public int PlayerId { get; set; }

    public string Name { get; set; }

    [ForeignKey("Team")]
    public int TeamId { get; set; }
    public virtual Team Team { get; set; }
}

Trying to create a migration based on this code I'm getting an error:

Unable to determine the relationship represented by navigation property 'Player.Team' of type 'Team'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

What am I missing? Is this even a correct approach to implement this?

Answer 1

I think that the problem is that EF Core cannot automatically detect what's the correct way of configure the mapping between the entities.

From the docs:

When there are multiple navigation properties defined between two types (that is, more than just one pair of navigations that point to each other) the relationships represented by the navigation properties are ambiguous. You will need to manually configure them to resolve the ambiguity.

Therefore, you should try to configure it manually in your DbContext. It would be something ish like this, but I might have a mistake somewhere since I didn't test it.

public class YourDatabaseContext : SqlContext
    {
        ...

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);

            modelBuilder.Entity<Team>()
                .HasOne(x => x.Goalkeeper )
                .WithOne()
                .HasForeignKey<Team>(t => t.GoalkeeperId);

            modelBuilder.Entity<Team>()
                .HasOne(x => x.Defender)
                .WithOne()
                .HasForeignKey<Team>(t => t.DefenderId);

            modelBuilder.Entity<Team>()
                .HasOne(x => x.Striker)
                .WithOne()
                .HasForeignKey<Team>(t => t.StrikerId);
        }
    }

PD: I usually prefer this method of configuring the relationships with the FluentAPI instead of polluting the model with Attributes. To further organize the code, you can create a class extending IEntityTypeConfiguration<SomeEntity> per entity where you put the configuration code, and then in your DbContext.OnModelCreating you can add this line to scan your assembly an add all configurations:

protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);

            modelBuilder.ApplyConfigurationsFromAssembly(Assembly.GetExecutingAssembly());
        }