ASP.NET C# EF Core migration error

When I run a migration I get the following error:

The given key 'System.Nullable1[System.Int32] ChildId' was not present in the dictionary.

Basically I want to create a relationship for a contact with another contact, e.g. Contact1 has a relationship with Contact2 of type Employee. One Contact can have many relationships with other Contacts. There is only one relationship type for each relationship. So under Contact1 you may see 3 relationship recrods, e.g. Contact2;Supervisor, Contact2;Friend, Contact3; Dentist.

My navigation properties are wrong, but I can't figure out how to set this up. Any help would be greatly appreciated :)

Contact Model

public class Contact
    {
        public int Id { get; set; }
        public string? FirstName { get; set; }
        public string? LastName { get; set; }

        [InverseProperty(nameof(Relationship.ParentId))]
        public virtual ICollection Parent { get; set; }

        [InverseProperty(nameof(Relationship.ChildId))]
        public virtual ICollection Child { get; set; }
    }

Relationship Model

public class Relationship
    {
        public int Id { get; set; }

        [ForeignKey(nameof(Parent)), Column(Order = 0)]
        public int? ParentId { get; set; }
        public virtual Contact? Parent { get; set; }

        [ForeignKey(nameof(Child)), Column(Order = 1)]
        public int? ChildId { get; set; }
        public virtual Contact? Child { get; set; }

        // Navigation for the relationship type
        public int? RelationshipId { get; set; }
        public virtual Relationship? Relationships { get; set; }
    }

The error message you're encountering suggests that there might be a problem with the keys or their mapping in the migration.

Let's correct the relationship setup and provide some guidance on how to configure the models:

Contact Model:

public class Contact
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

    // Navigation properties for relationships where this contact is the parent
    public virtual ICollection RelationshipsAsParent { get; set; }

    // Navigation properties for relationships where this contact is the child
    public virtual ICollection RelationshipsAsChild { get; set; }
}

Relationship Model:

public class Relationship
{
    public int Id { get; set; }

    // Foreign key and navigation property for the parent contact
    public int ParentId { get; set; }
    public virtual Contact Parent { get; set; }

    // Foreign key and navigation property for the child contact
    public int ChildId { get; set; }
    public virtual Contact Child { get; set; }

    // Navigation property for the relationship type (assuming there's a RelationshipType entity)
    public int RelationshipTypeId { get; set; }
    public virtual RelationshipType RelationshipType { get; set; }
}

In this setup, the Contact model has separate navigation properties to represent relationships where the contact is a parent or a child. The Relationship model includes foreign key properties for both the parent and child contacts.

Ensure you have properly configured the relationships in your DbContext:

public class YourDbContext : DbContext
{
    public DbSet Contacts { get; set; }
    public DbSet Relationships { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity()
            .HasOne(r => r.Parent)
            .WithMany(c => c.RelationshipsAsParent)
            .HasForeignKey(r => r.ParentId)
            .OnDelete(DeleteBehavior.Restrict);

        modelBuilder.Entity()
            .HasOne(r => r.Child)
            .WithMany(c => c.RelationshipsAsChild)
            .HasForeignKey(r => r.ChildId)
            .OnDelete(DeleteBehavior.Restrict);

        // Configure other relationships or constraints here

        base.OnModelCreating(modelBuilder);
    }
}

Make sure to adjust the relationship configurations based on your specific requirements and database setup.

Once you've updated the models and the DbContext configuration, regenerate the migration and update the database. This should resolve the issue with the key not being present in the dictionary.