Entity Framework Core 中两个实体之间的一对多和一对一关系

Question

I want to model Team and Players in Entity Framework Core 3.1. Each team has many players and every player has only one team. Also, each team has one captain from the player list. How can I write Fluent Api to the relation between Captain and Player (one to one)?

public class Team
{
    public int Id { get; set; }
    public string Name { get; set; }
    
    public int? CaptainId { get; set; }
    public Player Captain { get; set; }
    
    public ICollection<Player> Players { get; set; }
}

public class Player
{
    public int Id { get; set; }
    public string Name { get; set; }
    
    public int TeamId { get; set; }
    public Team Team { get; set; }
}

I tried the following code

builder.Entity<Team>()
    .HasOne(e => e.Captain)
    .WithOne()
    .HasForeignKey<Team>(e => e.CaptainId)
    .OnDelete(DeleteBehavior.Restrict);

builder.Entity<Team>()
    .HasMany(g => g.Players)
    .WithOne(s => s.Team)
    .HasForeignKey(s => s.TeamId);

But this code generates two one-to-many relationships ...

Answer 1

I've been trying something like this for the last few days, and after trying all sorts of Data Annotations and Fluent API nonsense, the cleanest solution I could come up with turned out to be very simple which requires neither. It only requires adding a 'private' constructor to the Team class (or a 'protected' one if you're using Lazy Loading) into which your 'DbContext' object is injected. Just set up your 'Team' and 'Player' classes as a normal one-to-many relationship, and with your database context now available from within the 'Team' entity, you can make the 'Captain' property simply return a query from the database using the Find() method. The Find() method also makes use of caching, so if you call the getter more than once, it will only make one trip to the database.

Here is the documentation on this ability: https://docs.microsoft.com/en-us/ef/core/modeling/constructors#injecting-services

Note: the 'Captain' property is read-only. To modify it, set the 'CaptainId' property.

class Team
{
    public Team() { }
    private MyDbContext Context { get; set; }
    // make the following constructor 'protected' if you're using Lazy Loading
    private Team(MyDbContext Context) { this.Context = Context; }

    public int ID { get; set; }
    public string Name { get; set; }
    public int? CaptainID { get; set; }
    public Player Captain { get { return Context.Players.Find(CaptainID); } }
}

class Player
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int TeamID { get; set; }
    public Team Team { get; set; }
}