我将如何在 EF Core 中创建这种多对多关系？

Question

So, I have the following 3 classes:

User.cs

public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }

    public virtual Timezone Timezone { get; set; }
    public virtual ICollection<UserChannel> UserChannels { get; set; }
}

Channel.cs

public class Channel
{
    public int Id { get; set; }
    [Column("Channel")]
    public string Name { get; set; }
}

UserChannel.cs

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

    public virtual User User { get; set; }
    public virtual Channel Channel { get; set; }
}

UserChannel is the medium between User and Channel .

The UserChannels table looks like this:

+----+--------+-----------+
| Id | UserId | ChannelId |
+----+--------+-----------+

Both UserId and ChannelId are foreign keys, for the User and Channel class.

Now, to get which Channels belong to a user, I would have to write something like this:

using var context = new TwitchContext();
var me = context.Users.FirstOrDefault(x => x.Id == 1);
var myChannels = me.UserChannels.Select(x => x.Channel);

Is there a way I can just get a collection of Channel directly instead of using UserChannel ?

Answer 1

Many to many relationship without referencing the join table / entity won't be available until EF Core 5 comes out, as per current roadmap:

https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-5.0/plan#many-to-many-navigation-properties-aka-skip-navigations

Feature request is tracked by this issue:

https://github.com/dotnet/efcore/issues/19003

that is included in EF Core 5.0.0 milestone.

EF Core 5 release is planned for November 2020.

Answer 2

In EF.Core, many-to-many without a join table is not yet supported . So you will need to rely on the solution with the UserChannel class and the Select call for now.

As your example does not explicitly specify that a channel can have multiple users, a one-to-many solution with a ICollection<Channel> property might work as well.