实体框架核心中的模型循环

Question

Imagine these models are a part of a social network application. User can make a Group and can add other users in it(as members) and a user can be a member of the group. I'm using .net core and also entity framework core.

Models:

public class User : BaseEntity
{
    public string UserName { get; set; }

    public IList<Group> OwnGroups { get; set; }
    public IList<GroupMember> MemberInGroups { get; set; }
}

public class Group : BaseEntity
{
    public string Name { get; set; }
    public int OwnerUserId { get; set; }

    [ForeignKey("OwnerUserId")]
    public User OwnerUser { get; set; }

    public IList<GroupMember> Members { get; set; }
}

public class GroupMember : BaseEntity
{
    public int GroupId { get; set; }
    public int UserId { get; set; }

    [ForeignKey("UserId")]
    public User User { get; set; }

    [ForeignKey("UserId")]
    public Group Group { get; set; }
}

Fluent API:

    modelBuilder.Entity<User>()
    .HasMany(x => x.OwnGroups)
    .WithOne(x => x.OwnerUser).HasForeignKey(x => x.OwnerUserId).IsRequired().OnDelete(DeleteBehavior.Cascade);

modelBuilder.Entity<User>()
    .HasMany(x => x.MemberInGroups)
    .WithOne(x => x.User).HasForeignKey(x => x.UserId).IsRequired().OnDelete(DeleteBehavior.Cascade);

modelBuilder.Entity<Group>()
    .HasMany(x => x.Members)
    .WithOne(x => x.Group).HasForeignKey(x => x.GroupId).IsRequired().OnDelete(DeleteBehavior.Cascade);

When I want to migrate to the database, this error happens:

Introducing FOREIGN KEY constraint 'FK_GroupMembers_Users_UserId' on table 'GroupMembers' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints. Could not create constraint or index. See previous errors.

However, I can make this model in SQL Server manually and there is no problem.

There are at least two ways to fix it. But I want to know why EF Core says, there is a cycle. What's the problem is?

Answer 1

I think your bridge table has a problem.

Try this

public class GroupMember: BaseEntity
{
public User User { get; set; }
[Key, Column(Order = 0)]
public int UserId { get; set; }

public Group Group { get; set; }
[Key, Column(Order = 1)]
public int GroupId { get; set; }
}

Answer 2

First of all: Get rid of the Fluent API part

//modelBuilder.Entity<User>()
//.HasMany(x => x.OwnGroups)
//.WithOne(x => x.OwnerUser).HasForeignKey(x => x.OwnerUserId).IsRequired().OnDelete(DeleteBehavior.Cascade);

//modelBuilder.Entity<User>()
//    .HasMany(x => x.MemberInGroups)
//    .WithOne(x => x.User).HasForeignKey(x => x.UserId).IsRequired().OnDelete(DeleteBehavior.Cascade);

//modelBuilder.Entity<Group>()
//    .HasMany(x => x.Members)
//    .WithOne(x => x.Group).HasForeignKey(x => x.GroupId).IsRequired().OnDelete(DeleteBehavior.Cascade);

And then, based on Microsoft Docs , choose one of these cases:

1. Change one or more of the relationships to not cascade delete(make it nullable)

    public int? OwnerUserId { get; set; }

2. The second approach instead, we can keep the OwnerUserId relationship required(non-nullable) and configured for cascade delete, but make this configuration only apply to tracked entities, not the database.

    modelBuilder .Entity<User>() .HasMany(e => e.OwnGroups) .WithOne(e => e.OwnerUser) .OnDelete(DeleteBehavior.ClientCascade);

In this way, if we want to delete a User and all its Groups , we should load both the User and its Groups in the application(RAM). Look at this: After running this code, the User and all its Groups will be removed.

var context = new MyDbContext();

var user = context.Users.Single(x => x.UserName == "SampleName");
var groups = context.Groups.Where(x => x.OwnerUser == user).ToList();

context.Users.Remove(user);
context.SaveChanges();

But the code below will throw an exception.

var context = new MyDbContext();

var user = context.Users.Single(x => x.UserName == "SampleName");

context.Users.Remove(user);
context.SaveChanges();

Microsoft.Data.SqlClient.SqlException (0x80131904): Introducing FOREIGN KEY constraint 'FK_Groups_Users_OwnerUserId' on table 'Groups' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.