EF Core刪除多對多關系數據

Question

我有以下表格：

public class Team 
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }

    public virtual ICollection<UserTeam> UserTeams { get; set; }
}

public class User
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }

    public virtual ICollection<UserTeam> UserTeams { get; set; }
}

public class UserTeam
{
    public long UserId { get; set; }
    public User User { get; set; }

    public long TeamId { get; set; }
    public Team Team { get; set; }
}

多對多關系是在上下文中定義的：

modelBuilder.Entity<UserTeam>()
    .HasKey(bc => new { bc.UserId, bc.TeamId });

modelBuilder.Entity<UserTeam>()
    .HasOne(bc => bc.User)
    .WithMany(b => b.UserTeams)
    .HasForeignKey(bc => bc.UserId)
    .OnDelete(DeleteBehavior.Restrict);

modelBuilder.Entity<UserTeam>()
    .HasOne(bc => bc.Team)
    .WithMany(c => c.UserTeams)
    .HasForeignKey(bc => bc.TeamId)
    .OnDelete(DeleteBehavior.Restrict);

我正在嘗試使用以下代碼從小組中刪除一些用戶：

public async Task RemoveUsersFromTeam(int teamId, List<long> users)
{
    Team existingTeam = await dbContext.Team.Include(x => x.UserTeams).FirstOrDefaultAsync(x => x.Id == teamId);
    foreach (var user in users)
    {
        existingTeam.UserTeams.Remove(new UserTeam { UserId = user });
    }

    await dbContext.SaveAsync();
}

但是此查詢不會刪除我通過的用戶。 有人知道為什么會這樣嗎？

Answer 1

您可以使用new和attach方法通過ID或通過傳遞實際實體來刪除對象。

傳遞實體

public async Task RemoveUsersFromTeam(int teamId, List<long> userIds)
{
    Team existingTeam = await dbContext.Team.Include(x => x.UserTeams).FirstOrDefaultAsync(x => x.Id == teamId);
    foreach (var userId in userIds)
    {
        var userTeam = existingTeam.UserTeams.FirstOrDefault(x => x.UserId == userId);
        if(userTeam != null)
        {
            existingTeam.UserTeams.Remove(userTeam);
        }
    }

    await dbContext.SaveAsync();
}

按編號刪除

public async Task RemoveUsersFromTeam(int teamId, List<long> userIds)
{
    Team existingTeam = await dbContext.Team.FirstOrDefaultAsync(x => x.Id == teamId);
    foreach (var userId in userIds)
    {
        var userTeam = new UserTeam { UserId = userId });
        dbContext.UserTeams.Attach(userTeam);
        existingTeam.UserTeams.Remove(userTeam);
    }

    await dbContext.SaveAsync();
}

選項2不需要從數據庫中選擇UserTeam，在這方面效率會更高。 但是，選項一可能更容易理解。 您應該選擇最適合您的情況。

我個人更喜歡第二種方法，因為include將選擇整個實體，在某些情況下可能會超出必要。