如何在Entity Framework Core中删除多对多关系？

Question

我有两个实体框架实体。1）员工2）案例

我已经在Entity Framework Core中建立了多对多关系模型。

我可以这样添加关系：

    [HttpPost]
    public async Task<Employee> AddCaseEmployee([FromBody]EmployeeCase employeeCase)
    {
        Case lcase = await _caseGenericService.GetSingleIncludingAsync(employeeCase.CaseId);

        lcase.EmployeeCases = new List<EmployeeCase>();
        lcase.EmployeeCases.Add(new EmployeeCase
            {
                CaseId = employeeCase.CaseId,
                EmployeeId = employeeCase.EmployeeId
            }
        );

        await _caseGenericService.UpdateAsync(lcase);

        Employee employee = await _employeeGenericService.GetSingleIncludingAsync(employeeCase.EmployeeId);
        return employee;
    }

现在，我试图删除这样的一种关系：

    [HttpDelete]
    public async Task<Employee> DeleteCaseEmployee(long employeeId, long caseId)
    {
        Case lcase = await _caseGenericService.GetSingleIncludingAsync(caseId,
            a => a.EmployeeCases);

        //lcase.EmployeeCases.Remove(new EmployeeCase
        //{
        //    CaseId = caseId,
        //    EmployeeId = employeeId
        //}
        //);
        EmployeeCase employeeCase = 
            lcase.EmployeeCases.FirstOrDefault(ec => ec.EmployeeId == employeeId && ec.CaseId == caseId);
        lcase.EmployeeCases.Remove(employeeCase);

        try
        {
            await _caseGenericService.UpdateAsync(lcase);
        }
        catch(Exception ex)
        {

        }

        Employee employee = await _employeeGenericService.GetSingleIncludingAsync(employeeId);
        return employee;
    }

您可以看到我的第一次尝试已被注释掉。因此，我尝试了第二种方法，即只取回要删除的记录的引用，将其从集合中删除，然后调用update。但是我得到这个错误：

“实体类型'Case'和'EmployeeCase'之间的关联已被切断，但是该关系的外键不能设置为null。如果应该删除从属实体，则设置该关系以使用级联删除。”

在这里，我不需要删除案例或雇员。我只需要删除加入记录。进入该记录的唯一方法是从案例或雇员进入关系。

如果我将EmployeeCase对象设为Dbset，则将无法建立模型。

这是我建立OnModelBuilding的关系：

    modelBuilder.Entity<EmployeeCase>()
            .HasKey(ec => new { ec.EmployeeId, ec.CaseId });

        modelBuilder.Entity<EmployeeCase>()
            .HasOne(ec => ec.Employee)
            .WithMany(e => e.EmployeeCases)
            .HasForeignKey(ec => ec.EmployeeId);

        modelBuilder.Entity<EmployeeCase>()
            .HasOne(ec => ec.Case)
            .WithMany(c => c.EmployeeCases)
            .HasForeignKey(ec => ec.CaseId);

        // Get rid of Cascading Delete Circular references error.
        var type = modelBuilder.Model.GetEntityTypes().Single(t => t.Name == "Hrsa.Core.Generic.Model.Lerd.EmployeeCase");
        foreach (var relationship in type.GetForeignKeys())
        {
            relationship.DeleteBehavior = DeleteBehavior.Restrict;
        }

我必须为联接对象EmployeeCase关闭Deletes，它不能是DbSet。如果不遵循这些步骤中的任何一个，则将不会建立多对多模型。

我需要做的就是能够删除该记录。我不需要删除员工或案例。

这是POCO。

public class Employee : BaseEntity
{
    [BindNever]
    public string Email { get; set; }
    [BindNever]
    public long OrganizationId { get; set; }
    [BindNever]
    public string Supervisor { get; set; }
    [BindNever]
    public string SupervisorEmail { get; set; }
    [BindNever]
    public string FirstName { get; set; }
    [BindNever]
    public string LastName { get; set; }
    public string Notes { get; set; }
    [BindNever]
    public  long BargainingUnitId { get; set; }
    [BindNever]
    public long PayPlanId { get; set; }
    [BindNever]
    public long GradeRankId { get; set; }
    [BindNever]
    public long PositionTitleId { get; set; }
    [BindNever]
    public long SeriesId { get; set; }
    public bool IsUnionEmployee { get; set; }
    public virtual Organization Organization { get; set; }
    public virtual BargainingUnit BargainingUnit { get; set; }
    public virtual PayPlan PayPlan { get; set; }
    public virtual GradeRank GradeRank { get; set; }
    public virtual PositionTitle PositionTitle { get; set; }
    public virtual  Series Series { get; set; }
    public virtual ICollection<UnionHours> UnionHours { get; set; }
    [NotMapped]
    public string UnionEmployeeYesNo => (IsUnionEmployee) ? "Yes" : "No";

    public virtual ICollection<EmployeeCase> EmployeeCases { get; set; }

}

public class Case : BaseEntity
{
    #region Common Case Properties
    public long InitialContactId { get; set; }
    public virtual Employee InitialContact { get; set; }
    [DataType(DataType.Date)]
    public DateTime? InitialConsultDate { get; set; }
    public string LerSpecialistId { get; set; }
    public virtual AppUser LerSpecialist { get; set; }
    public string Notes { get; set; }

    public bool IsDeleted { get; set; }
    public CaseType CaseType { get; set; }
    #endregion

    #region EmployeeCase Properties
    //public long PrimaryEmployeeId { get; set; }
    //public virtual Employee PrimaryEmployee {get;set;}
    public virtual ICollection<EmployeeCase> EmployeeCases { get; set; }
    #endregion

    #region NonEmployeeCase Properties
    //public long PrimaryNonEmployeeId { get; set; }
    //public virtual Employee PrimaryEmployee {get;set;}
    public virtual ICollection<NonEmployeeCase> NonEmployeeCases { get; set; }
    #endregion
}

和联接表（同样，它不是DbSet，但就添加关系而言，一切一直在起作用。现在无法删除。）

public class EmployeeCase
{
    [ForeignKey("Id")]
    public long EmployeeId { get; set; }
    public Employee Employee { get; set; }
    [ForeignKey("Id")]
    public long CaseId { get; set; }
    public Case Case { get; set; }

}

如何删除此多对多关系记录？

Answer 1

我遇到了同样的需求，并在这里找到了这篇文章。由于尚未发布真正的解决方案（使用EF），因此我决定添加以下答案：

另请参见在Entity Framework Core中更新多对多关系；这是用于更新，但删除的原理相同。

这是我的代码（我的情况是用户N？N个组）：

var groupToUpdate = _userGroupsContext.Groups.Include(g => g.UserGroups).Single(u => u.Id == userVm.groupsIds[0]);
var userToUpdate = _userGroupsContext.Users.Single(u => u.Id == userVm.user.Id);

groupToUpdate.UserGroups.Remove(groupToUpdate.UserGroups.Where(ugu => ugu.UserId == userToUpdate.Id).FirstOrDefault());
_userGroupsContext.SaveChanges();

该UserGroups字段是中间类（表）的列表包含group与groupId，user和userId特性。