entity.Items.Clear（）在EF中做什么？

Question

在一个项目中我的工作，每当我清算项目名单后打电话SaveChanges()，我得到这个错误：

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

我有这两个类具有一对多的关系（最简单的实现）：

public class Company 
{ 
    public int Id {get; set;} 
    public string Name {get; set;} 
    public virtual List<Vehicle> Vehicles {get; set;} 
} 

public class Vehicle 
{ 
    public int Id {get; set;} 
    public string Model {get; set;} 

} 

我猜测，调用company.Vehicles.Clear()应该更新Vehicle表中的自动生成列Company_Id，将其值设置为null，但似乎并不是上述错误的情况。 所以我想知道company.Vehicles.Clear()究竟在数据库端做了什么？

编辑

多亏了@McKabue的答案，他所提供的链接，我发现这是解决方案，这是完整的代码：

var company = context.Set<GENCompany>(). 
         Include(m => m.Vehicles). 
         Include(m => m.Quotes). 
         Include(m => m.Phones). 
         Include(m => m.Persons). 
         Include(m => m.Leads). 
         Include(m => m.Emails). 
         Include(m => m.Drivers). 
         Include(m => m.Deals). 
         Include(m => m.Comments). 
         Include(m => m.Branches). 
         Include(m => m.Addresses). 
         SingleOrDefault(m => m.Id == companyModel.Id); 

        context.Entry(company).State = EntityState.Modified; 

        company.Vehicles.Clear(); 
        company.Drivers.Clear(); 
        company.Quotes.Clear(); 
        company.Persons.Clear(); 
        company.Leads.Clear(); 
        company.Deals.Clear(); 
        company.Branches.Clear(); 

        var phones = context.CompanyPhones.Where(x => x.CompanyId == companyModel.Id); 
        context.CompanyPhones.RemoveRange(phones); 

        var emails = context.CompanyEmails.Where(x => x.CompanyId == companyModel.Id); 
        context.CompanyEmails.RemoveRange(emails); 

        var comments = context.CompanyComments.Where(x => x.CompanyId == companyModel.Id); 
        context.CompanyComments.RemoveRange(comments); 

        var addresses = context.CompanyAddresses.Where(x => x.CompanyId == companyModel.Id); 
        context.CompanyAddresses.RemoveRange(addresses); 

对于一些实体，他们被删除使用.Clear（）语法，其他实体通过获取其记录并将其全部删除（不仅是关系）而被删除。这在目前的设计中非常有意义，例如在公司的创建页面中，手机数据是手动添加的，而车辆是从下拉菜单中选择的，同时它使这个完美的感觉，我觉得很困惑，以确定为什么发生这种情况，例如这里是公司电话类：

public class GENCompanyPhone 
    { 
     [Key] 
     public long PhoneId { get; set; } 
     public long CompanyId { get; set; } 
     public long PhoneTypeId { get; set; } 
     public DateTime DateCreated { get; set; } = DateTime.Now; 
     public DateTime DateUpdated { get; set; } = DateTime.Now; 
     public Guid? CreatedById { get; set; } 
     public bool IsPublish { get; set; } = false; 
     public bool IsActive { get; set; } = false; 

     [MaxLength(255)] 
     [Index(IsUnique = true)] 
     public string PhoneNumber { get; set; } 
     [ForeignKey("CompanyId")] 
     public virtual GENCompany Company { get; set; } 
    } 

，另一方面，这是车辆类：

public class GENVehicle 
    { 
     public long Id { get; set; } 
     public long? BrandId { get; set; } 
     [ForeignKey("BrandId")] 
     public GENBrand Brand { get; set; } 
     public string Model { get; set; } 
     public DateTime? LicenseIssueDate { get; set; } 

     public Guid? CreatedById { get; set; } 

     [ForeignKey("VehicleTypeId")] 
     public virtual GENVehicleType VehicleType { get; set; } 
     public long? VehicleTypeId { get; set; }  
     public long? LicenseIssuerId { get; set; } 
     [ForeignKey("LicenseIssuerId")] 
     public GENEmployee LicenseIssuer { get; set; } 

    } 

我不能确定到底为什么EF与Vehicle和CompanyPhone

Answer 1

交易不同

这意味着您正在清除在另一个表中引用的记录。因此，EF警告你......有办法可以避免这种通过设定Cascade delete

编辑 要删除一个父表的子元素，实体需要的孩子被加载。

编辑2

var company = context.Set<GENCompany>().Where(m => m.Id == companyModel.Id); 
         Include(m => m.Vehicles). 
         Include(m => m.Quotes). 
         Include(m => m.Phones). 
         Include(m => m.Persons). 
         Include(m => m.Leads). 
         Include(m => m.Emails). 
         Include(m => m.Drivers). 
         Include(m => m.Deals). 
         Include(m => m.Comments). 
         Include(m => m.Branches). 
         Include(m => m.Addresses).SingleOrDefault(); 




        company.Vehicles.Clear(); 
        company.Drivers.Clear(); 
        company.Quotes.Clear(); 
        company.Persons.Clear(); 
        company.Leads.Clear(); 
        company.Deals.Clear(); 
        company.Branches.Clear(); 

，这将包括工作前

1. 查询
2. 包括要删除所有实体;
3. 清除/ RemoveRange /由一个来自父删除一个（目标公司）