我正在创建ASP.NET Core Web API,并且在查询上下文时遇到问题.
Web API连接到具有3个表的数据库：雇员,事件和事件责任.员工可以创建许多事件.此外,可以为许多员工在每个事件中分配许多事件的责任(因此链接了事件责任表).
员工表
+----+-----------------+
| Id | Name |
+----+-----------------+
| 1 | First Employee |
| 2 | Second Employee |
+----+-----------------+
活动表
+----+------------+---------------+
| Id | Creator_Id | Event_Name |
+----+------------+---------------+
| 1 | 1 | Random Event |
| 2 | 1 | Another Event |
+----+------------+---------------+
活动责任表
+----+-------------+----------+
| Id | Employee_Id | Event_Id |
+----+-------------+----------+
| 1 | 1 | 1 |
+----+-------------+----------+
这些模型包括…
事件模型
活动只能有一名雇员作为创建者.
一个事件也可以有许多承担不同职责的员工.
public int Id { get; set; }
public int? CreatorId { get; set; }
public string EventName { get; set; }
public Employee Creator { get; set; }
public ICollection<EventResponsibilities> EventResponsibilities { get; set; }
员工模式
员工可以成为许多事件的创建者.
员工可以在许多事件中承担责任.
public int Id { get; set; }
public string Name { get; set; }
public ICollection<Event> Event { get; set; }
public ICollection<EventResponsibilities> EventResponsibilities { get; set; }
事件_责任模型
多对多链接表模型.
public int Id { get; set; }
public int? EmployeeId { get; set; }
public int? EventId { get; set; }
public Employee Employee { get; set; }
public Event Event { get; set; }
return _context.Event.Include(e => e.EventResponsibilities).ThenInclude(e => e.Employee);
我希望得到一个JSON,其中包含事件的详细信息,其对应的事件职责以及与该职责相关的Employee.
我实际上得到的是以下json …
[
{
"Id": 1,"CreatorId": 1,"EventName": "Random Event","Creator": {
"Id": 1,"Name": "First Employee","Event": [],"EventResponsibilities": [
{
"Id": 1,"EmployeeId": 1,"EventId": 1
}
]
},"EventResponsibilities": [
{
"Id": 1,"EventId": 1,"Employee": {
"Id": 1,"EventResponsibilities": []
}
}
]
},{
"Id": 2,"EventName": "Another Event","Event": [
{
"Id": 1,"EventResponsibilities": [
{
"Id": 1,"EventId": 1
}
]
}
],"Event": {
"Id": 1,"EventResponsibilities": []
}
}
]
},"EventResponsibilities": []
}
]
这不是我想要或期望的…
如您所见,由于某种原因,即使我没有在查询中请求它,也未显示Event(事件)模型的Creator导航属性,因为我没有使用Include().
此外,如果您查看JSON中第二个返回的事件(“ Id”：2),即使没有EventResponsibilities,也将返回两个创建者行.我不认为这是一个延迟加载问题,因为导航属性不是虚拟的.
我本来希望Creator对象为null或为空,因为我没有将其包含在内.
所以我的问题是：为什么即使我没有加载/包含Creator导航属性也是如此？另外,我也想知道
不
包括相关的创建者数据,并且仅在JSON中显示事件责任数据.
Entity Framework Core will automatically fix-up navigation properties to any other entities that were prevIoUsly loaded into the context instance. So even if you don’t explicitly include the data for a navigation property,the property may still be populated if some or all of the related entities were prevIoUsly loaded.
通过做
.ThenInclude(e => e.Employee);
您请求EF提取此Employee,从而使它被急切地加载到它所引用的任何位置.并且由于在这种特定情况下,您的Event.Creator与您显式请求的EventResponsibilities.Employee相同,因此在将其提取到EventResponsibilities中之后,它也将提取到您的Event中.
如果您的Event.Creator是与EeventsResponsbilities.Employee不同的Employee,您将不会遇到这种异常情况.
这种意外行为和冗余数据是人们使用单独的DTO从控制器返回数据以确保API返回所需数量的正确数据的原因之一.
作为一种快速的解决方法,您只需在Creator属性上添加JsonIgnoreAttribute即可：
[JsonIgnore]
public Employee Creator { get; set; }
但是我个人不认为在同一类中混合DTO和Entity是一个很好的主意,因为每当任何实体发生变化时,都会引起各种麻烦.
