Entity Framework seeding causes insert statement conflict with foreign key

问题

I have a simple model:

class Employee {
 int EmployeeId {get;set;}
 List<Task> Tasks {get;set;}
}

class Manager {
 int ManagerId {get;set;}
 List<Task> Tasks {get;set;}
}

class Task {
 int TaskId {get;set;}
 EmployeeId {get;set;}
 ManagerId {get;set;}

 [ForeignKey("EmployeeId")]
 Employee Employee {get;set;}

 [ForeignKey("ManagerId")]
 Manager Manager {get;set;}
}

class Task1 : Task { }
class Task2 : Task { }

when I try to seed the database I get an exception, this is how I'm seeding:

Employee employee = new Employee();
context.Employees.Add(employee);
context.SaveChanges();

Manager manager = new Manager();
context.Managers.Add(manager);
context.SaveChanges();

List<Task> EmpTask = new List<Task>();
//EXCEPTION OCCURS HERE WHEN I TRY TO ADD NEW TASK ITEMS 
Task EmpOne = new Task1();
Task EmpTwo = new Task2();
EmpTask.Add(EmpOne);
EmpTask.Add(EmpTwo);

Employee.Tasks = EmpTask;
Context.Entry(employee).State = EntityState.Modified;
Context.SaveChanges();

List<Task> ManagerTask = new List<Task>();
Task ManagerOne = new Task1();
Task ManagerTwo = new Task2();
ManagerTask.Add(ManagerOne);
ManagerTask.Add(ManagerTwo);

Manager.Tasks = ManagerTask;
Context.Entry(manager).State = EntityState.Modified;
Context.SaveChanges();

The exception I keep getting is:

{"The INSERT statement conflicted with the FOREIGN KEY constraint \"FK_dbo.Task_dbo.Employee_EmployeeId\". The conflict occurred in database \"GIP\", table \"dbo.Employee\", column 'EmployeeId'.\r\nThe statement has been terminated."}

When I change my code to the following, however, I have no issues (seeding the same way, but no Manager portion):

class Employee {
 int EmployeeId {get;set;}
 List<Task> Tasks {get;set;}
}
class Task {
 int TaskId {get;set;}
 int EmployeeId {get;set;}
 [ForeignKey("EmployeeId")]
 Employee Employee {get;set;}
}

it seems like when the Task class has 2 foreign key constraints its getting confused or something...

Also, if I add the Manager class back, but REMOVE ALL ForeignKey constraints from the Task class, it has no issues because it creates it's own auto-generated foreign key columns in the database, with some employee foreign keys being Null for manager tasks, and manager foreign keys being null for employee tasks

回答1:

Make sure that your FK columns are nullable, eg. int? instead of int. If they are not null, then they are 0 by default, which will reference an entity with the 0 key, and that will violate the FK constraint.