Entity Framework: removing entity when One-to-One PK association is specified - entity-framework

Question

I have existing DB with the following structure:
I'm using EF fluent API to configure relationships between tables:
public GroupEntityConfiguration()
{
HasMany(x => x.Employees).WithRequired().HasForeignKey(x => x.GroupId).WillCascadeOnDelete(true);
}
public EmployeeEntityConfiguration()
{
HasOptional(x => x.InnerGroupMember).WithRequired();
}
With this configuration applied I can add new Employee, new InnerGroupMember or fetch data. The problem appears when I try to remove Employee. Then I get an exception:
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.
As far as I understand above exception is connected with GroupId foreign key. Trying to fix it I'm adding following line to EmployeeEntityConfiguration:
HasKey(x => new { x.Id, x.GroupId});
But after adding it I get another exception which I believe is connected with InnerGroupMember object:
Invalid column name 'Guest_Id'. Invalid column name 'Guest_GroupId'.
If I comment out InnerGroupMember navigation property and remove it's configuration, Employee can be removed.
Could you please give me a hint what I'm doing wrong and how to configure entities to be able to perform all needed operations? Thanks!

Answer 1

I have an existing Group entity and I want to remove Employee from the Employees Group collection:
var group = groupRepository.Find(groupId);
group.RemoveEmployee(employeeId);
_unitOfWork.Save();
RemoveEmployee function inside Group entity looks like this:
public void RemoveEmployee(int employeeId)
{
var employee = Employees.Single(n => n.Id == employeeId);
Employees.Remove(employee);
}
That's why I get an exeption:
The relationship could not be changed because one or more of the foreign-key properties is non-nullable....
After reading this post I wanted to fix it adding HasKey(x => new { x.Id, x.GroupId}); function inside EmployeeEntityConfiguration what leads to the second exception:
Invalid column name 'Guest_Id'. Invalid column name 'Guest_GroupId'.
Actually I made this step (I mean adding HasKey function) without changing DB structure. To make it work, inside Employees table I have to create composite key - combination of Id and GroupId which is also a foreign key. This modification forces changes inside InnerGroupMembers table. DB structure looks now as following:
Now I'm able to remove Employee in a way I showed at the beginning.
Anyway I'm not going for this solution. They are different ways to achieve what I want. Here are some links:
Removing entity from a Related Collection
Delete Dependent Entities When Removed From EF Collection
The relationship could not be changed because one or more of the
foreign-key properties is non-nullable

Answer 2

For one-to-one relationships cascading delete is not enabled by default, even not for required relationships (as it is the case for required one-to-many relationships, that is: The WillCascadeOnDelete(true) in your one-to-many mapping is redundant). You must define cascading delete for a one-to-one relationship always explicitly:
HasOptional(x => x.InnerGroupMember).WithRequired().WillCascadeOnDelete(true);
When you delete an Employee now, the database will delete the related InnerGroupMember as well and the exception should disappear.