c# - Prevent Entity Framework to Insert Values for Navigational Properties

Question

I am working on a WPF application using Entity Framework 4.0. When I tried to save the object, I got a primary key exception, but the primary key is an AutoIncremented field and I cannot understand the reason for the exception.

So after trying this and that, and a little debugging and using the SQL profiler, I found out that prior to inserting my object, a record must be inserted in the parent table, as I set the navigation property of that object.

So the crux is if an attempt to insert Employee object and set its department as Employee.Department = deptObject, then a new record is set to be inserted on department object.

Kindly suggest me someway by which navigational property objects won't be inserted in the database, any property or any method, Anything.

Thanks

Answer 1

This is the way how EF works if you incorrectly use detached entities. I suppose you are using something like this:

var employee = new Employee();
employee.Department = GetDepartmentFromSomewhere(departmentId);

...

using (var context = new YourContext())
{
    context.Employees.AddObject(employee);
    context.SaveChanges();
}

This code prepared employee entity, added reference to existing department and saved new employee to the database. Where is the problem? The problem is that AddObject doesn't add only employee but whole object graph. That is how EF works - you cannot have object graph where part of objects are connected to context and part of not. AddObject adds every object in the graph as a new one (new one = insert in database). So you must either change sequence of your operations or fix state of entities manually so that your context knows that department already exists.

First solution - use the same context for loading department and saving employee:

using (var context = new YourContext())
{
    var employee = new Employee();
    ...
    context.Employees.AddObject(employee);

    employee.Department = context.Departments.Single(d => d.Id == departmentId);
    context.SaveChanges();
}

Second solution - connect entities to the context separately and after that make reference between entities:

var employee = new Employee();
...

var department = GetDepartmentFromSomewhere(departmentId);

using (var context = new YourContext())
{
    context.Employees.AddObject(employee);
    context.Departments.Attach(department);
    employee.Department = department;

    context.SaveChanges();
}

Third solution - correct state of the department manually so that context doesn't insert it again:

var employee = new Employee();
employee.Department = GetDepartmentFromSomewhere(departmentId);

...

using (var context = new YourContext())
{
    context.Employees.AddObject(employee);
    context.ObjectStateManager.ChangeObjectState(employee.Department, 
                                                 EntityState.Unchanged);
    context.SaveChanges();
}