Entity Framework has a restriction that doesn't allow you to remove a child element from a collection, and must remove the entity from it's DbSet<> instead.

In the following models, we define a 1-to-many relationship between a user and their orders. A user can have multiple orders, but an order can only belong to a single user. It's important in this example (and for the purpose of this library), that the ParentId foreign key be non-nullable.

public class User
{
    public virtual int UserId { get; set; }
    
    public virtual ICollection<Order> Orders { get; set; }
}

public class Order
{
    public virtual int ChildId { get; set; }
    
    public virtual int OrderId { get; set; }
    
    public virtual User User { get; set; }
}

If you attempt to remove an order from the user directly, Entity Framework will throw an exception.

public void DeleteLastOrder(int userId)
{
    using (var db = new TestDbContext())
    {
        User user = db.Users.Include(u => u.Orders).Find(userId);
        
        var lastOrder = user.Orders.OrderByDescending(o => o.OrderDate).First();
        
        // do something with the order, maybe log a message
        
        user.Orders.Remove(lastOrder);
        
        db.SaveChanges(); // throws exception
    }
}

calling SaveChanges() will throw:

System.InvalidOperationException: 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.

For a detailed explanation of why, see [this post](http://blog.oneunicorn.com/2012/06/02/deleting-orphans-with-entity-framework/

In looking for a workaround, I found this great solution by brockallen

It works by hooking into the collection's Remove() event, and automatically marking removed entities as Deleted. Since I've used this in multiple projects, I've decided to write a generic implementation.

Inside your DbContext's constructor:

public class YourDbContext : DbContext
{
    public YourDbContext()
    {
        RemoveSemantics<User>.ForContext(this).DeleteOnRemove(i => i.Orders);
    }
}

Now removing a child entity from a collection should work, and delete the corresponding row in the database.

public void DeleteLastOrder(int userId)
{
    using (var db = new TestDbContext())
    {
        User user = db.Users.Include(u => u.Orders).Find(userId);
        
        var lastOrder = user.Orders.OrderByDescending(o => o.OrderDate).First();
        
        user.Orders.Remove(lastOrder);
        
        db.SaveChanges(); // this works now
    }
}