Error Deleting Child Entity in Entity Framework 6 - entity-framework

Question

We are using EF6 database first with AspNet Identity. AspNetUsers is our table of customers. It extends AspNet IdentityUser. Each customer has many devices. The abbreviated table structures are
CREATE TABLE [dbo].[AspNetUsers] (
[Id] NVARCHAR (128) NOT NULL
);
CREATE TABLE [dbo].[Devices] (
[DeviceID] INT IDENTITY (1, 1) NOT NULL,
[UserId] NVARCHAR (128) NOT NULL
);
ALTER TABLE [dbo].[Devices]
ADD CONSTRAINT [FK_Devices_AspNetUsers] FOREIGN KEY ([UserId]) REFERENCES [dbo].[AspNetUsers] ([Id]);
CustomersContext OnModelCreatingspecifies the relationship between AspNetUsers and Devices.
modelBuilder.Entity<ApplicationUser>()
.HasMany(e => e.Devices)
.WithRequired(e => e.User)
.HasForeignKey(e => e.UserId)
.WillCascadeOnDelete(false);
modelBuilder.Entity<Device>()
.HasKey(e => e.DeviceId);
modelBuilder.Entity<Device>().Property(e => e.UserId)
.IsRequired()
.HasMaxLength(450);
The following code
var userDevices = user.Devices.ToList();
foreach (Device device in userDevices)
{
user.Devices.Remove(device);
}
await customersContext.SaveChangesAsync();
fails with the error
Cannot insert the value NULL into column 'UserId', table 'Customers.dbo.Devices'
That appears to be trying to remove the relationship without deleting the device. How do I delete a device using EF?

Answer 1

You want to delete the device. It is not possible to just remove it from the Devices list, as it cannot exist stand-alone by itself. It has a non-nullable foreign key (from column UserId) referencing AspNetUsers(Id) - deletion would break referential integrity.
To delete the device, add following statement inside foreach loop to successfully delete the devices associated with a user.
customersContext.Entry(device).State = EntityState.Deleted;
Another way would be to place following code inside foreach loop to delete the device directly from the device repository itself, instead of going through Devices navigation property available on ApplicationUser entity:
customersContext.Devices.Remove(device);
If you don't want to delete the records from the Devices table and just want to remove the relationship between AspNetUsers and all the Devices in one go, then you can assign null to the navigation property and save the changes. This will remove the relation between the entities without physically deleting the records from database.
user.Devices = null;