EntityFramework: How to configure Cascade-Delete to nullify Foreign Keys

Question

EntityFramework\'s documentation states that the following behavior is possible:

If a foreign key on the dependent entity is nullable, Code First does

Answer 1

It works indeed as described but the article on MSDN misses to emphasize that it only works if the children are loaded into the context as well, not only the parent entity. So, instead of using Find (which only loads the parent) you must use eager loading with Include (or any other way to load the children into the context):

using (var dbContext = new TestContext())
{
    var master = dbContext.Set<TestMaster>().Include(m => m.Children)
        .SingleOrDefault(m => m.Id == 1);
    dbContext.Set<TestMaster>().Remove(master);
    dbContext.SaveChanges();
}

This will delete the master from the database, set all foreign keys in the Child entities to null and write UPDATE statements for the children to the database.

Answer 2

After following @Slauma's great answer I was still getting same error as OP.

So don't be as naive as me and think that the examples below will end up with same result.

dbCtx.Entry(principal).State = EntityState.Deleted;
dbCtx.Dependant.Where(d => d.PrincipalId == principalId).Load();

// code above will give error and code below will work on dbCtx.SaveChanges()

dbCtx.Dependant.Where(d => d.PrincipalId == principalId).Load();
dbCtx.Entry(principal).State = EntityState.Deleted;

First load the children into context before setting entity state to deleted (if you are doing it that way).