Question

I have an entity called Entry connected to multiple TimeWindows. I want to clear all time windows, then add new ones. At first I tried:

target.TimeWindows.Clear();

but this didn’t really delete them, and only tried to remove the relationship, which caused an exception since there is foreign key from TimeWindows to Entry. Then I thought I should do this:

foreach (var tw in target.TimeWindows)
    context.DeleteObject(tw);

but this throw an exception as well, since the collection was modified inside the foreach statement. So I thought of this:

while (target.TimeWindows.Count > 0)
    context.DeleteObject(target.TimeWindows.Last());

But now I am a bit concerned about using Count property, because it might cause a SQL SELECT COUNT statement to be executed. Does it? If yes, how can I delete all time windows in Entity Framework?

Answer 1

Calling count on navigation property will cause select only if lazy loading is enabled and the property is not loaded yet. So the first call can cause something like:

SELECT * FROM TimeWindows WHERE TargetId = @targetId

and all count evaluations will just execute on loaded data.

You can also use this to avoid the second exception:

foreach (var tw in target.TimeWindows.ToList())
    context.DeleteObject(tw);

Or you can change your database and model to support identifying relation (FK to parent will became part of TimeWindow’s PK) and in such case your first code snippet will work.