Deleting one of a collection on parent update - php

Question

I am trying to delete (or create) a models associated objects when it is updated. Each bar has several taps. When you create a bar, these tap objects are created, and you can update that number and additional taps will be created or deleted as necessary.
Originally I just wanted to use pop like this:
$taps=$bar->taps;
if ( $taps_old < $taps_new){
for ($i = $taps_old; $i < $taps_new; $i++) {
$tap = Growlertap::create(['growlerstation_id' => $id]);
}
}
elseif ($taps_old > $taps_new) {
for ($i = $taps_new; $i < $taps_old; $i++) {
$taps->pop();
}
which doesn't work but doesn't give me an error. I know the if statement is working fine because the code below works:
elseif ($taps_old > $taps_new) {
for ($i = $taps_new; $i < $taps_old; $i++) {
Beertap::where('bar_id', '=', $id)->first()->delete();
}
}
This seems to not be the simplest way to write this. Is there a better way to write this?
By the way, for those wondering, this is in my update function in my controller.

Answer 1

pop() will remove the last item in your local collection, but it won't persist that change to the database.
Assuming it's a Beertap object, something like this should work:
Warning: untested code
...
elseif ($taps_old > $taps_new) {
for ($i = $taps_new; $i < $taps_old; $i++) {
$delete_me = $taps->pop();
$delete_me->delete();
}
}
or more succinctly: $taps->pop()->delete();

Answer 2

This should work Unfortunately this doesn't work because the DELETE statement doesn't support offsets.
Beertap::where('bar_id')->skip($taps_new)->delete();
So it skips as many taps as you want to keep and deletes the rest. You might want to use orderBy if it matters which rows get deleted.
Update
This should totally work now. First get all id's to delete from the collection and then delete them with one query
$idsToDelete = $taps->slice($taps_new)->modelKeys();
Beertap::destroy($idsToDelete);
Update 2
You can optimize the creation process as well (so that it's done in a single query)
$data = [];
for ($i = $taps_old; $i < $taps_new; $i++) {
$data[] = ['growlerstation_id' => $id];
}
Beertap::insert($data);
Note that you will loose the Eloquent features like automatic timestamps and model events when choosing to use insert().