Laravel query builder whereIn doesn't work

Question

I have below Models and I need to get an array of district_id without field name. (like, [1, 2, 3, 4])

Province [id, description]

District [id, description, province_id]

$province_arr = DB::table('provinces')->pluck('id')->toArray();

$district_arr = DB::table('districts')->pluck('id')
     ->whereIn('province_id', $province_arr)->toArray();

But $district_arr is always empty. (i.e. dd($district_arr) always give [])

Database contains data to match the query. Can someone help me figure out why doesn't this work?

Answer 1

You should use pluck AFTER whereIn like so:

$district_arr = DB::table('districts')->whereIn('province_id', $province_arr)
    ->pluck('id')->toArray();

because pluck is here the method that gets data from database. If you add condition after getting from database you are not getting what you expected.

Otherwise you get data from database and then you run whereIn on Support Collection and there are no records with province_id because all of them have only id as you specified in pluck