Laravel groupBy Database query Builder not working - php

Question

I have this problem in a query using laravel groupBy, it simply return a groupBy error. I have read the documentation about this but can't really figure it out. I also read the same problem pointing that it is because of postgreSQL that I need to include all the columns in grouBy clause. I tried it but still it doesn't return the distinct values. Please help me with this. Below is my code. Thanks a lot.
Controller function
public function index(){
$purchases = Purchase::groupBy('purchase_order_id')->get();
return view('purchases/purchases_crud', ['allPurchases' => $purchases]);
}
Table to query
Error
QueryException in Connection.php line 680:
SQLSTATE[42803]: Grouping error: 7 ERROR: column "purchases.id" must appear
in the GROUP BY clause or be used in an aggregate function
LINE 1: select * from "purchases" group by "purchase_order_id"
^ (SQL: select * from "purchases" group by "purchase_order_id")

Answer 1

There you have it add group by "purchases.id" or restrict the select to only the operates that are needed.
->select("purchase_order_id","purchases.id")
->groupBy("purchases.id") // add if possible
Agreggates for your case should mean something like ->select("sum(po_total)")
If we group by id, we get all results as id is unique, my mistake. You want something like this
DB::table("purchases")->select("purchase_order_id", DB:raw("sum(po_total)"))->groupBy("purchase_order_id")->g‌​et();
Rule of thumb is you either select a field with Sum() or have it on the group by