SQL sum function to store as single variable

hi, i'm newby in using cakePHP.

i have 2 tables: customerbills and customerpurchases.

what i want to do is to make a sum query in customerpurchases and
update it in customerbills.
customerpurchases fields:
-id
-customerbill_id
-product
-amount

customerbills fields:
-id
-customer_id
-amountbill

Here's what i did in my customerpurchasesController:

$billid = 1;

$total = $this->Customerpurchase->query("SELECT
SUM(customerpurchases.amount) AS total From customerpurchases Group by
customerbill_id = $billid");

now i try this for updating:
$this->Customerpurchase->query("UPDATE bills SET amountbill = $total
Where id = $billid");

but i it doesn't work. I tried to display the variable $total in
session flash to check what's inside the variable and it displays
"arrays" ..

pls. help me. Thanks..

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