codeigniter model call not working from one of the controllers - php

Question

I'm getting a weird error when trying to call a model method from one of my controllers say problem.php, all the values are fine but $this->db->get(); returns false, even the query returned from $this->db->last_query() is fine.
And the main issue is, when i pass the same values from another controller say test.php, everything works fine. Is this some caching issue?
Heres my code for the model
function how_many_left($product_id)
{
var_dump($product_id);
$sql="select * from invite where product_id=".$product_id." and accepted = 1";
echo "<br/>Product id:".$product_id."<br/>";
$query = $this->db->query($sql);
echo $this->db->last_query();
$invites = $query->num_rows();
echo $invites;
$this->db->select('quantity');
$this->db->where('product_id', $product_id);
$query2 = $this->db->get_where('product_inventory');
$quantity = 0;
foreach ($query2->result() as $row)
{
$quantity = $row->quantity;
}
$inventory = $quantity - $invites;
return ($inventory);
}
the call from the controller which is not working
foreach ($user_list as $product_id => $value) {
if (!isset($product_wise_invites[$product_id]))
{
$product_wise_invites[$product_id] = array();
}
if($this->debug_mode())
{
echo "Product id to get error on :<br/>";
var_dump($product_id);
}
$invites_left = $this->Product->how_many_left($product_id);
output of the controller not working
int(1349375633)
select * from invite where product_id=1349375633 and accepted = 1
Call to a member function num_rows() on a non-object in /opt/bitnami/apache2/htdocs/application/models/internal.php on line 1135
the call from the working controller
function invite_server_test()
{
$product_id=1349375633;
$this->Product->how_many_left($product_id);
}
The output of the working controller:
int(1349375633)
Product id:1349375633
select * from invite where product_id=1349375633 and accepted = 1
0