Question

Why does this produce an object?

$sql = $this->db->query("
  SELECT 'product_id'
  FROM " . DB_PREFIX . "product_to_category
  WHERE category_id = '" . $this->db->escape($category['category_id']) . "'
");

And how to I get a string/array instead?

From Opencart
Related: Object of class stdClass could not be converted to string

Answer 1

You have an error in your query syntax, you put SELECT ‘product_id’ and you should not have apostrophies there.

$sql = $this->db->query("
  SELECT 'product_id'
  FROM " . DB_PREFIX . "product_to_category
  WHERE category_id = '" . $this->db->escape($category['category_id']) . "'
");

You need to make this change.

$sql = $this->db->query("
  SELECT product_id
  FROM " . DB_PREFIX . "product_to_category
  WHERE category_id = '" . $this->db->escape($category['category_id']) . "'
");