php - Fatal error: Call to a member function fetch_assoc() on a non-object

I'm trying to execute a few queries to get a page of information about some images. I've written a function

function get_recent_highs($view_deleted_images=false)
{
    $lower = $this->database->conn->real_escape_string($this->page_size * ($this->page_number - 1));
    $query = "SELECT image_id, date_uploaded FROM `images` ORDER BY ((SELECT SUM( image_id=`images`.image_id ) FROM `image_votes` AS score) / (SELECT DATEDIFF( NOW( ) , date_uploaded ) AS diff)) DESC LIMIT " . $this->page_size . " OFFSET $lower"; //move to database class
    $result = $this->database->query($query);
    $page = array();
    while($row = $result->fetch_assoc())
    {
        try
        {

            array_push($page, new Image($row['image_id'], $view_deleted_images));
        }
        catch(ImageNotFoundException $e)
        {
            throw $e;
        }
    }
    return $page;
}

that selects a page of these images based on their popularity. I've written a Database class that handles interactions with the database and an Image class that holds information about an image. When I attempt to run this I get an error.

Fatal error: Call to a member function fetch_assoc() on a non-object

$result is a mysqli resultset, so I'm baffled as to why this isn't working.

Answer

That's because there was an error in your query. MySQli->query() will return false on error. Change it to something like::

$result = $this->database->query($query);
if (!$result) {
    throw new Exception("Database Error [{$this->database->errno}] {$this->database->error}");
}

That should throw an exception if there's an error...