Escape object strings

Previously, if I had a class with a __toString method in it and passed
it to a query like this:

$q = $this->db->from('users')
->where('username', 'admin')
->where('password', new A())
->get();

the string returned by A's __toString() method would not be escaped in
the rendered query, allowing me to do sql injection using an object like
this:

class A{
public function __toString(){return "\"\" OR 1=1";}
}

and this would result with this SQL query being run:

SELECT * FROM users WHERE username = 'admin' AND password ="" OR 1=1

which would cause me to bypass the condition of the query or if I wrote
a bad query an SQL syntax error would occur.