关于CI 3加入的模糊字段

I have many branches and under one branch there are multiple cars.

So I need to join branch table with the car table .

My following code outputs an error like, "Column 'branch_id' in field list is ambiguous"

 $this->db->join("company","branch.company_id=company.company_id");
 $this->db->join("cars","branch.branch_id = cars.branch_id");

I have not used repeat of branch_id .

Any suggestion ?

Thank you.