Unknown column in where clause while parsing csv to update table - codeigniter

Question

I am parsing a CSV file to add records to a table. this is how I do it.
public function parse_csv($file) {
$this->load->library('CSVReader');
$csvData = $this->csvreader->parse_file($file);
foreach($csvData as $key => $row) {
$data_n[$key] = array(
'mf_date' => $row['mf_date'],
'mf_student_id' => $row['mf_student_id'],
'mf_sender' => $row['mf_sender'],
'mf_amount' => $row['mf_amount'],
'mf_trx_id' => $row['mf_trx_id'],
);
$this->db->insert('monthly_fee', $data_n[$key]);
$this->student_model->update_monthly_due($row['mf_student_id'], $row['mf_amount']);
}
}
I wish to update another table where there is three columns
1)md_student_id //unique
2)md_due
3)md_paid
I want to update that table while uploading the csv where mf_student_id and md_student_id matches with the model function below.
public function update_monthly_due($mf_student_id, $mf_amount)
{
$this->db->select('mf_student_id');
$this->db->from('monthly_fee');
$this->db->join('monthly_due', 'monthly_due.md_student_id = monthly_fee.mf_student_id');
$sql = "UPDATE monthly_due set md_due = md_due - " . $mf_amount . ", md_paid = md_paid + " . $mf_amount . " WHERE md_student_id = " . $mf_student_id;
$this->db->query($sql);
}
But when I upload a csv it generates the following error.
Error Number: 1054
Unknown column 'MCS20145B41' in 'where clause'
UPDATE monthly_due set md_due = md_due - 5500, md_paid = md_paid + 5500 WHERE md_student_id = MCS20145B41
Filename: F:\xampp\htdocs\student\system\database\DB_driver.php
Line Number: 330
What I am doing wrong?!
Thanks in advance

Answer 1

you should replace this line as follow
$sql = "UPDATE monthly_due set md_due = md_due - " . $mf_amount . ", md_paid = md_paid + " . $mf_amount . " WHERE md_student_id = '" . $mf_student_id . "'";
take attention to the end of this line

Answer 2

Why not use CI's active record instead of raw SQL queries.When doing it do it the correct way. This below:
Change this,
$sql = "UPDATE monthly_due set md_due = md_due - " . $mf_amount . ", md_paid = md_paid + " . $mf_amount . " WHERE md_student_id = " . $mf_student_id;
$this->db->query($sql);
To,
$this->db->where('md_student_id', $mf_student_id);
$this->db->set('md_due', "md_due - $mf_amount", FALSE);
$this->db->set('md_paid', "md_paid + $mf_amount", FALSE);
$this->db->update('monthly_due');
Explanation:
The reason this works is because the third (optional) FALSE parameter tells CodeIgniter not to protect the generated query with backticks ('). This means that the generated SQL will be:
UPDATE monthly_due set md_due = md_due - 5500, md_paid = md_paid + 5500 WHERE md_student_id = "MCS20145B41"