Save ID in related table from the main table

I have three tables in my database:

At first, I only had the contratos table and recently I needed to add the other two tables, as you will see, there is a field in pagos which is related contratos by fk .

And I need that, when it's in my part to add a "contract", this saves your table ID ( contratos ) in the pagos table. So it would be like sending to save data in two tables at once. Here is my driver code:

function agregar_contrato()
{
    $data = array(
        'contratos' => $this->Pagos_model->getContratos(),
    );
    $this->load->view("layouts/header");
    $this->load->view("layouts/aside");
    $this->load->view("admin/clientes/agregar_contrato", $data);
    $this->load->view("layouts/footer");
}

function registro_contrato()
{
    $fecha_de_contrato    = $this->input->post("fecha_de_contrato");
    $fecha_de_instalacion = $this->input->post("fecha_de_instalacion");
    $fecha_de_corte       = $this->input->post("fecha_de_corte");
    $fecha_de_pago        = $this->input->post("fecha_de_pago");

    $this->form_validation->set_rules("fecha_de_contrato", "\"Fecha del contrato\"", "trim|required");

    if ($this->form_validation->run()) {
        $data = array(
            'fecha_de_contrato'    => $fecha_de_contrato,
            'fecha_de_instalacion' => $fecha_de_instalacion,
            'fecha_de_corte'       => $fecha_de_corte,
            'fecha_de_pago'        => $fecha_de_pago,
            'borrado'              => "1",
        );

        if ($this->Clientes_model->save_contrato($data)) {
            redirect(base_url() . "clientes/clientes_controller"); //Me regresa a la vista principal

        } else {
            $this->session->set_flashdata("error", "No se pudo guardar la informacion");
            redirect(base_url() . "clientes/clientes_controller/agregar_contrato");
        }
    } else {
        $this->agregar_contrato();
    }
}

And my model:

function getContratos()
{
    $this->db->select("con.*, pag.*, fpagos.*");
    $this->db->from("contratos con");
    $this->db->join("pagos pag", "pag.contrato_id = con.id_contratos_d");
    $this->db->join("fechas_de_pagos fpagos", "fpagos.pagos_id = pag.id_pagos");
    $this->db->where("con.borrado", "1");
    $resultados = $this->db->get();
    return $resultados->result();
}

As I mentioned at the beginning contratos was there, and all the data keeps well (here I only put 4 data, really are more than 20). But I ask for help, since I've tried it, I think it would be worse to put it here. So, to avoid worse criticism, I leave it as I originally had it.