Codegniter - Internal error when acessing database - php

Question

I'm using CodeIgniter to access the database, but everytime I run the code it gets an internal error. This is my jQuery code:
function showCadeiras() {
$.ajax({
type: "POST",
url: base_url + "api/teacher/getCadeiras",
data: {id: localStorage.user_id},
success: function(data) {
for(var i = 0; i < data.cadeiras_id.length; i++) {
var cadeira_id = data.cadeiras_id[i].cadeira_id;
$.ajax({
type: "POST",
url: base_url + "api/teacher/getCadeiraInfo",
data: {id: cadeira_id},
success: function(data) {
console.log(data);
},
error: function(data) {
console.log("error");
}
})
}
},
error: function(data) {
console.log(data);
}
});
}
Here is the code inside the Controller:
public function getCadeiras() {
$user_id = $this->post('id');
$this->load->model('UserModel');
$data["cadeiras_id"] = $this->UserModel->getCadeiras($user_id);
$this->response($data, parent::HTTP_OK);
}
public function getCadeiraInfo(){
$cadeira_id = $this->post('cadeira_id');
$this->load->model('UserModel');
$data["info"] = $this->UserModel->getCadeiraInfo($cadeira_id);
$this->response($data, parent::HTTP_OK)
}
Lastly, the Model code:
public function getCadeiras($id) {
$this->db->select("cadeira_id");
$this->db->where(array('user_id =' => $id));
$query = $this->db->get('professor_cadeira');
return $query->result_array();
}
public function getCadeiraInfo($id) {
$query = $this->db->get_where('cadeira', array('id =' => $id));
return $query->result_array();
}
Everytime I comment the second function (getCadeiraInfo) the code works without a problem, however it doesn't work with the second function. They are written in a similar way, I don't understand the error.