I have an issue for getting the inserting multiple dropdown values. After submitting it gets a database error. : **View:**

<select class="form-control" data-placeholder="Choose a Category" tabindex="1" name="venues[]"  multiple >
                            <option value="">Select One</option>	
							<?php foreach($list_of_venues as $venues){ ?>
							<option value="<?php echo $venues->activity_venue_id; ?>"><?php echo $venues->venue_title; ?></option>
							<?php } ?>
							</select>

**Controller**

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Create_activity_session extends CI_Controller {

	function __construct()
    {
        parent::__construct();
        $this->load->model('vendor_model');
        $this->load->database();
    }
	public function index()
	{
		error_reporting(0);

		$session_data = $this->session->userdata('logged_in');
		$data['account_name'] = $session_data['account_name'];
		if($session_data['account_name']){
			
			$data['id'] = $session_data['id'];
			$id = $data['id'];
			$data['list_of_activities'] = $this->vendor_model->list_of_activities_by_vendor($id);
			$data['list_of_venues'] = $this->vendor_model->list_of_venues_by_vendor($id);
			$data['list_of_booking_fields'] = $this->vendor_model->list_of_booking_fields_by_vendor_by_venue($id);


			$this->form_validation->set_rules('activity_title','Activity Title','required');
			$this->form_validation->set_rules('venues[]','Selecting Venue','required');
			$this->form_validation->set_rules('start_date','Start Date','required');
			$this->form_validation->set_rules('end_date','End Date','required');
			$this->form_validation->set_rules('start_time','Start Time','required');
			$this->form_validation->set_rules('end_time','End Time','required');
			$this->form_validation->set_rules('number_of_seats','Number of Bookings','required');
			$this->form_validation->set_rules('price','Price','required');

           

			if($this->form_validation->run()==FALSE){
				$data['list_of_activities'] = $this->vendor_model->list_of_activities_by_vendor($id);
				$data['list_of_venues'] = $this->vendor_model->list_of_venues_by_vendor($id);
				$data['list_of_booking_fields'] = $this->vendor_model->list_of_booking_fields_by_vendor_by_venue($id);
    			$this->load->view('vendor/create_activity_session',$data);
    		} else {

				$end_date = $this->input->post('end_date');
    			$start_date = $this->input->post('start_date');

    			if($end_date < $start_date){
    				$data['error'] = 'Start Date should not be greater than the End Date.'; 
    				$this->load->view('vendor/create_activity_session',$data);
    			} else {
				
    			$activity_id = $this->input->post('activity_title');

               

                $venue_id = implode(',',$this->input->post('venues'));

				$data = array(
    					'activity_by_id' => $this->input->post('activity_title'),
    					'venue_id' => $venue_id,
    					'start_date' => $this->input->post('start_date'),
    					'end_date' => $this->input->post('end_date'),
    					'start_time' => $this->input->post('start_time'),
    					'end_time' => $this->input->post('end_time'),
    					'number_of_seats' => $this->input->post('number_of_seats'),
    					'price' => $this->input->post('price'),
    					'vendor_id' => $id,
    					'status' => 1,
    					'created_on' => date('Y-m-d h:i:s')

    			);

    			
    			
    			$result = $this->vendor_model->create_activity_dates_time($data);
    			

                $session_id = $this->db->insert_id();

             //   $venues = $this->input->post('venues');

                $data = array(
                        'venue_activity_id' => $venue_id,

                );
                $result2 = $this->vendor_model->update_activity($data,$activity_id);




                $venues = $this->input->post('venues');

                $venue_count = count($venues);

                for($i = 0;$i<=$venue_count; $i++){

              

               $data = array(
                        'session_by_id' => $session_id,
                        'venue_by_id' => $venues[$i]

                );


                $result3 = $this->vendor_model->create_venues_by_session($data); 

                }

    			$data['success'] = 'You have created the new Activity Session'; 


    			

    			$data['list_of_activities'] = $this->vendor_model->list_of_activities_by_vendor($id);
				$data['list_of_venues'] = $this->vendor_model->list_of_venues_by_vendor($id);
				$data['list_of_booking_fields'] = $this->vendor_model->list_of_booking_fields_by_vendor_by_venue($id);
    			 $this->load->view('vendor/create_activity_session',$data);
		}
		} 

	}  else {
			redirect('http://localhost/bookleisure/vendor/login');
		}
	}
}
I have an issue with inserting the multiple dropdown values that shows the database error such as "venue_by_id cannot be null. I wanted to insert the multiple dropdown rows. So please kindly resolve that issue.


  [1]: https://i.stack.imgur.com/5kZ99.jpg