cakephp3- unable to show count of values - php

Question

I am trying to show number of users present in a room. But when I try to do that, I get error.
This is the relationship.
Rooms has many users.
Rooms belong to groups.
public function initialize(array $config)
{
parent::initialize($config);
$this->table('rooms');
$this->displayField('name');
$this->primaryKey('id');
$this->belongsTo('Groups', [
'foreignKey' => 'group_id'
]);
$this->hasMany('Users', [
'foreignKey' => 'room_id'
]);
}
To achieve this, I wrote a controller function which looks like this:
public function index()
{
$this->loadComponent('Prg');
$this->Prg->commonProcess();
$params = $this->Prg->parsedParams();
$this->paginate = [
'limit' => 25,
'order' => ['id' => 'ASC'],
'finder' => ['RoomsList' =>['filter'=> $params]],
'sortWhitelist' => ['Rooms.id','Rooms.name'],
'extraOptions' =>['params' => $params]
];
$rooms = $this->paginate($this->Rooms);
$this->set(compact('rooms'));
$this->set('_serialize', ['rooms']);
}
This function paginates the list of rooms available. This is working fine but now when I try to show the count of users in each room, it throws error.
I modified my model function like this:
public function findRoomsList(Query $query, array $options)
{
$query->select(['id','name','modified','created'])
->contain([
'Groups'=> function ($query) {
return $query->select(['id','name']);
},
'Users' => function($query){
return $query->select(['status','full_name']);
//want to fetch total number of users
}
]);
echo $query;
return $query;
}
Now I'm getting this error:
You are required to select the "Users.room_id" field(s)
Can anyone tell me the mistakes I made here? I'm new to CakePHP3.