mongoDB的$ slice运算符在PHP中不起作用

Question

I have got this mongoDB query:

db.b_activity.find(
    {token_id:"71e32267108b163ccdd3f59ba06c66674b295499"},
    {activity_log:{$slice: -1}}
).pretty();

It gets all the desired results in the mongoDB terminal, but when I write the same query in PHP, it fetches the complete array every time.

MY PHP query:

$get_current_activity = $this->mongo_b_employee->b_activity->findOne(
    array('token_id'=>$token_id),
    array("activity_log" => array('$slice' => -1))
);

Any suggestions where I am doing something wrong?

PS: I am just trying to access the last array element.

Answer 1

The signature for MongoDB\Collection::findOne() is slightly different from the "shell". It requires the "projection" key in the $options as the second argument:

$get_current_activity=$this->mongo_b_employee->b_activity->findOne(
    array('token_id'=>$token_id),
    array(
      "projection" => array( "activity_log" => array('$slice' => -1) )
    )
)