Questions about the PHP query MongoDB limit return field

Recently wanted to do a front-end control Interface field return of a basic method, through the MongoDB find ($query, $field) query to specify the fields of the query, but encountered such a problem:

There are two encapsulation methods in the work code:

/**
* Query a record
* @param String collection name
* @param array Query criteria
* @param The fields to be returned by array
* Results of @return array query
*/
   Public Function Find_one ($coll _name, $condition = Array (), $ret _fields = Array (' _id ' = 0)} {
      return $this->docsdb->find_one ($coll _name, $condition, $ret _fields);
   }

/**
* Enquiry
* @param String collection name
* @param array query conditions array<br/> such as <i>array (col_a=>111) </i>
* @param Array Collection filter array<br/> full look like this: <i>array (Sort=>array (col_a=>1,col_b=>-1), skip=>100 , limit=>10, timeout=>5000, Immortal=>true) </i>, which means:
* <ol>
* <li>wrapped to wrapped as an array of keys, by default in order of arrays </li>
* <li>_id, the default does not return data with the Mongoid field, if you specify the return column is the same effect </li>
* <li>sort col_a to Asc,col_b for DESC, can be multi-column combination </li>
* <li>skip means to start fetching data from 101 records, skipping the first 100 </li>
* <li>limit the number of selected articles </li>
* <li>timeout indicates the time to wait for a response (temporarily not used) </li>
* <li>immortal indicates whether to maintain the link is permanent, default true, at this time timeout expires (temporarily not used) </li>
* </ol>
* @param array needs to return fields (usually returning only the necessary fields to speed up the response)
* Results of @return Mixed query
*/
Public function Find ($coll _name, $condition = Array (), $result _filter = Array (' wrapped ' = = ', ' with_objectid ' = = 0, ' Timeout ' = ' immortal ' = ' + ', $ret _fields = Array ()) {
return $this->docsdb->find ($coll _name, $condition, $result _filter, $ret _fields);
}

**

' With_objectid ' + 0 was set to 1 when the parameter was passed in

**
According to the above description, $ret _fields is the field in MongoDB find ($query, $field), but by testing, I have found such a problem if I want to go to display ID, passing in the parameter array

$ret _fields = Array (
' _id ' = 1,
    ' Account ' = 1,
    ' Card ' = 1
)，
and call the Find method, according to the theory, this time ' _id ' This field is also displayed, and I through this condition directly query MongoDB, is also displayed, but the printed data is not with the ID, the data is as follows:

{
"Account": "Qwe",
"Card": ""
}

Add a test result, if the Find method does not pass in the $ret_fields variable past, that is, does not limit the return field, is the existence of ' _id '

By modifying the conditions, found that the ' _id ' and 1 This condition does not write is useless, so I use the same

$ret _fields to call the Find_one method, this time unexpectedly returns the ' _id ' field, the data is as follows

{
"_id": {
"$id": "58f472672430e11817000152"
},
"Account": "Qwe",
"Card": ""
}

This problem first recorded in this, after the settlement of a record of the solution, if there is a great God can explain this reason, please advise, thank you!

Questions about the PHP query MongoDB limit return field