how to use $bit operator to query in mongodb with php - php

Question

i have a document like bellow, now i will to figure out which element in friendsR whose readable & 2 is grater than 0. anyone here can help me? thank you.
[code]
$doc = array('friendsR' => array(
array('friend_id'=>100, 'readable' => 7, 'bridge'=>5),
array('friend_id'=>100, 'readable' => 7, 'bridge'=>6),
array('friend_id'=>100, 'readable' => 7, 'bridge'=>7))
);
datamodel()->insert($doc);
$macher = array('$bit' => array( 'friendsR.$.readable' => array('and'=>2) ));
$cursor = datamodel()->find($macher);
[/code]

Answer 1

The $bit operator is an "update operator", so it is used to change values and not query them.
If you need this sort of query you need a JavaScript evaluation such as $where:
For a document that looks like this in the shell:
{
"friendsR": [
{ 'friend_id': 100, 'readable': 7, 'bridge': 5 },
{ 'friend_id': 100, 'readable': 7, 'bridge': 6 },
{ 'friend_id': 100, 'readable': 7, 'bridge': 7 }
]
}
To just find documents that have a matching element you would do this:
$macher = array(
'$where' => new MongoCode("function(){
return this.friendsR.some(function(x) {
return ( x.readable & 2 ) > 0
})
}")
);
To actually return "which elements" matched you would again have to do something like this with mapReduce. It will "mark" which items in the array met the condition and only return documents where at least one array member that matches.
$map = new MongoCode("function(){
this.friendsR.forEach(function(friend) {
friend.macthed = ( ( friend.readable & 2 ) > 0 );
});
if ( this.friendsR.some(function(x) { return x.matched }) )
emit( this._id, this );
}");
$reduce = new MongoCode("function(){}");
$result = $db->command(array(
"mapreduce" => "collection_name",
"map" => $map,
"reduce" => $reduce,
"out" => array( "inline" => 1 )
));
There are no built in operators for bitwise evaluation so JavaScript is your only option where it is available.
Note that the mapReduce is a command and must be run from a Db object, the result is either returned inline as shown or output to another collection. This does not return a cursor. See the official documentation on mapReduce for more information.