mongo db find function in php

Question

I am building a social network in mongo db. Following is the structure of document in the groups collection. It represents each social group present in the system. Users are a part of this group which is represented by the 'members' property which is an array if json objects.

{
  "group_name":"my group",
  "members": [
    {
      "user_id": ObjectId("4e29341986ba75dc14000000"),
      "joined": ISODate("2011-07-26T11:46:49.0Z")
    },
    {
      "user_id": ObjectId("4e2ea94286ba75f81500000e"),
      "joined": ISODate("2011-07-26T11:47:55.0Z")
    },
    {
      "user_id": ObjectId("4e2eaa0786ba75e815000003"),
      "joined": ISODate("2011-07-26T11:55:22.0Z")
    },
    {
      "user_id": ObjectId("4e2eab7f86ba75ec1500000a"),
      "joined": ISODate("2011-07-26T11:57:44.0Z")
    },
    {
      "user_id": ObjectId("4e2eac3586ba75dc15000000"),
      "joined": ISODate("2011-07-26T12:00:57.0Z")
    },
    {
      "user_id": ObjectId("4e2eacae86ba75dc15000004"),
      "joined": ISODate("2011-07-26T12:02:43.0Z")
    },
    {
      "user_id": ObjectId("4e2eadbb86ba75ec1500000c"),
      "joined": ISODate("2011-07-26T12:07:01.0Z")
    }
  ]

The problem I am facing is how do I check if a user belongs to a certain group. This is what I came up with but it doesn't seem to work. Please help.

    $criteria = array(
        '_id' => new MongoId($group_id),
        'members' => array('user_id' => new MongoId($user_id))
    );


   return $collection->find($criteria);

laurac · Accepted Answer · 2011-07-27 14:51:46Z

4

try with the following code:

$criteria = array(
        '_id' => new MongoId($group_id),
        'members.user_id' => new MongoId($user_id))
    );


return $collection->find($criteria);

see http://www.mongodb.org/display/DOCS/Dot+Notation+%28Reaching+into+Objects%29

answered Jul 27, 2011 at 14:51

laurac

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netcoder · Accepted Answer · 2011-07-27 14:36:52Z

Use the $elemMatch operator.

In Mongo shell, you'd do:

> criteria = { 
...   "_id" : ObjectId("group_id"), 
...   "members" : {
...      $elemMatch : {
...         "user_id" : ObjectId("user_id")
...      }
...   }
...}
> db.groups.find(criteria);

It will search the groups collection for an entry for which a members object has a user_id of ObjectId("user_id").

Haven't tested it in PHP, but since the structure is the same, this should do it:

$criteria = array(
    '_id' => new MongoId($group_id),
    'members' => array(
       '$elemMatch' => array(
           'user_id' => new MongoId($user_id)
       )
    )
);

return $collection->find($criteria);

Tip: Remove the _id clause completely and you can find out to which groups the user belong (instead of querying each one separately).

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