title: find() with mongoose on a dynamical path
body: <p>I would like to find every <code>group</code> documents about the <code>user_id</code> field in all my <code>members</code>array entries:</p>
  
  <pre><code>"group": {
    "name": 'group_1',
    "members": [
    {
      "user_id": ObjectId("51153212835b351826000001"),
      "right": 1,
    },{
      "user_id": ObjectId("51153212835b351826000002"),
      "right": 3,
    },
    ]
  }
  
  "group": {
    "name": 'group_2',
    "members": [
    {
      "user_id": ObjectId("51153212835b351826000001"),
      "right": 1,
    },{
      "user_id": ObjectId("51153212835b351826000002"),
      "right": 3,
    },
    ]
  }
  </code></pre>
  
  <p>I saw the operator <code>$in</code> but i am not sure i can use it in this case because i just have one 'user_id' and i need to use it to look for a "dynamical path" <code>group.members[x].user_id</code></p>
  
  <p>Is there a way to do this request dynamically ?</p>
  
  <p>Thanks !</p>