title: Querying two MongoDB collections with PHP, first returns correct result, but second is empty
body: <p>I run two MongoDB queries from PHP, one on a first collection (named "products"), second on another collection (named stats).</p>
  
  <p>Some datas of the two collections :</p>
  
  <pre><code>{
  "_id" : "20711122",
  "labels_hierarchy" : [ 
      "en:pgi"
  ],
  "languages_hierarchy" : [ 
      "en:french"
  ],
  "labels_prev_hierarchy" : [ 
      "en:pgi"
  ],
  "languages" : {
      "en:french" : 5
  },
  "countries_tags" : [ 
      "en:france"
  ],
  "purchase_places_debug_tags" : [],
  "photographers_tags" : [ 
      "tacite"
  ]
  }
  </code></pre>
  
  <p>Other collection :</p>
  
  <pre><code>{
  "_id" : "7613035010550",
  "purchases" : [ 
      {
          "date" : ISODate("2017-04-15T14:15:00.000Z"),
          "coords" : {
              "lon" : 43.729604,
              "lat" : 1.416017
          },
          "metar" : {},
          "quantity" : 1,
          "price" : 2.31
      }, 
      {
          "date" : ISODate("2017-05-02T16:23:00.000Z"),
          "coords" : {
              "lon" : 43.722862,
              "lat" : 1.415837
          },
          "metar" : {},
          "quantity" : 6,
          "price" : 12
      }, 
      {
          "date" : ISODate("2017-05-02T18:32:00.000Z"),
          "coords" : {
              "lon" : 46.307353,
              "lat" : 3.28937
          },
          "metar" : {},
          "quantity" : 2,
          "price" : 5
      }
  ],
  "rates" : [ 
      {
          "value" : 5
      }, 
      {
          "value" : 4
      }, 
      {
          "value" : 5
      }, 
      {
          "value" : 2
      }
  ]
  }
  </code></pre>
  
  <p>As u see, collections are different but the "_id" key...</p>
  
  <p>When i run a query on the first collection (products), everything is ok, but the second with the same "_key" returns empty results, here's the trace of the two queries (made with executeQuery() of MongoDB::Driver) :</p>
  
  <pre><code>&lt;pre&gt;&lt;code&gt;Interrogation de la collection : stats
  Avec le filtre :
  {"id":"7613035010550"}
  &lt;pre&gt;&lt;code&gt;Interrogation de la collection : products
  Avec le filtre :
  {"id":"7613035010550"}
  Results :
  {"ean":"7613035010550","title":"Eau Min\u00e9rale Naturelle","image":"https:\/\/static.openfoodfacts.org\/images\/products\/761\/303\/501\/0550\/front_fr.11.400.jpg","brands":"Vittel","categories":["Boissons","Eaux","Eaux min\u00e9rales","Eaux min\u00e9rales naturelles","Boissons non sucr\u00e9es"],"quantity":"1.5 l."}
  </code></pre>
  
  <p>As u can see, only the first query returns results, not the second...</p>
  
  <p>Any idea of this curious behaviour ?</p>
  
  <p>Thx 4 help,</p>
  
  <p>JL</p>