title: Unknown operator: $and in MongoDB
body: <p>I am querying mongodb through node.js code.
  My mongo documents collection (Patients Collection) has the following structure:+</p>
  
  <pre><code>Patient collection
  
  { 
      "_id" : ObjectId("59e5c28f37ce021e142e7ead"), 
      "MRN" : "00126389", 
      "Family_Name" : "Jones", 
      "First_Name" : "Lydia", 
      "Father_Name" : "Bob", 
      "Maiden_Name" : "", 
      "Mother_Name" : "n/a", 
      "Spouse_Name" : "", 
      "Address" : "", 
      "Telephone_Nbr" : "", 
      "Patient_Visit" : {
          "Department" : "ER", 
          "Hospital_Status" : "Active", 
          "Case_Nbr" : "17", 
          "Admission_Date" : "01/04/2011 12:00:00 AM", 
          "Admission_Time" : "14:02"
  
      }
  }
  </code></pre>
  
  <p>My query execution code is presented below:</p>
  
  <pre><code>mongoClient.connect(mongoConstr, function(err, db) {
      if (err) throw err;
      var query = {
          $and: [{
              "Patient_Visit.Department": "ER"
          }, {
              $or: [{
                  "Patient_Visit.Hospital_Status": "Active Left"
              }, {
                  "Patient_Visit.Hospital_Status": "Active"
              }]
          }]
      };
      var cursor = db.collection("tbl_Patients").find({
          query
      });
      cursor.forEach(function(doc) {
          console.log(JSON.stringify(doc));
      }, function(err) {
          db.close();
          throw (err);
      });
  });
  </code></pre>
  
  <p>When the request is executed I get the following error: </p>
  
  <pre><code>MongoError: unknown operator: $and
  </code></pre>
  
  <p>Any help would be appreciated.</p>