title: Populate on basis of condition in mongoose, mongoDB
body: <p>Here is my code to get one floorplan and populate all flats linked with this</p>
  
  <p>see the code below :</p>
  
  <pre><code>var floorplan = Floorplan.find({ 
      project: req.params.project, 
      tower: req.params.tower, 
      isDeleted: false 
  });
  floorplan.populate('flats').exec(function(err, floorplan) {
      if (err) { return res.send(err); }
      if (!floorplan) { return res.status(401).json(); }
      res.status(200).json(floorplan);
  });
  </code></pre>
  
  <p>But I want to populate only those flats where isDeleted : false 
  How to achive this ??</p>
  
  <p>Schema of floorplan</p>
  
  <pre><code>var FloorplanSchema = new Schema({
      project: { type: Schema.ObjectId, ref: "Project" },
      flats: [{ type: Schema.ObjectId, ref: "Flat" }],
      tower: [{ type: Schema.ObjectId, ref: "Tower" }],
      unitType: String,
      area: Number,
      floorPlan2D: String,
      floorPlan3D: String,
      livingRoomArea: Number,
      kitchenArea: Number,
      balconies: Number,
      bathRooms: Number,
      isDeleted: { type: Boolean, 'default': false },
      createdAt: { type: Date, 'default': Date.now }
  });
  </code></pre>
  
  <p>Schema of flat</p>
  
  <pre><code>var FlatSchema = new Schema({
      tower: { type: Schema.ObjectId, ref: "Tower" },
      floorplan: { type: Schema.ObjectId, ref: "Floorplan" },
      project: { type: Schema.ObjectId, ref: "Project" },
      status: String,
      floor: Number,
      size: String,
  
      superbuiltup_area: Number,
  
      directionFacing: String,
      furnishingState: String,
      flooringType: String,
      createdAt: { type: Date, 'default': Date.now },
      isDeleted: { type: Boolean, 'default': false },
  
  });
  </code></pre>