title: Moongoose aggregate $match does not match id's
body: <p>I want to show products by ids (<code>56e641d4864e5b780bb992c6</code> and <code>56e65504a323ee0812e511f2</code>) and show price after subtracted by discount if available.</p>
  
  <p>I can count the final price using aggregate, but this return all document in a collection, how to make it return only the matches ids</p>
  
  <pre><code>"_id" : ObjectId("56e641d4864e5b780bb992c6"), 
  "title" : "Keyboard", 
  "discount" : NumberInt(10),
  "price" : NumberInt(1000)
  
  "_id" : ObjectId("56e65504a323ee0812e511f2"), 
  "title" : "Mouse", 
  "discount" : NumberInt(0),
  "price" : NumberInt(1000)
  
  "_id" : ObjectId("56d90714a48d2eb40cc601a5"), 
  "title" : "Speaker", 
  "discount" : NumberInt(10),
  "price" : NumberInt(1000)
  </code></pre>
  
  <p>this is my query</p>
  
  <pre><code>productModel.aggregate([
          {
              $project: {
                  title   : 1,
                  price: {
                      $cond: {
                          if: {$gt: ["$discount", 0]}, then: {$subtract: ["$price", {$divide: [{$multiply: ["$price", "$discount"]}, 100]}]}, else: "$price"
                      }
  
                  }
              }
          }
      ], function(err, docs){
          if (err){
              console.log(err)
          }else{
              console.log(docs)
          }
      })
  </code></pre>
  
  <p>and if i add this <code>$in</code> query, it returns empty array </p>
  
  <pre><code>productModel.aggregate([
              {
                  $match: {_id: {$in: ids}}
              },
              {
                  $project: {
                      title   : 1,
                      price: {
                          $cond: {
                              if: {$gt: ["$discount", 0]}, then: {$subtract: ["$price", {$divide: [{$multiply: ["$price", "$discount"]}, 100]}]}, else: "$price"
                      }
  
                  }
              }
          }
      ], function(err, docs){
          if (err){
              console.log(err)
          }else{
              console.log(docs)
          }
      })
  </code></pre>