title: TypeError: ObjectId('') is not JSON serializable
body: <p>My response back from MongoDB after querying an aggregated function on document using Python, It returns valid response and i can print it but can not return it.</p>
  
  <p>Error: </p>
  
  <pre><code>TypeError: ObjectId('51948e86c25f4b1d1c0d303c') is not JSON serializable
  </code></pre>
  
  <p>Print:</p>
  
  <pre><code>{'result': [{'_id': ObjectId('51948e86c25f4b1d1c0d303c'), 'api_calls_with_key': 4, 'api_calls_per_day': 0.375, 'api_calls_total': 6, 'api_calls_without_key': 2}], 'ok': 1.0}
  </code></pre>
  
  <p>But When i try to return:</p>
  
  <pre><code>TypeError: ObjectId('51948e86c25f4b1d1c0d303c') is not JSON serializable
  </code></pre>
  
  <p>It is RESTfull call:</p>
  
  <pre><code>@appv1.route('/v1/analytics')
  def get_api_analytics():
      # get handle to collections in MongoDB
      statistics = sldb.statistics
  
      objectid = ObjectId("51948e86c25f4b1d1c0d303c")
  
      analytics = statistics.aggregate([
      {'$match': {'owner': objectid}},
      {'$project': {'owner': "$owner",
      'api_calls_with_key': {'$cond': [{'$eq': ["$apikey", None]}, 0, 1]},
      'api_calls_without_key': {'$cond': [{'$ne': ["$apikey", None]}, 0, 1]}
      }},
      {'$group': {'_id': "$owner",
      'api_calls_with_key': {'$sum': "$api_calls_with_key"},
      'api_calls_without_key': {'$sum': "$api_calls_without_key"}
      }},
      {'$project': {'api_calls_with_key': "$api_calls_with_key",
      'api_calls_without_key': "$api_calls_without_key",
      'api_calls_total': {'$add': ["$api_calls_with_key", "$api_calls_without_key"]},
      'api_calls_per_day': {'$divide': [{'$add': ["$api_calls_with_key", "$api_calls_without_key"]}, {'$dayOfMonth': datetime.now()}]},
      }}
      ])
  
  
      print(analytics)
  
      return analytics
  </code></pre>
  
  <p>db is well connected and collection is there too and I got back valid expected result but when i try to return it gives me Json error. Any idea how to convert the response back into JSON. Thanks</p>