title: Mongoose - Aggregation of two queries with condition
body: <p>I've two different collections that are connected by the id of the garden. I've a list of gardens and I've a list of allocations where it will be stored the start and the end date of the allocation. I can check if a garden is allocated by verifying if today is between both dates in the allocation table. </p>
  
  <p>Garden</p>
  
  <pre><code>{
      "_id": "5b98df3c9275f2291c0d7dc3",
      "id": "h1",
      "size": 43
  }
  </code></pre>
  
  <p>Allocation</p>
  
  <pre><code>{
      "_id": "5b9bcb8ecb9dee0015150549",
      "user": "5b9a2cd21eb58700141a3449",
      "garden": "5b98df5c9275f2291c0d7dc6",
      "start_date":"2018-09-14T00:00:00.000Z",
      "end_date": "2018-11-14T00:00:00.000Z"
  }
  </code></pre>
  
  <p>How can I return all the existing gardens with an aditional field 'ocupied' with true or false depending on if they exist on the allocation document between start_date and end_date?</p>
  
  <p>I'd like to get an array of gardens with the following data</p>
  
  <pre><code>{
      "_id": "5b98df3c9275f2291c0d7dc3",
      "id": "h1",
      "size": 43,
      "occupied": true
  }
  </code></pre>