title: mongodb join multiple collections
body: <p>I'd like to "join" 3 Collections in MongoDB by using mongochef. The collections are "Order", "Employee" and "City". I tried to use temp collections, but it is not effective.
  Now I am using var = a for the first "join".</p>
  
  <p>If I 'd like to show "a", there are displayed 20 results only.
  Do you have an idea or another solution?</p>
  
  <pre><code>        var a = db.Order.aggregate([
      { 
  
  
        $lookup: 
        {
          from: "City",
          localField: "City Key",
          foreignField: "City Key",
          as: "lsg"
        }
      },
      {
          $unwind: "$lsg"
      },
      {
          $project: 
          {
              "_id":1,
              "Salesperson Key":1,
              "City": "$lsg.City"
          }
      }
  
      ])
  
      a;
  
  var b = db.Employee.aggregate([
  { 
  
  
    $lookup: 
    {
      from: "a",
      localField: "Employee Key",
      foreignField: "Salesperson Key",
      as: "lsg2"
    }
  },
  {
      $unwind: "$lsg2"
  },
  {
      $project: 
      {
          "_id":1,
          "Employee":1
      }
  }
  
  ])
  </code></pre>
  
  <p>Thanks in advance for replies.</p>