mongodb join multiple collections
<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>