title: Using $match on computed field Mongodb
body: <p>I have this aggregation query in MongoDB:</p>
  
  <pre><code>db.questions.aggregate([
  { $project:{question:1,detail:1, choices:1, answer:1,
    percent_false:{
    $multiply:[100,{$divide:["$answear_false",{$add:["$answear_false","$answear_true"]}]}]},
    percent_true:{
    $multiply:[100,{$divide:["$answear_true",{$add:["$answear_false","$answear_true"]}]}]}    }}, {$match:{status:'active'} } 
  ]).pretty()
  </code></pre>
  
  <p>I want using $match on 2 computed fields "percent_true" and "percent_false" like this </p>
  
  <pre><code>$match : {percent_true:{$gte:20}}
  </code></pre>
  
  <p>How can i do ?</p>