title: $exists is not working with $match stage in MongoDb aggregation
body: <p>I am working on Mongodb v3.6. I am trying to run a query regarding customer reviews. In this query I need only that documents which have some value of "review" field. For this I have executed following query:</p>
  
  <pre><code>db.customer_reviews.aggregate([
  {$match: {rating: { $gte: 3}, review: {$ne: "", $exists: true}}},
  {$lookup: {
  "localField": "uid",
  "from": "users",
  "foreignField": "_id",
  "as": "review_fields"
  }},
  {"$unwind": "$review_fields"},
  ]).pretty()
  </code></pre>
  
  <p>I have some documents which do not contain review field or having value as empty string, like:</p>
  
  <pre><code>{
          "_id" : 1,
          "uid" : 8,
          "booking_id" : 1,
          "rating" : 4.5,
          "review" : "This is very fast.",
          "rated_by" : 1,
          "dont_send_notification" : false
  }
  {
          "_id" : 2,
          "uid" : 8,
          "booking_id" : 1,
          "rating" : 5,
          "review" : "",
          "rated_by" : 1,
          "dont_send_notification" : false
  }
  {
          "_id" : 3,
          "uid" : 8,
          "booking_id" : 17,
          "rating" : 5,
          "review" : "This team work's very fast and well. And I really appriciated there work and hard work. I really like this software.",
          "rated_by" : 1,
          "dont_send_notification" : false
  }
  {
          "_id" : 7,
          "uid" : 8,
          "booking_id" : 21,
          "rating" : 5,
          "rated_by" : 1,
          "dont_send_notification" : false
  }
  {
          "_id" : 8,
          "uid" : 8,
          "booking_id" : 21,
          "rating" : 5,
          "rated_by" : 1,
          "dont_send_notification" : false
  }
  </code></pre>
  
  <p>But in this query, I am still getting documents that either do not contain review field or having blank value for it.
  I have gone through the documentation of $exists operator.</p>
  
  <p><a href="https://docs.mongodb.com/v3.6/reference/operator/query/exists/" rel="nofollow noreferrer">https://docs.mongodb.com/v3.6/reference/operator/query/exists/</a></p>
  
  <p>I have also tried to run the query without $ne operator which results the same as before. Please guide what am I doing wrong or missing something ?</p>
  
  <p>Thanks!</p>