title: Mongodb $match and $isArray
body: <p>For this ample of data I want to match only the field that are array. But I would like to have the <code>$match</code> out of the project. </p>
  
  <p>This is the data sample:</p>
  
  <pre><code>/* 1 */
  {
      "_id" : ObjectId("5df8b596400289966e77e268"),
      "fieldIDA" : "1496461ahu-45196tfvj",
      "fieldIDB" : "4446rarr21-vau15ar6a"
  }
  
  /* 2 */
  {
      "_id" : ObjectId("5df9ee6014585733585abde6"),
      "fieldIDA" : "78a24da5ea-a0a84tf5t",
      "fieldIDB" : [ 
          "17a7a61a78-4649atfvj", 
          "149648a6a1-777777fvj", 
          "4846131prv-45aeda8vj"
      ]
  }
  
  /* 3 */
  {
      "_id" : ObjectId("5df9ee6014585733585abde9"),
      "fieldIDA" : null,
      "fieldIDB" : "7a25rayfg7-t69468a8v"
  }
  
  /* 4 */
  {
      "_id" : ObjectId("5df9ffe714585733585ac5b8"),
      "fieldIDA" : [ 
          "4946465434-vuhfyfuog", 
          "198fdyufu5-4654d43dc", 
          "aeadad596a-aead82a2a"
      ],
      "fieldIDB" : [ 
          "17a7a61a78-4649atfvj", 
          "149648a6a1-777777fvj", 
          "4846131prv-45aeda8vj"
      ]
  }
  </code></pre>
  
  <p>This is what I try but it return an syntax error on the line of the <code>$match</code> honestly I tried every syntax I could think of or look at on the mongo doc : </p>
  
  <pre><code>&gt; db.getCollection('Test').aggregate([ {
  &gt;     $project:{
  &gt;     "item":"$fieldIDA" 
  &gt;     }}
  &gt;     {$match : { $isArray: {"$item"} }:true}, ])
  </code></pre>
  
  <p>I know it's a simple request but I don"t find how to make it work. </p>