title: compare two collection in mongodb
body: <p>I have two different collection book and music in JSON .First I give a book collection example:</p>
  
  <pre><code>{
      "_id" : ObjectId("b1"),
      "author" : [
          "Mary",
      ],
      "title" : "Book1",
  }
  {
          "_id" : ObjectId("b2"),
          "author" : [
              "Joe",
              "Tony",
              "Mary"
          ],
          "title" : "Book2",
  }
  {
              "_id" : ObjectId("b3"),
              "author" : [
                  "Joe",
                  "Mary"
              ],
              "title" : "Book3",
  }
  .......
  </code></pre>
  
  <p>Mary writes 3 books, Joe write 2 books, Tony writes 1 book. Second I give a music collection example:</p>
  
  <pre><code> {
          "_id" : ObjectId("m1"),
          "author" : [
              "Tony"
          ],
          "title" : "Music1",
      }
      {
              "_id" : ObjectId("m2"),
              "author" : [
                  "Joe",
                  "Tony"
              ],
              "title" : "Music2",
      }
      .......
  </code></pre>
  
  <p>Tony has 2 musics, Joe has 1 music, Mary has 0 music.</p>
  
  <p>I hope to get the number of authors who write more books than music.</p>
  
  <p>Thus, Mary(3 > 0) and Joe(2 > 1) should take into consideration, but not Tony(1 &lt; 2). Thus the final result should be 2(Mary and Joe).</p>
  
  <p>I write down following code, but don't know how to compare:</p>
  
  <pre><code>db.book.aggregate([ 
       { $project:{ _id:0, author:1}},
       { $unwind:"$author" },     
       {$group:{_id:"$author", count:{$sum:1}}}  
       ]
       )
  
  db.music.aggregate([ 
       { $project:{ _id:0, author:1}},
       { $unwind:"$author" },     
       {$group:{_id:"$author", count:{$sum:1}}}  
       ]
       )
  </code></pre>
  
  <p>Is it so far right? How to do the following comparison? Thanks.</p>