retrieve array from mongodb using java with mongodb api

Question

I understand that there are many questions which as for the same and they are answered well. The problem is all those questions use MongoDBObject, MongoDBList to retrieve arrays. My problem is I am using http://api.mongodb.org/java/3.0/index.html?overview-summary.html api and I am having hard time retrieving array and adding elements to it. I have to use MongoCollection, MongoDatabase and MongoClient. I am trying to solve an assignment from mongodb course. The problem statement is to find an array and update it in mongod.

Here is what I have tried

      Document post = null; Bson filter = new Document("permalink",
      permalink); Bson projection = new Document("comments", true);
      List<Document> comments = postsCollection.find(filter)
      .projection(projection).into(new ArrayList<Document>());
      System.out.println(comments);

      post = postsCollection.find(Filters.eq("permalink",
      permalink)).first();

      Document newComment = new Document();

      newComment.append("author", name); newComment.append("body", body);
      if (email != null && (!email.equals(""))) {
      newComment.append("email", email); }

      comments.add(newComment);

      Bson filter2 = new Document("_id", post.get("_id"));
      System.out.println(comments); post =
      postsCollection.find(filter).first();

      postsCollection.updateOne(filter2, new Document("$unset",new
      Document("comments",true))); postsCollection.updateOne(filter2, new
      Document("$set", new Document( "comments", comments)));

This does not create a new comment. Instead, it creates another comments array in comments array itself. THe array should be updated in student

Here is the json data

{
"_id" : ObjectId("55d965eee60dd20c14e8573e"),
"title" : "test title",
"author" : "prasad",
"body" : "test body",
"permalink" : "test_title",
"tags" : [ 
    "test", 
    "teat"
],
"date" : ISODate("2015-08-23T06:19:26.826Z"),
"comments" : [ 
    {
        "_id" : ObjectId("55d965eee60dd20c14e8573e"),
        "comments" : [ 
            {
                "_id" : ObjectId("55d965eee60dd20c14e8573e"),
                "comments" : []
            }, 
            {
                "author" : "commented",
                "body" : "something in comment",
                "email" : "some@thing.com"
            }
        ]
    }, 
    {
        "author" : "commented",
        "body" : "something in comment",
        "email" : "some@thing.com"
    }
]

}

Answer 1

You need not write to this much code. Please check following code,

 public void addPostComment(final String name, final String email, final String body,
                           final String permalink) {
   Document post = findByPermalink(permalink);
   List<Document> comments = null;
   Document comment = new Document();
   if(post != null){
        comments = (List<Document>)post.get("comments");
        comment.append("author",name).append("body", body);

        if(email != null){
            comment.append("email", email);
        }
        comments.add(comment);
        postsCollection.updateOne(new Document("permalink",permalink), 
                                new Document("$set",new Document("comments",comments)));

        }
}