Mongodb：无法保存添加到BasicDBList中的新元素

Question

I am trying to add an element to a BasicDBList array and save it back to Mongodb, but when I check the result, it is not written. What did I do wrong? I use java driver version 2.7.2.

DBObject dbObject = coll.findOne(dbQuery);

BasicDBList unreadMsgs = (BasicDBList) dbObject.get("unreadMsgs");

Logger.debug("before incrementing unreadMsgs" + dbObject.toString());

unreadMsgs.add(new BasicDBObject("id", 1).append("unreadMsg", 1));
Logger.debug("after incrementing unreadMsgs : " + dbObject.toString());
coll.save(dbObject);
Logger.debug("check result: " + coll.findOne(dbQuery).toString());

before incrementing unreadMsgs{ "_id" : { "$oid" : "515c5eb88e3278e9c9d55867"} , "unreadMsgs" : [ ]}

after incrementing unreadMsgs : { "_id" : { "$oid" : "515c5eb88e3278e9c9d55867"} , "unreadMsgs" : [ { "id" : 1 , "unreadMsg" : 1}]}

check result: { "_id" : { "$oid" : "515c5eb88e3278e9c9d55867"} , "unreadMsgs" : [ ]}

Answer 1

The problem is that the coll.save(dbObject) is not updating anything.

It works as an insert and, since the _id already exists in the collection, you are getting a duplicateKey exception (you are just not seeing it because of configuration).

You have to use an update, here is how

Answer 2

The save call should work on that case, but I suggest you use an update with $addToSet operation.

Here's the code:

DBObject addToSetObj = BasicDBObjectBuilder.start()
    .push("$addToSet")
        .push("unreadMsgs")
            .add("id", 1)
            .add("unreadMsg", 1)
        .pop()
    .pop()
.get();

// addToSetObj will be { "$addToSet" : { "unreadMsgs" : { "id" : 1 , "unreadMsg" : 1}}}


coll.update(dbQuery, addToSetObj);
Logger.debug("check result: " + coll.findOne(dbQuery).toString());

Any doubts on how to use addToSet, check this out: http://docs.mongodb.org/manual/reference/operator/addToSet/

Answer 3

Thanks for everybody's answer. I found the real problem. It turns out that my collection is capped, and I am not allowed to insert more data into an existing document in a capped collection. I saw the exception after I changed WriteConcern to FSYNC_SAFE. I changed all my collections to uncapped, and the code works now.