MongoDB：检索集合中的第一个文档

Question

I'm new to Mongo, and I'm trying to retrieve the first document from a find() query:

> db.scores.save({a: 99});
> var collection = db.scores.find();
[ 
  {   "a" : 99,   "_id" : {   "$oid" : "51a91ff3cc93742c1607ce28"   }   }
]
> var document = collection[0];
JS Error: result is undefined

This is a little weird, since a collection looks a lot like an array. I'm aware of retrieving a single document using findOne() , but is it possible to pull one out of a collection?

Answer 1

The find method returns a cursor. This works like an iterator in the result set. If you have too many results and try to display them all in the screen, the shell will display only the first 20 and the cursor will now point to the 20th result of the result set. If you type it the next 20 results will be displayed and so on.

In your example I think that you have hidden from us one line in the shell.

This command

> var collection = db.scores.find();

will just assign the result to the collection variable and will not print anything in the screen. So, that makes me believe that you have also run:

> collection

Now, what is really happening. If you indeed have used the above command to display the content of the collection , then the cursor will have reached the end of the result set (since you have only one document in your collection) and it will automatically close. That's why you get back the error.

There is nothing wrong with your syntax. You can use it any time you want. Just make sure that your cursor is still open and has results. You can use the collection.hasNext() method for that.

Answer 2

Is that the Mongo shell? What version? When I try the commands you type, I don't get any extra output:

MongoDB shell version: 2.4.3
connecting to: test
> db.scores.save({a: 99});
> var collection = db.scores.find();
> var document = collection[0];

In the Mongo shell, find() returns a cursor, not an array. In the docs you can see the methods you can call on a cursor .

findOne() returns a single document and should work for what you're trying to accomplish.

Answer 3

So you can have several options.

Using Java as the language, but one option is to get a db cursor and iterate over the elements that are returned. Or just simply grab the first one and run.

DBCursor cursor = db.getCollection(COLLECTION_NAME).find();
List<DOCUMENT_TYPE> retVal = new ArrayList<DOCUMENT_TYPE>(cursor.count());
while (cursor.hasNext()) {
    retVal.add(cursor.next());
}
return retVal;

If you're looking for a particular object within the document, you can write a query and search all the documents for it. You can use the findOne method or simply find and get a list of objects matching your query. See below:

DBObject query = new BasicDBObject();
query.put(SOME_ID, ID);
DBObject result = db.getCollection(COLLECTION_NAME).findOne(query) // for a single object
DBCursor cursor = db.getCollection(COLLECTION_NAME).find(query) // for a cursor of multiple objects