Introduction to MongoDB cursor.objsLeftInBatch() Method

MongoDB is a popular document database with powerful querying capabilities. In scenarios where large amounts of data need to be queried, a cursor can be used to retrieve query results. The cursor.objsLeftInBatch() method in MongoDB is used to check if there are any unprocessed documents remaining in the query results.

Syntax

The syntax for the cursor.objsLeftInBatch() method is as follows:

db.collection.find().objsLeftInBatch()

Use Cases

When querying large amounts of data, MongoDB splits the query results into multiple batches for efficiency. The cursor.objsLeftInBatch() method can be used to check if there are any unprocessed documents in the current batch, allowing for reprocessing of the current batch or starting the next batch as needed.

Example

Suppose we have a collection called “products” with many documents, and we need to retrieve all of them. The following example code uses cursor.objsLeftInBatch() method to process the documents in batches:

var cursor = db.products.find()
var count = 0
while (cursor.hasNext()) {
  var doc = cursor.next()
  count++
  if (cursor.objsLeftInBatch() == 0) {
    print("Processed " + count + " documents in current batch")
    // Do something with the current batch
  }
}

In the example above, we first retrieve the query cursor for all documents using the db.products.find() method. We then iterate over all the documents, counting them. With each document, we check if there are any unprocessed documents remaining in the current batch. If there are none, we output the number of documents processed so far and perform the appropriate operations.

Here’s a more practical example. Suppose we need to copy all the documents from the “products” collection to a collection called “backup”. To avoid running out of memory, we’ll process a subset of the documents in each batch and write them to the “backup” collection after each batch is processed.

var cursor = db.products.find()
var count = 0
var batch = []
var batch_size = 1000
while (cursor.hasNext()) {
  var doc = cursor.next()
  count++
  batch.push(doc)
  if (cursor.objsLeftInBatch() == 0 || batch.length == batch_size) {
    db.backup.insertMany(batch)
    batch = []
    print("Processed " + count + " documents")
  }
}

In the example above, we set the batch size to 1000 documents. When the current batch has either reached the maximum batch size or there are no more unprocessed documents, we insert the current batch into the “backup” collection and output the number of documents processed so far.

Conclusion

The cursor.objsLeftInBatch() method is a useful tool for checking if there are any unprocessed documents in query results. By using this method, we can better control the processing flow of query results and more efficiently handle large amounts of data.