File write operations in mongo script?

Question

You could use print and then redirect output:

script.js:

cursor = db.users.find();
while(cursor.hasNext()){
    printjson(cursor.next());
}

then run the script and redirect output to a file:

mongo --quiet script.js > result.txt

http://www.mongodb.org/display/DOCS/Scripting+the+shell paragraph "Differences between scripted and interactive/ Printing".

./mongo server.com/mydb --quiet --eval "db.users.find().forEach(printjson);" > 1.txt

Whenever I need to write the result of a mongo query to a local file I generally use the the writeFile(pathToFile, stringContents) function.

Example: let's say that you quickly need to find the email of all registered users and send it to your buddy Jim in the marketing department.

$ mongo mongodb://my-fancy-mongo-server --ssl -u fancy_username -p fancy_password 
successfully connected to my-fancy-mongo-server!
> emails = db.users.distinct('email_address')
> writeFile("jims_email_list.json", tojson(emails))

or if Jim expect's a CSV file then

$ mongo mongodb://my-fancy-mongo-server --ssl -u fancy_username -p fancy_password 
successfully connected to my-fancy-mongo-server!
> emails = db.users.distinct('email_address')
> writeFile("jims_email_list.csv", emails.join("\n"))

You can now send Jim the list of emails and save the day!

To important things to notice about the writeFile function:

1. The second argument must be a string.
2. The first argument must be a file that doesn't already exist, otherwise you will get an error.

You can skip the while loop using forEach():

db.users.find().forEach(printjson);

Wouldn't it be simpler to use one of the Mongo drivers for a general purpose language (such as Python, Ruby, Java, etc) and write your results to a file that way, in a format you can use (such as CSV, etc.)?

UPDATE: According to the documentation for mongodump you can export a collection with a query:

$ ./mongodump --db blog --collection posts
-q '{"created_at" : { "$gte" : {"$date" : 1293868800000},
                      "$lt"  : {"$date" : 1296460800000}
                    }
    }'

However you would need to import that collection back into MongoDB to operate on it or use mongoexport to export as JSON or CSV using the same query flag (-q) as mongodump.