Pymongo: iterate over all documents in the collection

Question

I am using PyMongo and trying to iterate over (10 millions) documents in my MongoDB collection and just extract a couple of keys: "name" and "address", then output them to .csv file.

I cannot figure out the right syntax to do it with find().forEach()

I was trying workarounds like

cursor = db.myCollection.find({"name": {$regex: REGEX}})

where REGEX would match everything - and it resulted in "Killed". I also tried

cursor = db.myCollection.find({"name": {"$exist": True}})

but that did not work either.

Any suggestions?

Answer 1

I cannot figure out the right syntax to do it with find().forEach()

cursor.forEach() is not available for Python, it's a JavaScript function. You would have to get a cursor and iterate over it. See PyMongo Tutorial: querying for more than one document, where you can do :

for document in myCollection.find():
    print(document) # iterate the cursor

where REGEX would match everything - and it resulted in "Killed".

Unfortunately there's lack of information here to debug on why and what 'Killed' is. Although if you would like to match everything, you can just state:

cursor = db.myCollection.find({"name": {$regex: /.*/}}) 

Given that field name contains string values. Although using $exists to check whether field name exists would be preferable than using regex.

While the use of $exists operator in your example above is incorrect. You're missing an s in $exists. Again, unfortunately we don't know much information on what 'didn't work' meant to help debug further.

If you're writing this script for Python exercise, I would recommend to review:

• PyMongo Tutorial
• MongoDB Tutorial: query documents

You could also enrol in a free online course at MongoDB University for M220P: MongoDB for Python Developers.

However, if you are just trying to accomplish your task of exporting CSV from a collection. As an alternative you could just use MongoDB's mongoexport. Which has the support for :

• Exporting specific fields via --fields "name,address"
• Exporting in CSV via --type "csv"
• Exporting specific values with query via --query "..."

See mongoexport usage for more information.

Answer 2

I had no luck with .find().forEach() either, but this should find what you are searching for and then print it.

First find all documents that match what you are searching for

cursors = db.myCollection.find({"name": {$regex: REGEX}})

then iterate it over the matches

for cursor in cursors
    print(cursor.get("name"))