Mongo Query to CSV Download (Express JS)

The Problem

I recently worked on an app which uses MongoDB and Express JS. The app had to allow for the export of a large Mongo query result (50K+ of documents). In this post, we’ll see how to do it succinctly and “in-flight” with two popular CSV libraries.

Query Cursor

To keep things simple, I used the Standard MongoDB driver. My ultimate goal was to stream documents from a query to a CSV formatter, which can then stream the output to the HTTP response. As a first step we need to acquire a dabatase cursor. Assuming the Mongo DB driver instance is called mongodb our code should be similar to this:

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// Your collection name
const collectionName = ...

// Your Mongo query object - e.g. {name: "john"}
const query = ... 

// Your Mongo query selection object - e.g. {name: 1, _id: 0}
// Select as few fields as possible!
const selection = ...

// Every query (e.g. find/aggregate) returns a cursor
const cursor = mongodb.collection(collectionName).find(query, selection)

Cursors can be pipe-d into other streams and are handy for “in-flight” processing.

Transformation Function

We’ll also need a function which transforms the JSON documents, returned by the cursor, into objects with a suitable schema/object model. The function should look like this:

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transform(doc) {
    // Return an object with all fields you need in the CSV
    // For example ...
    return {
        Address: doc.address,
        State: doc.state.abbreviation
    };
}

Using Fast-CSV

Let’s see how to transform the cursor’s data with the fast-csv library.

First we obtain the cursor and the transformation function. Then we need to set the appropriate download headers, which also include the name of the downloaded file, and send/flush them to the response. Next, we create a CSV transformer sream, and use it to “pipe” the cursor’s documents the response. The following snippet demonstrates the idea:

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const fastCsv = require('fast-csv');

downloadCSV(request, response) {
    // Obtain your cursor ...
    const cursor = ...
    // The transformer function
    const transformer = ...;

    // Name of the downloaded file - e.g. "Download.csv"
    const filename = ...;

    // Set approrpiate download headers
    response.setHeader('Content-disposition', `attachment; filename=${filename}`);
    response.writeHead(200, { 'Content-Type': 'text/csv' });

    // Flush the headers before we start pushing the CSV content
    response.flushHeaders();

    // Create a Fast CSV stream which transforms documents to objects
    var csvStream = fastCsv
        .createWriteStream({headers: true})
        .transform(transformer)

    // Pipe/stream the query result to the response via the CSV transformer stream 
    cursor.stream().pipe(csvStream).pipe(response);
}

Using Node-CSV

The fast-csv library worked pretty well, but I was also willing to try the node-csv library. I was hoping to get better performance without much extra efforts. Its interface is similar, so without much change the code looked like this:

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var csv = require('csv');

download(request, response) {
    // Obtain your cursor ...
    const cursor = ...
    // The transformer function
    const transformer = ...;

    // Name of the downloaded file - e.g. "Download.csv"
    const filename = ...;

    // Set approrpiate download headers
    response.setHeader('Content-disposition', `attachment; filename=${filename}`);
    response.writeHead(200, { 'Content-Type': 'text/csv' });

    // Flush the headers before we start pushing the CSV content
    response.flushHeaders();

    // Pipe/stream the query result to the response via the CSV transformer stream 
    cursor.stream()
        .pipe(csv.transform(transformer))
        .pipe(csv.stringify({header: true}))
        .pipe(response)
}

However, the performance of the two libraries turned out to be very similar.

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