Let's be frank: backwards-breaking API changes are painful. Especially when there's no good way for the library to tell you when you're using the API incorrectly. This was my dilemma in upgrading a codebase from MongoDB Node driver 2.1.x to 3.x. In particular, dropping support for the fields parameter was brutal because this syntax was used everywhere in the code.

// In MongoDB Driver 2.x, `{ key: 1 }` would mean key2 was excluded from the
// results. Not in MongoDB Driver 3.x.
const doc = await db.collection('Test').findOne({}, { key1: 1 });

// Same for `find()`. MongoDB driver 3.x prints:
// `[ { _id: 5b9ae07d9b6dcf3725f3d74d, key1: 'foo', key2: 'bar' } ]`
// But 2.x prints:
// `[ { _id: 5b9ae07d9b6dcf3725f3d74d, key1: 'foo' } ]`
const docs = await db.collection('Test').find({}, { key1: 1 }).toArray();

I'm sure there are a lot of ways to migrate this code, but since we already had a robust test suite and used monogram, I used Monogram's middleware to throw an error every time find() and findOne() were used incorrectly.

An Overview of Middleware in Monogram

The key idea behind everything in Monogram is to create an object representation of a function call. Whenever you call any collection function in Monogram, like find() or findOne(), Monogram creates an object called an action that contains the most pertinent information about that function call:

  • The function's name
  • The name of the collection it is operating on
  • What parameters were passed in
  • The original stack trace

Monogram actions are a similar concept to Redux actions. Monogram then passes the action through a list of user-defined middleware functions. For example, below is a middleware function that prints out the function your code is calling and its parameters.

const { connect } = require('monogram');
const { inspect } = require('util');

run().catch(error => console.error(error.stack));

async function run() {
  const db = await connect('mongodb://localhost:27017/test', {
    useNewUrlParser: true

  await db.dropDatabase();

  // Monogram middleware takes in a single parameter, the action, an
  // object representation of the function being called.
  db.collection('Test').pre(action => {
    const paramsAsString = action.params.map(inspect).join(', ');
    // Prints:
    // "insertOne({ key1: 'foo', key2: 'bar' })"
    // "findOne({}, { key1: 1 })"

  await db.collection('Test').insertOne({ key1: 'foo', key2: 'bar' });

  const doc = await db.collection('Test').findOne({}, { key1: 1 });

Now that you can define middleware that can see all function calls on a given collection, let's take a look at how to actually solve the original problem of catching code that's using the obsolete find() syntax.

Middleware to Detect Bad Function Calls

Among other things, Monogram middleware lets you throw errors when your function call is using an outdated API. The general idea is to check if the 2nd argument to find() and findOne() contains a property that isn't a valid option for findOne(). In MongoDB Node driver 3.x, the 2nd parameter to findOne() is always treated as an options object. Below is the middleware I used to throw an error if the outdated API is used.

db.collection('Test').pre(/^(find|findOne)$/, action => {
  const opts = action.params[1];
  const allowedOptions = ['projection', 'sort', 'skip', 'limit', 'hint'];
  if (opts != null &&
      Object.keys(opts).find(option => !allowedOptions.includes(option))) {
    throw new Error('MongoDB driver 3.x does not allow passing projection ' +
      'as 2nd arg to find(). Use `projection` instead. Got ' +

Monogram bubbles this error up with a neat stack trace. Even if this middleware was async, Monogram would still retain the original stack trace from the initial function call.

$ node test.js
Error: MongoDB driver 3.x does not allow passing projection as 2nd arg to find(). Use `projection` instead. Got { key1: 1 }
    at Object.db.collection.pre.action [as fn] (/home/project/test.js:16:13)
    at /home/project/node_modules/monogram/lib/collection.js:109:24
    at Generator.next (<anonymous>)
    at onFulfilled (/home/project/node_modules/co/index.js:65:19)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)

Moving On

Monogram provides a neat lightweight middleware API on top of the MongoDB Node.js Driver for teams that find Mongoose too heavyweight. If you're using the MongoDB Node.js Driver directly and are stuck on 2.x, give Monogram a shot and save yourself the pain of tracking down every single place where you use the 2.x-style projection syntax.

Found a typo or error? Open up a pull request! This post is available as markdown on Github
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