I've gotten a lot of requests for a guide to Mongoose internals, because it's hard to contribute unless you have the time and patience to read through the codebase. In this series of tutorials, I'll provide an overview of how Mongoose works geared towards developers that want to contribute to the project.

In part 1, I covered how the Schema class works. In this tutorial, I'll cover how Mongoose does change tracking: how Mongoose determines what properties changed on a document.

Compiling a Schema

Calling mongoose.model(name, schema) tells Mongoose to compile a model based on schema. Here's where mongoose.model() calls Model.compile() and here's the implementation of Model.compile(). The Model.compile() function creates a new model class that extends from Mongoose's built in Mongoose class, and applies middleware and query helpers to the new class. But it also calls the $__setSchema() function to tell the new model class which paths to watch for changes on.

First, let's take a quick look at how change tracking looks from a user perspective. Suppose you have a simple model with 2 properties: name and age.

const UserModel = mongoose.model('User', mongoose.Schema({ name: String, age: String }));

Suppose you load a user from the database and change the user's name, but not their age. Mongoose tracks this change and only sends the changed name when you call save(). You can check what Mongoose will send to MongoDB when you call save() using the getChanges() method:

const { _id } = await UserModel.create({ name: 'test', age: 29 });

const user = await UserModel.findOne({ _id });
user.name = 'test2';

user.getChanges(); // { $set: { name: 'test2' }, $unset: {} }

The $__setSchema() method calls the compile() helper, which is responsible for calling Object.defineProperty() on every path in the schema so Mongoose can watch for changes. Below is some simplified pseudo-code that demonstrates how compile() works, excluding recursion and nested paths.

function compile(schema, model) {
  const keys = Object.keys(schema.tree); // `tree` contains the paths of the schema as a nested structure
  const len = keys.length;
  let limb;
  let key;

  for (let i = 0; i < len; ++i) {
    key = keys[i];

    defineKey(model, key);

function defineKey(model, key) {
  Object.defineProperty(model.prototype, key, {
    enumerable: true,
    configurable: true,
    get: function() {
      // `this` refers to the `UserModel` instance, like `user` in the previous example
      return this.get(path);
    set: function(v) {
      this.set(key, v);

The key lesson here is that, excluding nested paths, user.name = 'test2' is equivalent to user.set('name', 'test2') because Mongoose registers a name property on UserModel.prototype that calls this.set('name', v) whenever someone assigns a value to user.name.

From a performance perspective, Object.defineProperty() is very slow. Mongoose mostly avoids this performance impact by only calling Object.defineProperty() when compiling the schema into a model (again excluding nested paths). We assume that developers will typically only compile models once when the process starts, as opposed to compiling models on every request, which would be very slow. Once we've defined a property on the model class's prototype, we get change tracking for every instance of this model class without having to run defineProperty() every time we create a new instance of UserModel.

Path State

So the compile() function is responsible for compiling a schema into a model class that has built-in change tracking, and change tracking works by watching all paths in the schema and calling get() / set() whenever someone accesses or assigns to a particular property. So the core logic of change tracking is really the set() method.

The lib/document.js file defines a $set() method. It's called $set() as opposed to set() to allow people to use set as a schema path name, although set() is an alias for $set() by default.

The $set() function is pretty complex because it has support for setting multiple paths, setting nested paths, manual population, adhoc paths, strict mode, immutable paths, and numerous other features. But for the purposes of this tutorial, it is responsible for:

  1. Getting the SchemaType of the path being set
  2. Getting the current value of the path
  3. Casting the value we're trying to set to the schematype
  4. Storing any error that occurred
  5. Determining whether the value changed and setting the value on _doc (via $__set() and $__shouldModify())

Below is a heavily simplified pseudocode implementation of $set():

Document.prototype.$set = function(path, val) {
  // 1. Get the current SchemaType
  const schematype = this.$__path(path);

  // 2. Get the current value
  const priorVal = this.$__getValue(path);

  try {
    // 3. Cast the value we're setting
    val = schema.applySetters(val, this, false, priorVal);

  } catch (err) {
    // 4. If an error occurred when casting, store the error internally and return
    this.invalidate(path, new mongoose.CastError(schema.instance, val, path, err));

  // 5. If value changed, mark the path as modified
  if (val !== priorVal) {

The Document#markModified() method is responsible for marking a path as modified. Modulo subdocuments, this.markModified(path) is equivalent to this.$__.activePaths.modify(path).

The this.$__ property is an instance of Mongoose's InternalCache class in lib/internal.js. This property stores the Mongoose document's internal state, including change tracking. Mongoose often checks for the presence of $__ to check if an arbitrary object is a document. The activePaths property is an instance of the ActiveRoster class, which is a state machine that tracks the state of every path in the document.

Calling this.$__.activePaths.modify(path) tells the active roster that path is now in the 'modified' state. A path can be in one of 5 states:

  1. 'require'
  2. 'modify'
  3. 'init'
  4. 'default'
  5. 'ignore'

To check whether a path is modified, you check this.$__.activePaths.states.modify.hasOwnProperty(path). Note that this is a check for whether a path is "direct modified" if you have subdocuments or nested paths: if you set doc.a, doc.a.b is modified even though you haven't directly modified doc.a.b. We consider a path direct modified if this.$__.activePaths.states.modify.hasOwnProperty(path). Similarly, to see what paths are direct modified, you use Object.keys(this.$__.activePaths.states.modify).

Casting and Cast Errors

In Mongoose, casting is the process of converting an arbitrary value to the type specified in the schema. For example, if you set a number path to the string '42', Mongoose is smart enough to cast that string to the number 42.

One of the key nuances of $set() is that it does not throw an error if casting failed. For example, if you try to set a number path to a string, you don't get any errors.

const User = mongoose.model('User', Schema({ name: String, age: Number }));

const doc = await User.create({ name: 'Jean-Luc Picard', age: 59 });

// Doesn't throw an error
doc.$set('age', 'oops!');

doc.age; // 59, Mongoose doesn't overwrite the current value if an error occurred.

await doc.validate(); // Throws ValidationError

Remember that $set() wraps applySetters(), which is responsible for casting, and calls the invalidate() method if casting or setters throw an error.

  try {
    // 3. Cast the value we're setting
    val = schema.applySetters(val, this, false, priorVal);

  } catch (err) {
    // 4. If an error occurred when casting, store the error internally and return
    this.invalidate(path, new mongoose.CastError(schema.instance, val, path, err));

The Document#invalidate() method is responsible for marking paths as invalid. Below is the implementation of invalidate():

Document.prototype.invalidate = function(path, err, val, kind) {
  if (!this.$__.validationError) {
    this.$__.validationError = new ValidationError(this);

  if (this.$__.validationError.errors[path]) {

  if (!err || typeof err === 'string') {
    err = new ValidatorError({
      path: path,
      message: err,
      type: kind || 'user defined',
      value: val

  if (this.$__.validationError === err) {
    return this.$__.validationError;

  this.$__.validationError.addError(path, err);
  return this.$__.validationError;

The invalidate() function is responsible for setting properties on this.$__.validationError.errors, which is where Mongoose stores all paths that had failed $set() calls. This property also stores validation errors, which we'll explain in a future tutorial. But, for now, all you need to know is that, when you run validate(), Mongoose returns this.$__.validatorError. So if casting fails, await doc.validate() will throw an error.

get() and _doc

That's the overview of how the set() function and setting a property works. What about get()? Mongoose documents have a get() method that returns the current value for a given path.

The get() function supports several advanced features, like dotted paths and adhoc types. But, for the purposes of this tutorial, below is a heavily simplified version.

Document.prototype.get = function(path) {
  const val = this._doc[path];

  const schema = this.$__path(path);
  if (schema != null) {
    return schema.applyGetters(val, this);

  return val;

The _doc property is where Mongoose stores the "raw" value of the document. A document itself has getters/setters, the $__ property, etc. But _doc is just a POJO representation of the document, with no getters/setters, methods, or anything Mongoose-specific. The only difference is that single nested subdocuments are still fully fledged Mongoose documents in _doc.

You might also be wondering where the $set() function modifies _doc. There's a $__set() method that's responsible for the internals of setting the value on the _doc property.

Moving On

So far, we've looked at the basics of Schemas and how Mongoose compiles a schema into a model with change tracking. But we've only looked at simple schemas so far: schema paths so far have been strings and numbers, not objects or arrays. Next up, we'll take a look at nested paths, subdocuments, arrays, and other more complex schemas.

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