Mongoose _id exclusion from JSON serialization without removing document identity

Answer

Yes. Define a schema‑level toJSON (and optionally toObject) transform that deletes _id from the returned object. The original document retains its _id for internal tracking and middleware.

Confirmed facts

• Setting { _id: false } at the schema level removes the identifier from the document itself, which can break plugins that expect _id to exist.
• A transform in toJSON runs after all document middleware (e.g., pre('save'), post('save')), so middleware still sees the full document with _id intact.
• The toJSON/toObject transform API has not changed between Mongoose 6.x and 7.x; behavior is identical in both versions.

Steps to hide _id from JSON output

1. In your schema definition, add a toJSON options object with a transform function:

const mongoose = require('mongoose');
const { Schema } = mongoose;

const userSchema = new Schema({
  name: String,
  email: String,
  // other fields...
}, {
  toJSON: {
    transform: (doc, ret) => {
      delete ret._id;   // remove _id from serialized output
      return ret;
    }
  },
  // optional: also hide _id from toObject() if you use it elsewhere
  toObject: { transform: (doc, ret) => { delete ret._id; return ret; } }
});

module.exports = mongoose.model('User', userSchema);

2. Create a document and verify:

const User = require('./user');
(async () => {
  const user = new User({ name: 'Ada', email: '[contact removed]' });
  await user.save();

  // Middleware still sees _id
  user.pre('save', function(next) {
    console.log('pre‑save _id:', this._id); // <- present
    next();
  });

  // Serialized output lacks _id
  const json = user.toJSON();
  console.log('toJSON output:', json); // <- no _id field
  console.log('document _id still:', user._id); // <- present
})();

3. If you want the behavior globally, either:
• Apply the same toJSON transform to every schema via a plugin, or
• Set the transform as a default schema option using mongoose.Schema.set('toJSON', { transform: ... }) before defining schemas.

Likely explanation (why this works)

The transform function receives the raw document (doc) and a plain object (ret) that Mongoose will return. By deleting _id from ret only, you alter the serialized representation without mutating the source document. Because the transform executes after all middleware hooks, any middleware that reads _id continues to operate on the unchanged document.