Find By ID in Mongoose

In Mongoose, the Model.findById() function is used to find one document by its _id. The findById() function takes in a single parameter, the document id. It returns a promise that resolves to the Mongoose document if MongoDB found a document with the given id, or null if no document was found.

const schema = new mongoose.Schema({ _id: Number }, { versionKey: false });
const Model = mongoose.model('MyModel', schema);

await Model.create({ _id: 1 });

// `{ _id: 1 }`
await Model.findById(1);

// `null` because no document was found
await Model.findById(2);

When you call findById(_id), Mongoose calls findOne({ _id }) under the hood. That means findById() triggers findOne() middleware.

const schema = new mongoose.Schema({ _id: Number }, { versionKey: false });
schema.pre('findOne', function() {
  console.log('Called `findOne()`');
});
const Model = mongoose.model('MyModel', schema);
await Model.create({ _id: 1 });

// Prints "Called `findOne()`" because `findById()` calls `findOne()`
await Model.findById(1);

Mongoose casts queries to match your schema. That means if your _id is a MongoDB ObjectId, you can pass the _id as a string and Mongoose will convert it to an ObjectId for you.

const _id = '5d273f9ed58f5e7093b549b0';
const schema = new mongoose.Schema({ _id: mongoose.ObjectId }, { versionKey: false });
const Model = mongoose.model('MyModel', schema);

await Model.create({ _id: new mongoose.Types.ObjectId(_id) });

typeof _id; // 'string'
// `{ _id: '5d273f9ed58f5e7093b549b0' }`
const doc = await Model.findById(_id);

typeof doc._id; // 'object'
doc._id instanceof mongoose.Types.ObjectId; // true

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