Query Casting

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The first parameter to Model.find(), Query#find(), Model.findOne(), etc. is called filter. In older content this parameter is sometimes called query or conditions. For example:

const query = Character.find({ name: 'Jean-Luc Picard' });
query.getFilter(); // `{ name: 'Jean-Luc Picard' }`
// Subsequent chained calls merge new properties into the filter
query.find({ age: { $gt: 50 } });
query.getFilter(); // `{ name: 'Jean-Luc Picard', age: { $gt: 50 } }`

When you execute the query using Query#exec() or Query#then(), Mongoose casts the filter to match your schema.

// Note that `_id` and `age` are strings. Mongoose will cast `_id` to
// a MongoDB ObjectId and `age.$gt` to a number.
const query = Character.findOne({
  _id: '5cdc267dd56b5662b7b7cc0c',
  age: { $gt: '50' }
});
// `{ _id: '5cdc267dd56b5662b7b7cc0c', age: { $gt: '50' } }`
// Query hasn't been executed yet, so Mongoose hasn't casted the filter.
query.getFilter();
const doc = await query.exec();
doc.name; // "Jean-Luc Picard"
// Mongoose casted the filter, so `_id` became an ObjectId and `age.$gt`
// became a number.
query.getFilter()._id instanceof mongoose.Types.ObjectId; // true
typeof query.getFilter().age.$gt === 'number'; // true

If Mongoose fails to cast the filter to your schema, your query will throw a CastError.

const query = Character.findOne({ age: { $lt: 'not a number' } });
const err = await query.exec().then(() => null, err => err);
err instanceof mongoose.CastError; // true
// Cast to number failed for value "not a number" at path "age" for
// model "Character"
err.message;

The strictQuery Option

By default, Mongoose does not cast filter properties that aren’t in your schema.

const query = Character.findOne({ notInSchema: { $lt: 'not a number' } });
// No error because `notInSchema` is not defined in the schema
await query.exec();

You can configure this behavior using the strictQuery option for schemas. This option is analogous to the strict option. Setting strictQuery to true removes non-schema properties from the filter:

mongoose.deleteModel('Character');
const schema = new mongoose.Schema({ name: String, age: Number }, {
  strictQuery: true
});
Character = mongoose.model('Character', schema);
const query = Character.findOne({ notInSchema: { $lt: 'not a number' } });
await query.exec();
query.getFilter(); // Empty object `{}`, Mongoose removes `notInSchema`

To make Mongoose throw an error if your filter has a property that isn’t in the schema, set strictQuery to 'throw':

mongoose.deleteModel('Character');
const schema = new mongoose.Schema({ name: String, age: Number }, {
  strictQuery: 'throw'
});
Character = mongoose.model('Character', schema);
const query = Character.findOne({ notInSchema: { $lt: 'not a number' } });
const err = await query.exec().then(() => null, err => err);
err.name; // 'StrictModeError'
// Path "notInSchema" is not in schema and strictQuery is 'throw'.
err.message;

Implicit $in

Because of schemas, Mongoose knows what types fields should be, so it can provide some neat syntactic sugar. For example, if you forget to put $in on a non-array field, Mongoose will add $in for you.

// Normally wouldn't find anything because `name` is a string, but
// Mongoose automatically inserts `$in`
const query = Character.findOne({ name: ['Jean-Luc Picard', 'Will Riker'] });
const doc = await query.exec();
doc.name; // "Jean-Luc Picard"
// `{ name: { $in: ['Jean-Luc Picard', 'Will Riker'] } }`
query.getFilter();