MongoDB: Filter documents against array length

Find by array length

// Find all records that have 2 employees
db.Firm.find({ employees: { "$size": 2 }})

Using $where (not efficient)

Not as efficient but useful when you’re querying using the Mongo shell:

Find all documents that have more than 5 employees
db.Firm.find({ $where: "this.employees.length > 5"})

Using $exists (efficient)

// Find all records that have at least 2 employees
db.Firm.find({ 'employees.1': { $exists: true } })