mongodb limit array documents

mongo_array.txt

collection: people
{
  name: 'some name',
  age: 23,
  ...
  groups: [
    {
      id: 1,
      ...
    },
    ...
  ]
}

to find all persons having groups.id = 1 in groups

db.people.find({ 'groups.id': 1 })

to find all persons having groups.id = 1 in groups, returning only matching groups

db.people.find({ 'groups.id': 1 }, { groups: { $elemMatch: { id: 1 }}})

https://docs.mongodb.org/manual/reference/operator/projection/elemMatch/#proj._S_elemMatch