Suppose we have collection of documents having array and we want to update document having array on the basis of some condition.

Insert document in movies collection

db.movies.insertMany([
  {
    title: "Phone Bhoot",
    actors: [
      { age: 30, sex: "f", name: "n" },
      { age: 20, sex: "f", name: "c" },
      { age: 35, sex: "m", name: "s" },
      { age: 40, sex: "m", name: "a" },
    ],
  },
  {
    title: "Sita Raman",
    actors: [
      { age: 30, sex: "m", name: "se" },
      { age: 20, sex: "f", name: "cn" genre:['action', 'romance', 'thrill', 'suspence]},
      { age: 25, sex: "f", name: "sn" },
      { age: 30, sex: "m", name: "an" },
    ],
  },
  {
    title: "Bharamsastra",
    actors: [
      { age: 50, sex: "m", name: "ser" genre:['family', 'thrill']},
      { age: 40, sex: "f", name: "cnr" },
      { age: 25, sex: "f", name: "snr" },
      { age: 30, sex: "m", name: "anr" , genre:['action', 'romance', 'thrill']},
    ],
  },
]);

The $[<identifier>] operator facilitates updates to arrays that contain embedded documents. To access the fields in the embedded documents, use the dot notation with the $[<identifier>].

db.collection.updateMany(
   { <query selector> },
   { <update operator>: { "array.$[<identifier>].field" : value } },
   { arrayFilters: [ { <identifier>: <condition> } } ] }
)

Example 1: Increment age by 5 where tilte “Phone Bhoot” and age less than 30

db.movies.updateMany(
  { title: "Phone Bhoot" },
  { $inc: { "actors.$[elem].age": 5 } },
  { arrayFilters: [{ "elem.age": { $lt: 30 } }] }
);

Example 2: set “senior” to “yes”, those age greater than 50

db.movies.updateMany(
  {},
  { $set: { "actors.$[elem].senior": "yes" } },
  { arrayFilters: [{ "elem.age": { $gt: 40 } }] }
);

Example 3: set “diva” to “yes”, those age greater than equal 20 and less than equal to 30 and sex is f

db.movies.updateMany(
  {},
  { $set: { "actors.$[elem].diva": "yes" } },
  { arrayFilters: [{ "elem.age": { $gt: 18, $lte: 30 }, "elem.sex": "f" }] }
);

Example 4: add “action” to genre whose age greater than equal 20 and less than equal to 30 and sex is m

db.movies.updateMany(
  {},
  { $push: { "actors.$[elem].genre": "action" } },
  { arrayFilters: [{ "elem.age": { $gte: 20, $lte: 30 }, "elem.sex": "m" }, ] }
);

How it works ?

The filtered positional operator $[<identifier>] identifies the array elements that match the arrayFilters conditions for an update operation, Used in conjunction with the arrayFilters option, the $[<identifier>] operator has the following form:

{ <update operator>: { "<array>.$[<identifier>]" : value } },
{ arrayFilters: [ { <identifier>: <condition> } ] }

The <identifier> must begin with a lowercase letter and contain only alphanumeric characters.

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Source : MongoDB Doc