MongoDB update with condition

Starting Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update/creation of a field based on another field:

// { a: "Hello", b: "World" }
// { a: "Olleh", b: "Dlrow" }
db.collection.updateMany(
  {},
  [ { $set: { active: { $eq: [ "$a", "Hello" ] } } } ]
)
// { a: "Hello", b: "World", active: true  }
// { a: "Olleh", b: "Dlrow", active: false }

• The first part {} is the match query, filtering which documents to update (in our case all documents).

• The second part [ { $set: { active: { $eq: [ "$a", "Hello" ] } } } ] is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline). $set is a new aggregation operator and an alias of $addFields. Then any aggregation operator can be used within the $set stage; in our case a conditional equality check on which depends the value to use for the new active field.