MongoDB - How to update a particular element in nested array

It is due to flight.fare is an array, by setting the value with flight.fare.baseFare, it will update the basefare value of each document in flight.fare array.

You need $map operator to iterate each document in flight.fare array and update the basefare value if the document fulfilled the condition.

Solution 1

db.collection.aggregate([
  {
    $unwind: "$flights"
  },
  {
    $match: {
      "flights.fare": {
        $elemMatch: {
          travelClass: "Economy",
          baseFare: {
            $lt: 3800
          }
        }
      }
    }
  },
  {
    $set: {
      "flights.fare": {
        $map: {
          input: "$flights.fare",
          as: "fare",
          in: {
            $cond: {
              if: {
                $and: [
                  {
                    $eq: [
                      "$$fare.travelClass",
                      "Economy"
                    ]
                  },
                  {
                    $lt: [
                      "$$fare.baseFare",
                      3800
                    ]
                  }
                ]
              },
              then: {
                $mergeObjects: [
                  "$$fare",
                  {
                    baseFare: 3800
                  }
                ]
              },
              else: "$$fare"
            }
          }
        }
      }
    }
  }
])

Demo Solution 1 @ Mongo Playground

Solution 2: Without $unwind

db.collection.aggregate([
  {
    $set: {
      flights: {
        $map: {
          input: "$flights",
          as: "flight",
          in: {
            $mergeObjects: [
              "$$flight",
              {
                fare: {
                  $map: {
                    input: "$$flight.fare",
                    in: {
                      $cond: {
                        if: {
                          $and: [
                            {
                              $eq: [
                                "$$this.travelClass",
                                "Economy"
                              ]
                            },
                            {
                              $lt: [
                                "$$this.baseFare",
                                3800
                              ]
                            }
                          ]
                        },
                        then: {
                          $mergeObjects: [
                            "$$this",
                            {
                              baseFare: 3800
                            }
                          ]
                        },
                        else: "$$this"
                      }
                    }
                  }
                }
              }
            ]
          }
        }
      }
    }
  }
])

Demo Solution 2 @ Mongo Playground

Note: Both aggregation queries will result in the value being updated for the projection, but won't update the document in the collection.