MongoDB - How to update a particular element in nested array

Wasim Akhtar Khan 0
Viewed 17
{
    "source": "Chennai",
    "destination": "Kolkata",
    "flights": [
        {
            "flightId": "E6-2145",
            "seatsAvailable": 10,
            "fare": [
                {
                    "travelClass": "Economy",
                    "baseFare": 3588
                },
                {
                    "travelClass": "Business",
                    "baseFare": 9999
                }
            ]
        }
    ]
}

I want to update the basefare of Economy class to 3800 if it is less than 3800.

I tried:

db.Flights.aggregate({
  $unwind: "$flights"
},
{
  $match: {
    "flights.fare": {
      $elemMatch: {
        travelClass: "Economy",
        baseFare: {
          $lt: 3800
        }
      }
    }
  }
},
{
  $set: {
    "flights.fare.basefare": 3800
  }
})

But it is updating both basefares of the flight i.e Economy as well as Business to 3800.

Please help me to solve this.

aggregation-framework mongodb-query database mongodb
1 Answers

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.

Yong Shun0
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