Question 264067 in Mongodb Question posted in
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Advanced Data Aggregation with MongoDB and Python

I have a task involving the aggregation and processing of data stored in a MongoDB collection named flight_data. This collection holds a wealth of information about flights, encompassing details such as flight numbers, departure airports, arrival airports, fuel consumption, and flight distances.

Here’s a glimpse of the document structure within the flight_data collection:

{
    "_id": ObjectId("3f94fb7faff0b71ae5e3cb8d"),
    "flight_number": "SWA4146",
    "departure_airport": "SNA",
    "arrival_airport": "SMF",
    "fuel_consumption_gallons": 700.4,
    "flight_distance_miles": 450
}

I’m trying to find a way to calculate the total fuel consumption for each unique flight route, and I aim to identify the routes that have consumed the most fuel. My desired output format is:

{
    "route": "SNA-SMF",
    "total_fuel_consumption_gallons": THE_CALCULATED_AMOUNT
},
{
    "route": "SFO-ORD",
    "total_fuel_consumption_gallons": THE_CALCULATED_AMOUNT
}

Here’s my Python script:

from pymongo import MongoClient

client = MongoClient("mongodb://localhost:27017/")
db = client["aviation_data"]
collection = db["flight_data"]

pipeline = [
    {
        "$group": {
            "_id": {
                "departure_airport": "$departure_airport",
                "arrival_airport": "$arrival_airport"
            },
            "total_fuel_consumption_gallons": {
                "$sum": "$fuel_consumption_gallons"
            }
        }
    },
    {
        "$project": {
            "_id": 0,
            "route": {
                "$concat": ["$_id.departure_airport", "-", "$_id.arrival_airport"]
            },
            "total_fuel_consumption_gallons": 1
        }
    },
    {
        "$sort": {
            "total_fuel_consumption_gallons": -1
        }
    },
    {
        "$limit": 5
    }
]

result = list(collection.aggregate(pipeline))
print(result)

However, when I execute the script, I encounter this error:

pymongo.errors.OperationFailure: The field 'departure_airport' must be an accumulator object

Does anyone know where the error is here? I’ve seen some similar questions, but I couldn’t fix my script using the answers that I found.

Tags:

2

Answers


  1. 0

    So, this error typically indicates that MongoDB aggregation framework is interpreting a field in an unexpected manner.

    The problem lies in the $project stage where you attempt to concatenate the ‘departure_airport‘ and ‘arrival_airport‘ fields. To mitigate this error, you should reshape the pipeline as follows:

    from pymongo import MongoClient

client = MongoClient("mongodb://localhost:27017/")
db = client["aviation_data"]
collection = db["flight_data"]

pipeline = [
    {
        "$group": {
            "_id": {
                "departure_airport": "$departure_airport",
                "arrival_airport": "$arrival_airport"
            },
            "total_fuel_consumption_gallons": {
                "$sum": "$fuel_consumption_gallons"
            }
        }
    },
    {
        "$addFields": {
            "route": {
                "$concat": [
                    "$_id.departure_airport",
                    "-",
                    "$_id.arrival_airport"
                ]
            }
        }
    },
    {
        "$project": {
            "_id": 0,
            "route": 1,
            "total_fuel_consumption_gallons": 1
        }
    },
    {
        "$sort": {
            "total_fuel_consumption_gallons": -1
        }
    },
    {
        "$limit": 5
    }
]

result = list(collection.aggregate(pipeline))
print(result)

    The $addFields stage is used to create the ‘route‘ field, eliminating the error and ensuring that you can achieve the intended results.

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  2. 0

    I can’t figure out why the other answer (LLM generated?) was upvoted and accepted. It is not correct.

    In a comment I posted that seems to have been removed, I asked if the aggregation as written in the question was yielding the error message shared. This playground example, which uses the sample document and the aggregation (directly copied from the question) completes successfully.

    Typically error messages stating that the system is expecting an accumulator object are related to the $group stage (not the $project stage as the other answer claims). This is because accumulator operators are used in the $group stage. From the documentation the syntax of $group is:

    {
 $group:
   {
     _id: <expression>, // Group key
     <field1>: { <accumulator1> : <expression1> },
     ...
   }
 }

    And indeed that is exactly what is provided in the description of this question. However if we improperly move the departure_airport field outside of the _id definition in the $group stage as follows:

      {
    "$group": {
      "_id": {
        "arrival_airport": "$arrival_airport"
      },
      "departure_airport": "$departure_airport",
      "total_fuel_consumption_gallons": {
        "$sum": "$fuel_consumption_gallons"
      }
    }

    Then we get the exact error message provided in the question:

    query failed: (Location40234) The field 'departure_airport' must be an accumulator object

    The playground demonstration of this can be found here.

    The $project stage defined in the question is syntactically valid and just fine. Moreover, the other answer "solves" the supposed problem with generating the route field in the $project stage by adding a preceding $addFields stage to handle it. That answer claims that this "eliminat[es] the error and ensur[es] that you can achieve the intended results."But the problem with the logic in that claim is that it relies on those two stages being ‘different’ in some meaningful way. However per the documentation:

    The $addFields stage is equivalent to a $project stage that explicitly specifies all existing fields in the input documents and adds the new fields.

    So to summarize:

    1. I don’t see evidence that the aggregation stage provided in the question generates the error in the question. (If it does, please share a link to a mongoplayground so that we can investigate further).
    2. The currently accepted answer neither does anything to solve the problem nor is the logic it is based on correct.
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