mongodb python , quick pipeline code check

Question

i am a beginner to mongodb and i have the assignment to write pipeline code. MY goal is to find the Region in India has the largest number of cities with longitude between 75 and 80? I hope anybody can help me to point out my misconceptions and/or mistakes, it is a very short code, so i am sure the pros will spot it right away.

Here is my code, i will post how the datastructure looks like under it :

    pipeline = [
    {"$match" :  {"lon": {"$gte":75, "$lte" : 80}},
                 {'country' : 'India'}},
    { '$unwind' : '$isPartOf'},
    { "$group":
    {
                "_id": "$name",
                "count" :{"$sum":{"cityname":"$name"}} }},

    {"$sort": {"count": -1}},
     {"$limit": 1}

]


{
    "_id" : ObjectId("52fe1d364b5ab856eea75ebc"),
    "elevation" : 1855,
    "name" : "Kud",
    "country" : "India",
    "lon" : 75.28,
    "lat" : 33.08,
    "isPartOf" : [
        "Jammu and Kashmir",
        "Udhampur district"
    ],
    "timeZone" : [
        "Indian Standard Time"
    ],
    "population" : 1140
}

chridam · Accepted Answer · 2015-05-04 10:16:17Z

The following pipeline will give you the desired result. The first $match pipeline operator uses standard MongoDB queries to filter the documents (cities) whose longitude is between 75 and 80 and as well as the ones only in India based on the country field. Since each document represents a city, the $unwind operator on the isPartOf deconstructs that array field from the filtered documents to output a document for each element. Each output document replaces the array with an element value. Thus for each input document, outputs n documents where n is the number of array elements and this operation is rather useful in the next $group operator stage since that's where you can calculate the number n through $sum group accumulator operator. The next pipeline stages will then transform your final document structure by introducing new replacement fields Region and NumberOfCities + sorting the documents in descending order and then returning the top 1 document which is your region with the largest number of cities:

pipeline = [
    {
        "$match": {
            "lon": {"$gte": 75, "$lte": 80},
            "country": "India"
        }
    },
    {
        "$unwind": "$isPartOf"
    },
    {
        "$group": {
            "_id":  "$isPartOf",
            "count": {
                "$sum": 1
            }
        }
    },
    {
        "$project": {
            "_id": 0,
            "Region": "$_id",
            "NumberOfCities": "$count"
        }
    },
    {
        "$sort": {"NumberOfCities": -1}
    },
    { "$limit": 1 }
]

thegreenogre · Accepted Answer · 2015-05-04 10:19:06Z

There are some syntax and logical errors in your pipeline.

{"$match" : {"lon": {"$gte":75, "$lte" : 80}}, {'country' : 'India'}},

The Syntax here is wrong, you should just use comma to seperate key value pairs in `$match.

"_id": "$name",

You are grouping based on city name and not on the region.

{"$sum":{"cityname":"$name"}}

You need to send a numeric values to the $sum operator that result from applying a specified expression. {"cityname":"$name"} will be ignored.

The correct pipeline would be :-

[
    {"$match" :  {"lon": {"$gte":75,"$lte" : 80},'country' : 'India'}},
    { '$unwind' : '$isPartOf'},
    { "$group":
        {
                "_id": "$isPartOf",
                "count" :{"$sum":1}
         }
    },
    {"$sort": {"count": -1}},
    {"$limit": 1}

]

If you want to get all the cities in that region satisfying your condition as well ,you can add "cities": {'$push': '$name'} in the $group stage.

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