How to show null sum in MongoDB aggregate [duplicate] - mongodb

Question

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Fill missing dates in records
(5 answers)
Closed 2 years ago.
I'm trying to get a small table from mongodb aggregate, for example, number of fatal accidents by years.
I want to get all the years , even if sum is null (zero).
MongoDB query:
[
{"$match": {"city": "myCity"}},
{"$group": {
"_id": "$accident_year",
"count": {"$sum": 1}}
},
{"$sort": {"_id": 1}}
]
actual result:
[
{"_id": "2015", "count": 2},
{"_id": "2017", "count": 4},
{"_id": "2018", "count": 6},
{"_id": "2019", "count": 2}
]
desired result:
[
{"_id": "2015", "count": 2},
{"_id": "2016", "count": 0},
{"_id": "2017", "count": 4},
{"_id": "2018", "count": 6},
{"_id": "2019", "count": 2}
]
Thank you

Answer 1

I don't think it's possible to get an output of 0 for years that are not listed in the collection the way you have the aggregation pipeline created now. I don't see a way for the $group stage to know what values don't exist.
Since you are sorting the results, your application code that receives the output could check if years are listed and manipulate the aggregation pipeline results to include missing years. This is probably the best option.
Here is a hack to get around this if you really want the results in your aggregation pipeline:
Create a document for every year in your collection. Inside of those documents add a field called something like isAccident and set it to 0. For example:
{
{"_id":{"$oid":"5e662fca1c9d440000c1aa71"},
"accident_year":"2018",
"isAccident":"0"
}
Then you can update your pipeline to have a $project stage before the $group stage that adds the isAccident field to all of the documents that don't have the isAccident field and assigns them a value of 1. In your $project stage you can sum on the $isAccident field.
[{$project: {
_id: 1,
accident_year: 1,
isAccident: { $ifNull: ["$isAccident", 1] }
}}, {$group: {
_id: "$accident_year",
count: {
$sum: "$isAccident"
}
}}]
This will give you the results you're expecting. Beware that if others after you come to group and sum the accidents in this collection and don't realize you've created this extra documents for the years, their calculations will be off by one.