How to get most repeated string array in Pymongo? - python-3.x

Question

How can I get most repeated value for gender and age respectively?
My data:
[{ "_id": ObjectId("5dff27c0ac2d1547d87a1fe7"), "time": "2019-12-20 21:09:53",
"object": [{"Id": 1,"gender": "female","age": "0-10"},
{"Id": 2,"gender": "female","age": "20-30"}]
},
{ "_id": ObjectId("5dff27c0ac2d1547d87a1fe8"), "time": "2019-12-20 21:09:53",
"object": [{"Id": 1,"gender": "male","age": "0-10"},
{"Id": 2,"gender": "female","age": "30-40"}]
} ,
{ "_id": ObjectId("5dff27c0ac2d1547d87a1fe9"), "time": "2019-12-20 21:09:53",
"object": [{"Id": 1,"gender": "male","age": "10-15"},
{"Id": 2,"gender": "female","age": "30-40"},
{"Id": 3,"gender": "male","age": "0-10"}]
},
{ "_id": ObjectId("5dff27c0ac2d1547d87a1fea"), "time": "2019-12-20 21:09:53",
"object": [{"Id": 2,"gender": "male","age": "40-50"},
{"Id": 3,"gender": "male","age": "0-10"},
{"Id": 4,"gender": "male","age": "0-10"}]
}]
I have written below query,
mongo.db.xyz.aggregate([
{ "$unwind" : "$object"},
{"$group" : {"_id" : "$object.Id","_gen":{"$push":"$object.gender"},"_age":{"$push":"$object.age"}}},
{ "$project": { "_id" : "$_id", "gender":"$_gen","age":"$_age"}}
])
Below is the result I am getting,
[{"_id": 3,"age": ["0-10","0-10"],"gender": ["male","male"]},
{"_id": 2,"age": ["20-30","30-40","30-40","40-50"],"gender": ["female","female","female","male"]},
{"_id": 4,"age": ["0-10"],"gender": ["male"]},
{"_id": 1,"age": ["0-10","0-10","10-15"],"gender": ["female","male","male"]}
]
But I want the output to be ,
[{"_id": 3,"age": "0-10","gender": "male"},
{"_id": 2,"age": "30-40","gender": "female"},
{"_id": 4,"age": "0-10","gender": "male"},
{"_id": 1,"age": "0-10","gender": "male"}
]

Answer 1

Thinking about this problem I realized it was not so simple to get the mode of some fields independently in an array of objects with just one db query. To solve that I created a query to do it generically, based on your sample data.
db.collection.aggregate([
{
$unwind: {
path: "$arr"
}
},
{
$project: {
arr: {
$objectToArray: "$arr"
}
}
},
{
$unwind: {
path: "$arr"
}
},
{
$group: {
_id: {
_id: "$_id",
k: "$arr.k",
v: "$arr.v"
},
count: {
$sum: 1
}
}
},
{
$sort: {
count: -1
}
},
{
$group: {
_id: {
_id: "$_id._id",
k: "$_id.k"
},
v: {
$first: "$_id.v"
},
count: {
$first: "$count"
}
}
},
{
$group: {
_id: {
_id: "$_id._id"
},
arr: {
$push: {
k: "$_id.k",
v: {
mode: "$v",
count: "$count"
}
}
}
}
},
{
$project: {
arr: {
$arrayToObject: "$arr"
}
}
}
])
The secret to do that is to use the $objectToArray and the $arrayToObject operations, along with the $unwind and the correct $group stages. If you need a more detailed response on any stage please ask in the comments.
The output of the sample data is:
[
{
"_id": {
"_id": ObjectId("5dff27c0ac2d1547d87a1fea")
},
"arr": {
"Id": {
"count": 1,
"mode": 3
},
"age": {
"count": 2,
"mode": "0-10"
},
"gender": {
"count": 3,
"mode": "male"
}
}
},
{
"_id": {
"_id": ObjectId("5dff27c0ac2d1547d87a1fe7")
},
"arr": {
"Id": {
"count": 1,
"mode": 2
},
"age": {
"count": 1,
"mode": "0-10"
},
"gender": {
"count": 2,
"mode": "female"
}
}
},
{
"_id": {
"_id": ObjectId("5dff27c0ac2d1547d87a1fe9")
},
"arr": {
"Id": {
"count": 1,
"mode": 2
},
"age": {
"count": 1,
"mode": "30-40"
},
"gender": {
"count": 2,
"mode": "male"
}
}
},
{
"_id": {
"_id": ObjectId("5dff27c0ac2d1547d87a1fe8")
},
"arr": {
"Id": {
"count": 1,
"mode": 2
},
"age": {
"count": 1,
"mode": "30-40"
},
"gender": {
"count": 1,
"mode": "female"
}
}
}
]
After running it for a collection that has an array of objects named "arr" (edit the query if in your collection it has a different name) it will return the mode value and the number of occurrences of that value for each field. Objects that has that field unset will be not considered, but the ones with "null" will.