MongoDB find count of all possible 'columns' within a collection - mongodb

Question

Is there a way to find all possible number of 'columns' or json properties available in a collection? (I know it's not correct to call them columns, but just for the ease of understanding)
For example, all the following documents are in the same collection called 'people':
{"Name": "bob", "Profession": "IT", "Height": 200},
{"Name": "simon", "Weight": 100, "IQ": 120},
{"Name": "james", "Weight": 130, "Glasses": "Yes"}
The possible 'columns' here are: Name, Profession, Height, Weight, IQ and Glasses. A total of 6.
Is there any way I can do an operation which gets this count of 6? (extra useful if there's also a pymongo variant)
I'm wanting to transfer data from MongoDB into a table format, and knowing the overall number of columns the table can have is useful.

Answer 1

You can use this aggregation query to get your desired result:
The trick here is to use $objectToArray to get the keys as values. Then remove the key _id (if exists) and group to get the total.
db.collection.aggregate([
{
"$project": {
"keys": {
"$objectToArray": "$$ROOT"
}
}
},
{
"$unwind": "$keys"
},
{
"$match": {
"keys.k": {
"$ne": "_id"
}
}
},
{
"$group": {
"_id": "$keys.k",
"total": {
"$sum": 1
}
}
},
{
"$group": {
"_id": null,
"total": {
"$sum": 1
}
}
}
])
Example here
Edit:
Another way to avoid $unwind and double $group id this query:
The idea is the same as before, use $objectToArray to get the keys as key.k and then $group all values and add into an array.
Then get the size of the array after to do some calculations: A $reduce to flatten the array and $filter to not get the _id field.
Note that if you want to count the _id you can simply remove the $filter stage like this example
db.collection.aggregate([
{
"$project": {
"keys": {
"$objectToArray": "$$ROOT"
}
}
},
{
"$group": {
"_id": null,
"keys": {
"$addToSet": "$keys.k"
}
}
},
{
"$project": {
"_id": 0,
"keys": {
"$size": {
"$filter": {
"input": {
"$reduce": {
"input": "$keys",
"initialValue": [],
"in": {
"$setUnion": [
"$$value",
"$$this"
]
}
}
},
"cond": {
"$ne": [
"$$this",
"_id"
]
}
}
}
}
}
}
])
Example here