Mongodb group by consults in colab - mongodb

Question

I have to make a consult on a data base in Mongodb, but usin colab (google colab), i found that the existing documentation (oficial and every other site) have a similar way of doing the consult but not de same (in colab with "less tan" operator yo have to do "$lt" and $lt doesn't work) thas why i am here asking how can i translate a group by and sum consult.
For more detail i want to group by name of publisher and sum another field (weeks on best seller)
Query = collection.aggregate(
[
{'$group':{'_id': 'publisher', 'Cantidad_total': { "$sum": 'weeks_on_list' }}}])
for elemento in Query:
pprint.pprint(elemento)
this is what i came up with (that doesn´t fail) but give this
{'Cantidad_total': 0, '_id': 'publisher'}

Answer 1

You just need a couple of $.
From the docs:
Expressions:
Expressions can include field paths , literals , system variables ,
expression objects , and expression operators . Expressions can be
nested.
Field Paths:
Aggregation expressions use field path to access fields in the input
documents. To specify a field path, prefix the field name or the
dotted field name (if the field is in the embedded document) with a
dollar sign $. For example, "$user" to specify the field path for the
user field or "$user.name" to specify the field path to "user.name"
field.
So for your particular example, you want your aggregation pipeline to be:
[
{
"$group": {
"_id": "$publisher",
"Cantidad_total": {
"$sum": "$weeks_on_list"
}
}
}
]