MongoDB - objects? Why do I need _id in aggregate

Question

Here is an example from MongoDB tutorial (here it collection ZIP Code db:

db.zipcodes.aggregate( [
   { $group: { _id: \"$state\", totalPop: { $sum: \"$pop\"         


        

Answer 1

In a $group stage, _id is used to designate the group condition. You obviously need it.

If you're familiar with the SQL world, think of it as the GROUP BY clause.

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_{Please note, in that context too, _id is really an unique identifier in the generated collection, as by definition $group cannot produce two documents having the same value for that field.}