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" } } },
   { $match: { totalPop: { $gte: 10*1000*1000 } } }
] )

if I replace _id with something else like word Test, I will get error message:

"errmsg" : "exception: the group aggregate field 'Test' must be defined as an expression inside an object",
"code" : 15951,
"ok" : 0

Could anybody help me understand why I need _id in my command? I thought MongoDB assigns IDs automatically, if used does not provide it.

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.}

Answer 2

The _id field is mandatory, but you can set it to null if you do not wish to aggregate with respect to a key, or keys. Not utilising it would result in a single aggregate value over the fields. It is thus acting a 'reserved word' in this context, indicating what the resulting 'identifier'/key is for each group.

In your case, grouping by _id: "$state" would result in n aggregate results of totalPop, provided there there are n distinct values for state (akin to SELECT SUM() FROM table GROUP BY state). Whereas,

$group : {_id : null, totalPop: { $sum: "$pop" }}}

would provide a single result for totalPop (akin to SELECT SUM() FROM table).

This behaviour is well described in the group operator documentation.