"errmsg" : "exception: the group aggregate field '_state' must be defined as an expression inside an object",

[janwen lou]

I try to do the examples based on the dataset:http://media.mongodb.org/zips.json

and i see the  docs about two phase of mongodb aggregate:http://docs.mongodb.org/manual/core/aggregation-pipeline/#aggregation-pipeline

so i try to match the state equals "LA",and group the total pop within the state,the shell is:

db.zips.aggregate({$match:{state:"LA"}},{$group:{_state:"$state",total:{$sum:"$pop"}}})

like the doc says,phrase one:match the "LA" state docs,and the sum the pop,and the get the error like post title:

Error: Printing Stack Trace

    at printStackTrace (src/mongo/shell/utils.js:37:15)

    at DBCollection.aggregate (src/mongo/shell/collection.js:897:9)

    at (shell):1:9

Sun Nov  3 04:08:19.542 aggregate failed: {

"errmsg" : "exception: the group aggregate field '_state' must be defined as an expression inside an object",

"code" : 15951,

"ok" : 0

 

after that error,i remenber r that mysql group syntax, if i want to do that i need use similar sql:select * from zips group by state having state='LA'

and then get the example from doc:http://docs.mongodb.org/manual/tutorial/aggregation-zip-code-data-set/#return-states-with-populations-above-10-million

but i am just wondering why i can use the db.zips.aggregate({$match:{state:"LA"}},{$group:{_state:"$state",total:{$sum:"$pop"}}})

to do my request like the example said  

[Nasir Rasul]

just a thought: i think you are required to have _id which is a special field.

try: db.zips.aggregate({$match:{

state:"LA"}},{$group:{_id:"$state",total:{$sum:"$pop"}}})

- Nasir

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