Aggregation query and read preference

[Bhupen Sinha]

Hello All

like we can indicate the read preference with a normal FIND() query, can we do the same with an aggregation query? I tried but failed

db.marks.find({marks:{$gte:50}}).comment(‘spref’).readPref(‘secondaryPreferred’) .. this works

but 

the following does not work ... Any solution pls?

 db.zipcode.aggregate ( [

    { $group: { _id: { state: "$state", city: "$city" }, pop: { $sum: "$pop" } } },

    { $group: { _id: "$_id.state", avgCityPop: { $avg: "$pop" } } },

    { $out : “zipout" } 

 ] ).readPref(‘secondary’)

thanks!

[Asya Kamsky]

You can set your read preference to secondary (i.e. in the shell: db.getMongo().setReadPref('secondary’)) but I’m afraid this aggregation cannot run on secondary since it has “$out” stage and therefore it *must* be run on the primary.

Asya

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Asya Kamsky
Lead Product Manager
MongoDB
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