Query MongoDB for single field and insert into array

Question

I'm currently use Mongoid to talk to my mongodb database. I have a query like the following:

ClassDoc.only(:class_id).where(:recurrs => true)

I'm only interested in the IDs as I exclude them in a subsequent query. However, the subsequent query needs the IDs in an array so I have to do this:

first_result_ids = ClassDoc.only(:class_id).where(:recurrs => true).collect { |class| class.class_id }

This takes quite a long time since AFAIK 'collect' loops over every result. It can take up to 10s to loop over just 1000 results.

Is there a way to pull the IDs out directly into an array?

If you only need class_id i would try to use an covered indexe for that query, so the result will come from the index only. this will speed up the queries a lot. see: docs.mongodb.org/manual/core/read-operations/… it may work with .only() in mongoid — Marc
– Marc, Commented Dec 12, 2012 at 10:16

abhas · Accepted Answer · 2012-12-05 15:16:06Z

1

try this

first_result_ids = ClassDoc.only(:class_id).where(:recurrs => true).map(&:class_id)

answered Dec 5, 2012 at 15:16

abhas

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1 Comment

Thanks for the try but no (or negligible) time difference between the two.