Use field from previous query result in another query in mongodb

Question

I have below query, in 2nd query I get null for que.name.

var que  = db.que.find({ _id: u.que_id }, {name:1})
db.documents.update(u, {$set: {que_name: que.name}})

I can see data for que as below

 { "_id" : "general", "name" : "General" }

but no data for que.name. What's wrong?

what are you trying to achieve actually? transfer name attribute of que collection to documents collection as que_name ? — erdimeola
– erdimeola, Commented Aug 20, 2013 at 13:42

Sergey Berezovskiy · Accepted Answer · 2013-08-20 14:34:41Z

2

find returns cursor to the matching documents. So que is not a single document - it is cursor. Cursor do not have field name - that's why you have no data for que.name. Cursor points to documents which you have found (yes, there could be several documents). If you will iterate over cursor, you will get each document. Just to show what is cursor:

var que  = db.que.find({ _id: u.que_id }, { name: 1 }) // que is a cursor
que.forEach(function(d) { print(d.name); })

If you want to get document, then use findOne method, which returns one document matching your search criteria:

var que  = db.que.findOne({ _id: u.que_id }, { name: 1 }) // now que is document
que.name // prints name value

edited Aug 20, 2013 at 14:34

answered Aug 20, 2013 at 14:09

Sergey Berezovskiy

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Comments

Makis Tsantekidis · Accepted Answer · 2013-08-20 14:11:20Z

1

db.collection.find returns a cursor. Try using findOne()

var que = db.que.findOne( { _id: u.que_id }, {name:1})

answered Aug 20, 2013 at 14:11

Makis Tsantekidis

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2 Comments

Sergey Berezovskiy Over a year ago

That is exactly what I wrote 0_o

Makis Tsantekidis Over a year ago

Excuse me i didn't refresh before posting... I didn't remove it after i saw your post because i think mine is smaller and more concise. If you think i should remove it, i can, but in no way i copied your answer. (MongoDB documentation is no secret)

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