Atomic inc of multiple mongoid hash values

Question

I have a Mongoid model with a hash field that looks like this:

class DimensionStat
 include ::Mongoid::Document
 include ::Mongoid::Timestamps

 field :data, type: Hash

 attr_accessible :data
end

I pretend on runtime fill data with something like

data: {
    'a' => 1,
    'b' => 2,
    ...
}

I need to perform increments on multiple keys atomically such as:

'a' => -1,
'b' => 5

Somewhere I found that:

instance.collection.find(_id: my_id).update("$inc" => {'data.a' => -1,
'data.b' => 5})

will do the trick but it doesn't, what am I doing wrong?

UPDATE: I'm using mongoid 3.1.6

What do you mean by "doesn't work"? I do that sort of thing all the time and it works fine. What does `data` look like inside that document inside MongoDB? BTW, `:$inc` is a valid symbol so you can say `:$inc => { ... }` if you'd like. — mu is too short, Jun 26 '14 at 05:13
But what does `data` look like inside MongoDB *before* you try to do anything to it? — mu is too short, Jun 26 '14 at 18:06
It does not exists on the document, it does not get dynamically created with mongoid when trying to update this field? — Carlos Castellanos, Jun 26 '14 at 19:05
No, MongoDB won't autovivify the hash for you. You use `:$inc` to increments things that are already there. — mu is too short, Jun 26 '14 at 19:17
@mu is too short field :data, type: Hash, default: {}, worked, put it as an answer for me to accept it — Carlos Castellanos, Jun 27 '14 at 11:48

score 3 · Accepted Answer · answered Dec 02 '14 at 19:36

The .inc method works perfectly with non-existing keys, but the document is not automatically refreshed from the database. Try:

dimension_stat.inc('data.a' => -1, 'data.b' => 5)
puts dimension_stat.data['a'] # not changed
dimension_stat.reload
puts dimension_stat.data['a'] # changed

you will see that the counters have changed.

Atomic inc of multiple mongoid hash values

1 Answers1