How do I mimic a "SELECT _id" in mongoid and ruby? - mongodb

Question

Currently I am doing the following:
responses = Response.where(user_id: current_user.uid)
qids = []
responses.each { |r| qids << r._id}
return qids
Any better way of doing this?

Answer 1

Use .only() to retrieve less data.
quids = Response.only(:_id).where(user_id: current_user.uid).map(&:_id)

Answer 2

Response.where(user_id: current_user.uid).map { |r| r._id }
That's a bit more idiomatic.
As far as mongoid is concerned, the only 'mapping' type functionality that mongoid offers are custom map-reduce filters. You can check out the documentation.
In this case, writing such a filter will not be to your advantage. You are loading the entire dataset (lazy-loading doesn't help) and you aren't reducing anything.

Answer 3

Straight to the point better solution for this problem
In case you want to get the id or something that's unique in the result set, then it is functionally equivalent to use the distinct method. That way you save the mapping operation and it seems to be much faster (The tests and why you should perhaps take them with a bit of a precaution is explained at the bottom).
Response.where(user_id: current_user.uid).distinct(:_id)
So only use this if you want to get something non-unique and for some reason want to get duplicate results. I.e. if your responses could be liked and if you wanted to get an array of all likes (say you wanted to calculate some statistics about likings):
Response.where(user_id: current_user.uid).map { |r| r.likes }
Testing...
Here's some random tests, but for more trust worthy results one should commit the tests with a large database, instead of repeating the action. I mean for all I know there can be any sort of optimizations for repeating the same query all over again (where obviously the map can't have any such optimizations).
Benchmark.measure { 1000.times { Organization.where(:event_labels.ne =[]).map(&:_id) } }
=> 6.320000 0.290000 6.610000 ( 6.871498)
Benchmark.measure { 1000.times { Organization.where(:event_labels.ne => []).only(:_id).map(&:_id) } }
=> 5.490000 0.140000 5.630000 ( 5.981122)
Benchmark.measure { 1000.times { Organization.where(:event_labels.ne => []).distinct(:_id) } }
=> 0.570000 0.020000 0.590000 ( 0.773239)
Benchmark.measure { 1000.times { Organization.where(:event_labels.ne => []).only(:_id) } }
=> 0.140000 0.000000 0.140000 ( 0.141278)
Benchmark.measure { 1000.times { Organization.where(:event_labels.ne => []) } }
=> 0.070000 0.000000 0.070000 ( 0.069482)
Doing map without only takes a bit longer, so using only is beneficial. Though using it seems to actually slightly hurt the performance if you don't do map at all, but having less data seems to make map run a bit faster. Anyhow, according to this test, it seems distinct is about 10 times faster on all metrics (user, system, total, real) than using the only and map combo though it's slower than using only without map.