-
Type:
Task
-
Resolution: Done
-
Affects Version/s: None
-
Component/s: None
-
None
Had to debug a performance issue that broke our app yesterday.
Turned out it was due to Mongoid making a non wanted query (on a non indexed field) when requesting a belongs_to association with a reverse has_one declaration.
Reproduce on Mongoid 3.1.2 with:
class Video include Mongoid::Document belongs_to :screenshot end class Image include Mongoid::Document end class Screenshot < Image has_one :video end Video.delete_all Screenshot.delete_all Video.create(screenshot: Screenshot.create) video = Video.last Moped.logger = Logger.new($stdout) video.screenshot # Normal query: MOPED: 127.0.0.1:27017 QUERY database=analytics_test collection=images selector={"$query"=>{"_id"=>"516e3cbac3b5a07a2c000001", "_type"=>{"$in"=>["Screenshot"]}}, "$orderby"=>{:_id=>1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil (0.4778ms) # Extra query: MOPED: 127.0.0.1:27017 QUERY database=analytics_test collection=vs selector={"$query"=>{"screenshot_id"=>"516e3cbac3b5a07a2c000001"}, "$orderby"=>{:_id=>1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil (0.2599ms)
Without the has_one declaration the extra query disappears:
# ... class definitions ... class Screenshot < Image # has_one :video <- removed this end # ... object stuff ... video.screenshot # Normal query only: MOPED: 127.0.0.1:27017 QUERY database=analytics_test collection=images selector={"$query"=>{"_id"=>"516e3cbac3b5a07a2c000001", "_type"=>{"$in"=>["Screenshot"]}}, "$orderby"=>{:_id=>1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil (0.4778ms)
We survived by adding an index on screenshot_id. But I guess that query should never have happened in the first place.