Consider that we have two models;
SurveryPoints . A Well represent an oil/gas/water well, each with multiple SurveyPoints. A SurveyPoint represent the well’s coordinate at a certain depth.
class Well < ActiveRecord::Base has_many :survey_pointsendclass SurveyPoint < ActiveRecord::Base belongs_to :wellend
Say, we’d like to implement a filtration mechanism that allows us to query the wells – and for those set of wells we’d like to conditionally present the set of
survey_points for those set of filtered wells.
find_for_wells would returns the set of wells we’re interested in ( “ )
Using the power of AREL chaining, we could then conditionally extract the
SurveyPoints using the following:
def SurveyPoint.find_for_wells(wells) scope.where(:well_id => wells.select(:id))end
This roughly translates into the query:
SELECT * FROM SURVEY_POINTS WHERE WELL_ID IN (SELECT ID FROM WELLS WHERE ….)
The challenge here is that if both results is large, depending on the cardinality – the database engine might not be able to make use of defined indexes and the outer query might not be optimizable.
Instead – if we use pluck instead of select:
def self.find_for_wells(wells) scope.where(:well_id => wells.pluck(:id))end
Rails would fire two queries instead of an inner one. Pluck would first extract the list of ids (val_1, val_2, …) in an array before passing it to the main query. This roughly translates into:
SELECT * FROM SURVEY_POINTS WHERE WELL_ID IN (val_1, val_2, val_3, ….)
No inner query is used in that case and the main one is easy to optimize.