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# Here's a contrived example of a LEFT JOIN using ARel. This is an example of | |
# the mechanics, not a real-world use case. | |
# NOTE: In the gist comments, @ozydingo linked their general-purpose ActiveRecord | |
# extension that works for any named association. That's what I really wanted! | |
# Go use that! Go: https://gist.github.com/ozydingo/70de96ad57ab69003446 | |
# == DEFINITIONS | |
# - A Taxi is a car for hire. A taxi has_many :passengers. | |
# - A Passenger records one person riding in one taxi one time. It belongs_to :taxi. | |
class Taxi < ActiveRecord::Base | |
# This scope LEFT JOINs the Passenger table. You might use this if you wanted | |
# to select taxis by a set of taxi and passenger IDs (as with the | |
# SQL "taxis.id IN () OR passengers.id IN ()"). | |
def self.left_join_passengers | |
taxis = Taxi.arel_table | |
passengers = Passenger.arel_table | |
# The key here is providing the join type (Arel::Nodes::OuterJoin) and | |
# grabbing the ARel join object itself from Arel::SelectManager#join_sources | |
# to feed to ActiveRecord's #joins method. | |
# SQL equivalent: "LEFT OUTER JOIN passengers ON taxis.id = passengers.taxi_id" | |
taxi_passengers = taxis.join(passengers, Arel::Nodes::OuterJoin). | |
on(taxis[:id].eq(passengers[:taxi_id])). | |
join_sources | |
joins(taxi_passengers) | |
end | |
end | |
# Sources that almost documented this: | |
# http://blog.donwilson.net/2011/11/constructing-a-less-than-simple-query-with-rails-and-arel/ |
Why method without self. ?
Good point, @araslanov-e. My Rails is pretty rusty now, but I think you're right; the method should be def self.left_join_passengers
. Fixed.
Thank you for this, join_sources is my new best friend! I've generalized this to do essentially the same method but for any named association: https://gist.github.com/ozydingo/70de96ad57ab69003446
@ozydingo That's fantastic! That's what I actually wanted in ActiveRecord. Thanks!
👍 ❤️
Thank you.
Thank you for this. I have implemented this in our code base. I found one issue however, in that that it didn't work when on Polymorphic associations because original relation's bind values were not present on the new relation. https://gist.github.com/braindeaf/bceb8244416dc9c7035ad47a76b439b9 This seems to resolve things.
Thank you.
Thanks! 👍
Thanks!
Can we further join taxi_passengers
with another table ?
ah, I've been looking for the
join_sources
thing! thanks for the gist!