Reputation: 4169
I've been digging around for this for awhile... I can't find a graceful solution. I have loans
and loans has_many :decisions
. decisions
has an attribute that I care about, called risk_rating
.
I'd like to sort loans
based on the most recent decision (based on created_at, per usual), but by the risk_rating
.
Loan.includes(:decisions).references(:decisions).order('decisions.risk_rating DESC')
doesn't work...
I want loans... sorted by their most recent decision's risk_rating. This seems like it should be easier than it is.
I'm currently doing this outside of the database like this, but it's chewing up time and memory:
Loan.all.sort do |x,y|
x.decisions.last.try(:risk_rating).to_f <=> y.decisions.last.try(:risk_rating).to_f
end
I'd like to show the performance I'm getting with the proposed answer, along with an inaccuracy...
Benchmark.bm do |x|
x.report{ Loan.joins('LEFT JOIN decisions ON decisions.loan_id = loans.id').group('loans.id').order('MAX(decisions.risk_rating) DESC').limit(10).map{|l| l.decisions.last.try(:risk_rating)} }
end
user system total real
0.020000 0.000000 0.020000 ( 20.573096)
=> [0.936775, 0.934465, 0.932088, 0.922352, 0.921882, 0.794724, 0.919432, 0.918385, 0.916952, 0.914938]
The order isn't right. That 0.794724
is out of place.
To that extent... I'm only seeing one attribute in the proposed answer. I don't see the connection =/
Upvotes: 1
Views: 255
Reputation: 80041
Alright, it looks like I'm working late tonight because I couldn't help but jump in:
class Loan < ApplicationRecord
has_many :decisions
has_one :latest_decision, -> { merge(Decision.latest) }, class_name: 'Decision'
end
class Decision < ApplicationRecord
belongs_to :loan
def latest
t1 = arel_table
t2 = arel_table.alias('t2')
# Self join based on `loan_id` prefer latest `created_at`
join_on = t1[:loan_id].eq(t2[:loan_id]).and(
t1[:created_at].lt(t2[:created_at]))
where(t2[:loan_id].eq(nil)).joins(
t1.create_join(t2, t1.create_on(join_condition), Arel::Nodes::OuterJoin)
)
end
end
Loan.includes(:latest_decision)
This doesn't sort, just provides the latest decision for each loan. Throwing an order
that references access_codes
messes things up because of the table aliasing. I don't have the time to work that kink out now, but I bet you can figure it out if you check out some of the great resources on Arel and how to use it with ActiveRecord. I really enjoy this one.
Upvotes: 2
Reputation: 2860
At first let's write sql-query which will select necessary data. SO contains a question which may helps here: Select most recent row with GROUP BY in MySQL. My best version:
SELECT loans.*
FROM loans
LEFT JOIN (
SELECT loan_id, MAX(id) as id
FROM decisions
GROUP BY loan_id) d ON d.loan_id = loans.id
LEFT JOIN decisions ON decisions.id = d.id
ORDER BY decisions.risk_rating DESC
This code suppose MAX(id)
gives id
of the recent row in group.
You may do the same query by this Rails code:
sub_query =
Decision.select('loan_id, MAX(id) as id').
group(:loan_id).to_sql
Loan.
joins("LEFT JOIN (#{sub_query}) d ON d.loan_id = loans.id").
joins("LEFT JOIN decisions ON decisions.id = d.id").
order("decisions.risk_rating DESC")
Unfortunately, I don't have MySQL at hand and I can't try this code. Hope it will work.
Upvotes: 1