ActiveRecord/ARel modify `ON` in a left out join from includes - sql

Question

I'm wondering if it's possible to specify additional JOIN ON criteria using ActiveRecord includes?
ex: I'm fetching a record and including an association with some conditions
record.includes(:other_record).where(:other_record => {:something => :another})
This gives me (roughly):
select * from records
left outer join other_records on other_records.records_id = records.id
where other_records.something = another
Does anyone know how I can specify an extra join condition so I could achieve something like.
select * from records
left outer join other_records on other_records.records_id = records.id
and other_records.some_date > now()
where other_records.something = another
I want my includes to pull in the other_records but I need additional criteria in my join. Anything using ARel would also be great, I've just never known how to plug a left outer join from ARel into and ActiveRecord::Relation

Answer 1

I can get you close with ARel. NOTE: My code ends up calling two queries behind the scenes, which I'll explain.
I had to work out LEFT JOINs in ARel myself, recently. Best thing you can do when playing with ARel is to fire up a Rails console or IRB session and run the #to_sql method on your ARel objects to see what kind of SQL they represent. Do it early and often!
Here's your SQL, touched up a bit for consistency:
SELECT *
FROM records
LEFT OUTER JOIN other_records ON other_records.record_id = records.id
AND other_records.some_date > now()
WHERE other_records.something = 'another'
I'll assume your records model is Record and other_records is OtherRecord. Translated to ARel and ActiveRecord:
class Record < ActiveRecord::Base
# Named scope that LEFT JOINs OtherRecords with some_date in the future
def left_join_other_in_future
# Handy ARel aliases
records = Record.arel_table
other = OtherRecord.arel_table
# Join criteria
record_owns_other = other[:record_id].eq(records[:id])
other_in_future = other[:some_date].gt(Time.now)
# ARel's #join method lets you specify the join node type. Defaults to InnerJoin.
# The #join_sources method extracts the ARel join node. You can plug that node
# into ActiveRecord's #joins method. If you call #to_sql on the join node,
# you'll get 'LEFT OUTER JOIN other_records ...'
left_join_other = records.join(other, Arel::Nodes::OuterJoin).
on(record_owns_other.and(other_in_future)).
join_sources
# Pull it together back in regular ActiveRecord and eager-load OtherRecords.
joins(left_join_other).includes(:other_records)
end
end
# MEANWHILE...
# Elsewhere in your app
Record.left_join_other_in_future.where(other_records: {something: 'another'})
I bottled the join in a named scope so you don't need to have all that ARel mixed in with your application logic.
My ARel ends up calling two queries behind the scenes: the first fetches Records using your JOIN and WHERE criteria, the second fetches all OtherRecords "WHERE other_records.record_id IN (...)" using a big list of all the Record IDs from the first query.
Record.includes() definitely gives you the LEFT JOIN you want, but I don't know of a way to inject your own criteria into the join. You could use Record.joins() instead of ARel if you wanted to write the SQL yourself:
Record.joins('LEFT OUTER JOIN other_records' +
' ON other_records.record_id = records.id' +
' AND other_records.some_date > NOW()')
I really, really prefer to let the database adapter write my SQL, so I used ARel.
If it were me, I'd consider putting the additional join criterion in the WHERE clause. I assume you're asking because putting the additional criterion on the join makes the query's EXPLAIN look better or because you don't want to deal with NULLs in the other_records.some_date column when there aren't any related other_records.

Answer 2

If you have a simple (equality) extra join condition it could simply be
record.includes(:other_record).where(:other_record => {:something => :another,
:some_date => Time.now})
But if you need the greater than comparison the following should do it.
record.includes(:other_record).where([
'other_records.something = ? and other_records.some_date > ?',
another, Time.now])
Hope that helps.