Unique join table reference inside model scope - ruby-on-rails

Question

I have 2 models with a has_many association:
class Log < ActiveRecord::Base
has_many :log_details
end
and
class LogDetail < ActiveRecord::Base
belongs_to :log
end
The Log table has an action_type string column. The LogDetail table has 2 columns: key and value, both string, and a reference back to the Log table with a log_id.
I want to write 3 scopes on the Log model to query for some details joining the Log table with LogModel twice. Here's a sample:
has_many :payment_gateways, -> {where(key: 'payment_gateway')}, class_name: 'LogDetail', foreign_key: :log_id
has_many :coupon_codes, -> {where(key: 'coupon_code')}, class_name: 'LogDetail', foreign_key: :log_id
scope :initiate_payment, -> {where(action_type: 'INITIATE PAYMENT')}
scope :payment_gateway, -> (pg) {joins(:payment_gateways).where(log_details: {value: pg}) unless pg.blank?}
scope :coupon_code, -> (cc) {joins(:coupon_codes).where(log_details: {value: cc}) unless cc.blank?}
Using the above scopes, if I try to query for
Log.initiate_payment.payment_gateway('sample_pg').coupon_code('sample_cc')
I get the SQL query:
SELECT
`logs`.*
FROM
`logs`
INNER JOIN
`log_details` ON `log_details`.`log_id` = `logs`.`id`
AND `log_details`.`key` = 'payment_gateway'
INNER JOIN
`log_details` `coupon_codes_logs` ON `coupon_codes_logs`.`log_id` = `logs`.`id`
AND `coupon_codes_logs`.`key` = 'coupon_code'
WHERE
`logs`.`action_type` = 'INITIATE PAYMENT'
AND `log_details`.`value` = 'sample_pg'
AND `log_details`.`value` = 'sample_cc'
instead of: (notice the difference in the last AND condition)
SELECT
`logs`.*
FROM
`logs`
INNER JOIN
`log_details` ON `log_details`.`log_id` = `logs`.`id`
AND `log_details`.`key` = 'payment_gateway'
INNER JOIN
`log_details` `coupon_codes_logs` ON `coupon_codes_logs`.`log_id` = `logs`.`id`
AND `coupon_codes_logs`.`key` = 'coupon_code'
WHERE
`logs`.`action_type` = 'INITIATE PAYMENT'
AND `log_details`.`value` = 'sample_pg'
AND `coupon_codes_logs`.`value` = 'sample_cc'
The first query, because it doesn't resolve the join table references properly, gives me zero results.
How can I modify my scopes/models in such a way to generate the correct query? I think I need a reference to the join table alias inside the scope's where clause, but I'm not sure how to get that reference.

Answer 1

Sadly, ActiveRecord has no built-in way to specify the alias used when joining an association. Using merge to try merging the two scopes also fails as the condition is overridden.
3 solutions:
Use Arel to alias a joins, but that's a bit hard to read, and you still need to repeat the association definition for payment_gateways and coupon_codes
Join directly in SQL:
scope :payment_gateway, -> (pg) { joins(<<-SQL
INNER JOIN log_details payment_gateways
ON payment_gateways.log_id = logs.id
AND payment_gateways.key = 'payment_gateway'
AND payment_gateways.value = #{connection.quote(pg)}
SQL
) if pg.present? }
But you need to add manually the conditions already defined in the associations
Finally, my favorite, a solution that sticks to ActiveRecord:
scope :payment_gateway, -> (pg) do
where(id: unscoped.joins(:payment_gateways).where(log_details: {value: pg})) if pg.present?
end
scope :coupon_code, -> (cc) do
where(id: unscoped.joins(:coupon_codes).where(log_details: {value: cc})) if cc.present?
end
Gotcha #1: if you use Rails < 5.2, you might need to use class methods instead of scopes.
Gotcha #2: Solution #3 might be less performant than #2, make sure to EXPLAIN ANALYZE to see the difference.