Using Scopes on Arel Join Queries

scopes_on_joins.rb

# Let's say you have a join between Address and Job
# You want to use a scope on Job to limit the jobs that have to be joined otherwise you would join all jobs.
# So you try this
Address.joins('INNER JOIN jobs ON jobs.id = addresses.addressable_id AND addresses.addressable_type = "Job" AND ' \ 
'addresses.address_type = "physical"').where(Job.in_the_future)

# What you get is an error like so
#  Job Load (1.1ms)  SELECT `jobs`.* FROM `jobs` WHERE (start_datetime > '2014-04-28 16:06:12')
# (pry) output error: #<TypeError: Cannot visit ActiveRecord::Relation::ActiveRecord_Relation_Job>

# You see the condition is correct in the SQL but the query is referencing Job
# What you need to be able to extract the conditions out of the scope
Address.joins
('INNER JOIN jobs ON jobs.id = addresses.addressable_id AND addresses.addressable_type = "Job" AND ' \ 
'addresses.address_type = "physical"').where(Job.in_the_future.arel.where_clauses)

# Now you'll have a SQL query that runs properly and looks like this
"SELECT `addresses`.* FROM `addresses` INNER JOIN jobs ON jobs.id = addresses.addressable_id AND addresses.addressable_type = \"Job\" AND addresses.address_type = \"physical\" WHERE ((start_datetime > '2014-04-28 16:13:21'))"