ActiveRecord join through two associations - ruby-on-rails

Question

How do I get an ActiveRecord collection with all the unique Jobs that exist for a particular User given that there are two possible associations?
class User
has_many :assignments
has_many :jobs, through: assignments
has_many :events
has_many :jobs_via_events, through: :events, source: :jobs
end
class Assignment
belongs_to :user
belongs_to :job
end
class Event
belongs_to :user
belongs_to :job
end
class Job
has_many :assignments
has_many :events
end
The best I could come up with so far is using multiple joins, but it isn't coming back with correct results:
Job.joins("INNER JOIN assignments a ON jobs.id = a.job_id").joins("INNER JOIN events e ON jobs.id = e.job_id").where("a.user_id = ? OR e.user_id = ?", userid, userid)
Any advice?

Answer 1

A simple approach is to collect all job-ids first and then fetch all jobs for those ids.
We first ask for an array of all job-ids for a user:
event_job_ids = Event.where(user_id: user).select(:job_id).map(&:job_id)
assignment_job_ids = Assignment.where(user_id: user).select(:job_id).map(&:job_id)
all_job_ids = (event_job_ids + assignment_job_ids).uniq
note that, depending on the rails version you use, it is better to replace the select(:job_id).map(&:job_id) by a .pluck(:job_id) (which should be more efficient, but the select.map
Getting the jobs for the assembled ids is then straightforward:
jobs = Job.where(id: all_job_ids)
This is the naive approach, how can we approve upon this?
If I would write a query I would write something like the following
select * from jobs
where id in (
select job_id from assignments where user_id=#{user_id}
union
select job_id from events where user_id=#{user_id}
)
so how do we convert this to a scope?
class User
def all_jobs
sql = <<-SQL
select * from jobs
where id in (
select job_id from assignments where user_id=#{self.id}
union
select job_id from events where user_id=#{self.id}
)
SQL
Job.find_by_sql(sql)
end
Of course this is untested code, but this should get you started?