How do I write a UNION chain with ActiveRelation? - ruby-on-rails

Question

I need to be able to chain an arbitrary number of sub-selects with UNION using ActiveRelation.
I'm a little confused by the ARel implementation of this, since it seems to assume UNION is a binary operation.
However:
( select_statement_a ) UNION ( select_statement_b ) UNION ( select_statement_c )
is valid SQL. Is this possible without doing nasty string-substitution?

Answer 1

You can do a bit better than what Adam Lassek has proposed though he is on the right track. I've just solved a similar problem trying to get a friends list from a social network model. Friends can be aquired automatically in various ways but I would like to have an ActiveRelation friendly query method that can handle further chaining. So I have
class User
has_many :events_as_owner, :class_name => "Event", :inverse_of => :owner, :foreign_key => :owner_id, :dependent => :destroy
has_many :events_as_guest, :through => :invitations, :source => :event
def friends
friends_as_guests = User.joins{events_as_guest}.where{events_as_guest.owner_id==my{id}}
friends_as_hosts = User.joins{events_as_owner}.joins{invitations}.where{invitations.user_id==my{id}}
User.where do
(id.in friends_as_guests.select{id}
) |
(id.in friends_as_hosts.select{id}
)
end
end
end
which takes advantage of Squeels subquery support. Generated SQL is
SELECT "users".*
FROM "users"
WHERE (( "users"."id" IN (SELECT "users"."id"
FROM "users"
INNER JOIN "invitations"
ON "invitations"."user_id" = "users"."id"
INNER JOIN "events"
ON "events"."id" = "invitations"."event_id"
WHERE "events"."owner_id" = 87)
OR "users"."id" IN (SELECT "users"."id"
FROM "users"
INNER JOIN "events"
ON "events"."owner_id" = "users"."id"
INNER JOIN "invitations"
ON "invitations"."user_id" =
"users"."id"
WHERE "invitations"."user_id" = 87) ))
An alternative pattern where you need a variable number of components is demonstrated with a slight modification to the above code
def friends
friends_as_guests = User.joins{events_as_guest}.where{events_as_guest.owner_id==my{id}}
friends_as_hosts = User.joins{events_as_owner}.joins{invitations}.where{invitations.user_id==my{id}}
components = [friends_as_guests, friends_as_hosts]
User.where do
components = components.map { |c| id.in c.select{id} }
components.inject do |s, i|
s | i
end
end
end
And here is a rough guess as to the solution for the OP's exact question
class Shift < ActiveRecord::Base
def self.limit_per_day(options = {})
options[:start] ||= Date.today
options[:stop] ||= Date.today.next_month
options[:per_day] ||= 5
queries = (options[:start]..options[:stop]).map do |day|
where{|s| s.scheduled_start >= day}.
where{|s| s.scheduled_start < day.tomorrow}.
limit(options[:per_day])
end
where do
queries.map { |c| id.in c.select{id} }.inject do |s, i|
s | i
end
end
end
end

Answer 2

Because of the way the ARel visitor was generating the unions, I kept getting SQL errors while using Arel::Nodes::Union. Looks like old-fashioned string interpolation was the only way to get this working.
I have a Shift model, and I want to get a collection of shifts for a given date range, limited to five shifts per day. This is a class method on the Shift model:
def limit_per_day(options = {})
options[:start] ||= Date.today
options[:stop] ||= Date.today.next_month
options[:per_day] ||= 5
queries = (options[:start]..options[:stop]).map do |day|
select{id}.
where{|s| s.scheduled_start >= day}.
where{|s| s.scheduled_start < day.tomorrow}.
limit(options[:per_day])
end.map{|q| "( #{ q.to_sql } )" }
where %{"shifts"."id" in ( #{queries.join(' UNION ')} )}
end
(I am using Squeel in addition to ActiveRecord)
Having to resort to string-interpolation is annoying, but at least the user-provided parameters are being sanitized correctly. I would of course appreciate suggestions to make this cleaner.

Answer 3

I like Squeel. But don't use it. So I came to this solution (Arel 4.0.2)
def build_union(left, right)
if right.length > 1
Arel::Nodes::UnionAll.new(left, build_union(right[0], right[1..-1]))
else
Arel::Nodes::UnionAll.new(left, right[0])
end
end
managers = [select_manager_1, select_manager_2, select_manager_3]
build_union(managers[0], managers[1..-1]).to_sql
# => ( (SELECT table1.* from table1)
# UNION ALL
# ( (SELECT table2.* from table2)
# UNION ALL
# (SELECT table3.* from table3) ) )

Answer 4

There's a way to make this work using arel:
tc=TestColumn.arel_table
return TestColumn.where(tc[:id]
.in(TestColumn.select(:id)
.where(:attr1=>true)
.union(TestColumn.select(:id)
.select(:id)
.where(:attr2=>true))))