How to create Rails query that (I think) requires PostgreSQL subquery? - ruby-on-rails

Question

Not sure how to do this so title may not be correct.
Each User has a field country of type String.
Given an array of user_id, country tuples for the query, find all the records that match. Each User must be found with it's own country.
For example, here is the array of tuples.
[1, 'us'],
[2, 'mexico'],
[3, 'us']
This would return User 1 if it exists and its country is 'us'.
It should also return User 2 if it exists and its country is 'mexico'.
The query should return all matching results.
Rails 4.2

Answer 1

class User < ApplicationRecord
def self.query_from_tuples(array_of_tuples)
array_of_tuples.inject(nil) do |scope, (id, country)|
if scope
scope.or(where(id: id, country: country))
else
where(id: id, country: country) # this handles the initial iteration
end
end
end
end
The resulting query is:
SELECT "users".* FROM "users"
WHERE (("users"."id" = $1 AND "users"."country" = $2 OR "users"."id" = $3 AND "users"."country" = $4) OR "users"."id" = $5 AND "users"."country" = $6)
LIMIT $7
You could also adapt kamakazis WHERE (columns) IN (values) query by:
class User < ApplicationRecord
def self.query_from_tuples_2(array_of_tuples)
# just a string of (?,?) SQL placeholders for the tuple values
placeholders = Array.new(array_of_tuples.length, '(?,?)').join(',')
# * is the splat operator and turns the tuples (flattened) into
# a list of arguments used to fill the placeholders
self.where("(id, country) IN (#{placeholders})", *array_of_tuples.flatten)
end
end
Which results in the following query which is a lot less verbose:
SELECT "users".* FROM "users"
WHERE ((id, country) IN ((1,'us'),(2,'mexico'),(3,'us'))) LIMIT $1
And can also perform much better if you have a compound index on [id, country].

Answer 2

I know this would work in pure SQL: e.g.
SELECT * FROM user
WHERE (id, country) IN ((1, 'us'), (2, 'mexico'), (3, 'us'))
Now I don't know how Rails would handle the bind parameter if it was a list of pairs (list of two elements each). Perhaps that would work.

Answer 3

You can construct a raw sql and use active record. Something like this:
def self.for_multiple_lp(arr=[])
# Handle case when arr is not in the expected format.
condition = arr.collect{|a| "(user_id = #{a[0]} AND country = #{a[1]})"}.join(" OR ")
where(condition)
end
Edit: Improved Solution
def self.for_multiple_lp(arr=[])
# Handle case when arr is not in the expected format.
condition = arr.collect{|a| "(user_id = ? AND country = ?)"}.join(" OR ")
where(condition, *(arr.flatten))
end
This should work.