Cleaner Code for custom query

Question

Is there a cleaner way to do the following?

friend_ids = [1,2,3,4,5]
friendIDsQuery = ""
friend_ids.each_with_index do |friend_id, index|
  friendIDsQuery += "SELECT id FROM test WHERE user_id = #{friend_id}"

  if index != friend_ids.size - 1
    friendIDsQuery += " INTERSECT "
  end
end

Basically I'm passing in an array of IDs, and building a custom INTERSECT query.

Answer 1

This would be the more idiomatic Rails form, without so much SQL.

friend_ids.
  map {|fid| Test.select(:id).where(:user_id => fid) }.
  reduce {|a,e| a = a & e }

If you really need the sql, then append:

.to_sql

Answer 2

query = friend_ids.map {|id| "SELECT id FROM test WHERE user_id=#{id}"}.join " INTERSECT "

Bam.

Edit: Ok, well, not just bam. First of all, if you're using Rails, go with something like NewAlexandria's answer, because it doesn't involve raw string interpolation (which quickly leads to SQL injection vulnerabilities). To make the above a little more safe (and assuming the IDs are integers), you should probably do

query = friend_ids.map { |id|
  "SELECT id FROM test WHERE user_id = #{id.to_i}"
}.join " INTERSECT "