Patch on ActiveRecord to randomize all select without order

database_randomizer.rb

# frozen_string_literal: true

# As PostgreSQL documentation states:
#   https://www.postgresql.org/docs/9.1/queries-order.html
#   The actual order in that case will depend on the scan and join plan types and the order on disk,
#   but it must not be relied on.
#
# Problem is, in 99% of the CI run, in tests, the order will be the same.
#   And in 1% of the run, regardless how many retries, the order will be different and the test will fail.
# Solution here is to "really" randomize requests so that retries will work,
#   and test will be marked as flaky instead of breaking the build.
#
# All these test cases queries should return a different order at each call:
#   Patient.pluck(:id, :last_name)
#   Patient.find_by(email: '')
#   Patient.first.update!(last_name: rand().to_s)
#   Patient.where(master_patient: MasterPatient.where.not(id: nil))
#   Patient.distinct
#   Patient.all.select('DISTINCT ON (email) id')
#   Patient.from(Patient.arel_table.create_table_alias(Patient.all.arel.union(Patient.all), :patients))
#
# But the use case where we inject to_sql in an union is impossible to handle, and should not be used in the codebase:
#   "select count(*) from (#{directory_doctors_query.select('1').to_sql} "\
#     "union all #{profiles_query.select('1').to_sql}) as doctors"
#
module DatabaseRandomizer
  RANDOM_AREL = Arel.sql('random()')

  # Add a random order on every non ordered select request.
  module RelationRandomOrder
    def build_order(arel)
      orders = order_values.uniq
      orders.reject!(&:blank?)
      if orders.empty? &&
        !distinct_value &&
        select_values.none? { |select_value| select_value.to_s.downcase.include?('distinct') }
        orders = [RANDOM_AREL]
      end
      arel.order(*orders)
    end
  end
  ActiveRecord::Relation.prepend(RelationRandomOrder)

  # But remove that order if we union the select (union do not support order by values).
  module ArelUnionRemovesRandomOrder
    def union(operation, other = nil)
      orders.delete(RANDOM_AREL)
      operation.arel.orders.delete(RANDOM_AREL)
      other.arel.orders.delete(RANDOM_AREL) if other.present?
      super(operation, other)
    end

    def intersect(other)
      orders.delete(RANDOM_AREL)
      other.orders.delete(RANDOM_AREL)
      super(other)
    end

    def except(other)
      orders.delete(RANDOM_AREL)
      other.orders.delete(RANDOM_AREL)
      super(other)
    end
  end
  Arel::SelectManager.prepend(ArelUnionRemovesRandomOrder)
end