module Gitlab
  module SQL
    # Class for building SQL UNION statements.
    #
    # ORDER BYs are dropped from the relations as the final sort order is not
    # guaranteed any way.
    #
    # Example usage:
    #
    #     union = Gitlab::SQL::Union.new(user.personal_projects, user.projects)
    #     sql   = union.to_sql
    #
    #     Project.where("id IN (#{sql})")
    class Union
      def initialize(relations, remove_duplicates: true)
        @relations = relations
        @remove_duplicates = remove_duplicates
      end

      def to_sql
        # Some relations may include placeholders for prepared statements, these
        # aren't incremented properly when joining relations together this way.
        # By using "unprepared_statements" we remove the usage of placeholders
        # (thus fixing this problem), at a slight performance cost.
        fragments = ActiveRecord::Base.connection.unprepared_statement do
          @relations.map { |rel| rel.reorder(nil).to_sql }.reject(&:blank?)
        end

        if fragments.any?
          fragments.join("\n#{union_keyword}\n")
        else
          'NULL'
        end
      end

      def union_keyword
        @remove_duplicates ? 'UNION' : 'UNION ALL'
      end
    end
  end
end