module Gitlab
  module Database
    module DateTime
      # Find the first of the `end_time_attrs` that isn't `NULL`. Subtract from it
      # the first of the `start_time_attrs` that isn't NULL. `SELECT` the resulting interval
      # along with an alias specified by the `as` parameter.
      #
      # Note: For MySQL, the interval is returned in seconds.
      #       For PostgreSQL, the interval is returned as an INTERVAL type.
      def subtract_datetimes(query_so_far, end_time_attrs, start_time_attrs, as)
        diff_fn = if Gitlab::Database.postgresql?
                    Arel::Nodes::Subtraction.new(
                      Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(end_time_attrs)),
                      Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(start_time_attrs)))
                  elsif Gitlab::Database.mysql?
                    Arel::Nodes::NamedFunction.new(
                      "TIMESTAMPDIFF",
                      [Arel.sql('second'),
                       Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(start_time_attrs)),
                       Arel::Nodes::NamedFunction.new("COALESCE", Array.wrap(end_time_attrs))])
                  end

        query_so_far.project(diff_fn.as(as))
      end
    end
  end
end