Question

If I pass a String into Datetime column while creating new AR object, it will be automatically parse:

1.9.2p290 :011 > Movie.new(:release_date=>"21-Nov-1990")
 => #<Movie id: nil, release_date: "1990-11-21 00:00:00", created_at: nil, updated_at: nil>

How does Rails, or ActiveRecord, do this magic? Which method does it use?

Answer 1

Rails adds a to_date method to String. Its source is simple:

# File activesupport/lib/active_support/core_ext/string/conversions.rb, line 42
def to_date
  return nil if self.blank?
  ::Date.new(*::Date._parse(self, false).values_at(:year, :mon, :mday))
end

Date._parse is native to Ruby (the same method is called by Date.parse) and it’s where the real work is done.

It first uses a regular expression to remove extraneous symbols from the string, then passes it to other methods like _parse_eu, _parse_iso, _parse_dot and so on. Each of these uses its own regular expressions and other methods to see if it’s a date that it understands and extract the meaningful information from it. Once one of them “works” (i.e. returns true), the rest are skipped. Finally, back in _parse, the extracted information is used to build a date and time, doing a little more work to figure out things like checking for the day of the week and whether a year value of “12” should mean 1912 or 2012.

The docs call this a heuristic method, which could be taken to mean it throws a bunch of possibilities at the wall to see what sticks. It’s pretty poorly-documented but works remarkably well.