Rails：Date.today - 1.day

Question

Using the rails console, I just got bit by this:

Assume today is December 11.

Date.today-1.day   # December 10  (no spaces)
Date.today - 1.day # December 10  (a space on both sides of the minus sign)
Date.today -1.day  # December 11  whaaaat?
Date.today -5.days # Still december 11!

Can someone explain what's going on here? I am a little worried about how easy this could be missed in code. Any other suggestions on how to code this?

Answer 1

The difference you are seeing is caused by the way how ruby parses your code. In your case, you have stumbled about a syntactic nuance which greatly changes how the code is evaluated.

Your first two examples are actually expressions involving an operation on an object. Thus

Date.today-1.day
Date.today - 1.day

are both equivalent to

Date.today.send(:-, 1.day)
# a "fancy" way to write
Date.today - 1.day

Your second two examples

Date.today -1.day
Date.today -5.days

are actually direct method calls on the Date.today method and are thus equivalent to

Date.today( (-1).day )

This is because the minus is interpreted as a prefix to the 1 (and thus is parsed as the number -1). This interpretation is possible and doesn't throw any errors because of two properties of ruby:

• Parenthesis are optional when calling methods as long as the result is unambiguous
• Date.today accepts an optional argument denoting calendar used

The actual results you receive depend a bit on your environment as Rails seems to override some Date methods here. In plain Ruby, you would get different dates back but it would still work.

Answer 2

When you don't add a space after the minus sign, ruby takes it as a parameter for the function today. This function can receive one parameter. See here

Answer 3

Your problem is a little syntax problem.

This works fine for me

Date.today
# => Sun, 18 May 2014

Date.today() - 1.day
# => Sat, 17 May 2014

An prettier option is

Date.yesterday
# => Sat, 17 May 2014

Good luck!