Object # 在 Rails 中的存在意义是什么？

Here's the point:

''.presence
# => nil

so if params[:state] == '':

region = params[:state].presence || 'US'
# => 'US'
region = params[:state] || 'US'
# => ''

What's more, it works in similar way (that is, returns nil if object is 'empty') on every object that responds to empty? method, for example:

[].presence
# => nil

Here's the documentation, for reference:

http://api.rubyonrails.org/classes/Object.html#method-i-presence

I just used it in a useful way I found neat. My variable is a string, if it's the empty string i want nil, otherwise I want it converted to an integer.

 x.presence.try(&:to_i)


"".presence.try(&:to_i) # => nil
"22".presence.try(&:to_i) # => 22

presence is very useful when you want to return nil if object is not present and the object itself if the object is present. In other words you want a code that looks like this:

object.present? object : nil

Instead of the line above you can simply call object.presence and the method will do the work for you.

As another example, presence lets me present my favorite FizzBuzz solution:

puts 1.upto(100).map { |n| "#{'Fizz' if n%3==0}#{'Buzz' if n%5==0}".presence || n }

The real point of using #presence is that it expands the notion of falsey values to handle web and http scenarios. The docs don't make this purpose clear ... instead simply focussing on the method's API: The what but not the why. Web and HTTP is different from normal programming because a blank string is often what you get instead of a nil from a request.

In plain Ruby, however, an empty string is truthy. This makes web developers write a lot of redundant boilerplate code like the docs for Object.presence uses as its example, as others here have quoted.

The bottom line for people writing web applications in Rails is that we can (should) now use #present? and #presence with the standard Ruby short-circuiting or, ||:

# Check for a param like this
@name = params[:name].presence || 'No name given'

That line properly handles everything the web server packs into the request params for us. While this plain old ruby does not:

# DON'T DO THIS
@name = params[:name] || 'No name given'