Is there built in support in rails for the default value substitution idiom?

I often write code to provide a default value upon encountering nil/empty value.

E.g:

category = order.category || "Any"
#  OR
category = order.category.empty? ? "Any" 开发者_如何学编程: order.category

I am about to extend the try method to handle this idiom.

category = order.try(:category, :on_nill => "Any")
#  OR
category = order.try(:category, :on_empty=> "Any")

I am wondering if Rails/Ruby has some method to handle this idiom?

Note:

I am trying to eliminate repetition of || / or / ? operator based idioms.

Essentially I am looking for a equivalent of try method for handling default substitution scenarios.

Without try method:

product_id = user.orders.first.product_id unless user.orders.first.nil? 

With try method:

product_id = user.orders.first.try(:product_id)

It is easy to implement a generic approach to handle this idiom, but I want to make sure I do not reinvent the wheel.

---

See this question. ActiveSupport adds a presence method to all objects that returns its receiver if present? (the opposite of blank?), and nil otherwise.

Example:

host = config[:host].presence || 'localhost'

---

Perhaps this might serve:

class Object
  def subst_if(condition, replacement)
    condition = send(condition) if condition.respond_to?(:to_sym)
    if condition
      replacement
    else  
      self
    end
  end
end

Used like so:

p ''.subst_if(:empty?, 'empty')       # => "empty"
p 'foo'.subst_if(:empty?, 'empty')    # => "foo"

It also takes stand-alone conditions, not related to the object:

p 'foo'.subst_if(false, 'bar')    # => 'foo'
p 'bar'.subst_if(true,  'bar')    # => 'bar'

I'm not crazy about the name subst_if. I'd borrow whatever name Lisp uses for this function, if I knew it (assuming it exists).

---

Pretty sure its not baked in. Here is a link to a similar question/answer. It is the approach that I take. Leveraging the ruby: ||= syntax

An aside: This question also reminds me of the first Railscasts of all time: Caching with instance variables which is a useful screencast if you need to do this kind of operation in a Controller

---

fetch will work in the case that you are looking for values by index values of an array or by keys in a hash ( or ActionController::Parameters aka params ). Otherwise, you will have to use the ||= or attempted_value || default_value as described in the other answers.

The current accepted answer's example is rewritten here:

assuming: config = { my_host_ip: '0.0.0.0' }

 config.fetch(:my_host_ip, 'localhost')
 # => "0.0.0.0" 

 config.fetch(:host, 'localhost')
 # => "localhost"

 a_thru_j = ('a'..'j').to_a
 # => ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"]

 a_thru_j.fetch(1, 'not here')
 # => "b"

 a_thru_j.fetch(10, 'not here')
 # => "not here"
 

Note: without the default, you will raise an error when the key/index is not present:

 config.fetch(:host)
 # => KeyError: key not found: :host

 params.fetch(:host)
 # => ActionController::ParameterMissing: param is missing or the value is empty: host

 a_thru_j.fetch(10)
 # => index 10 outside of array bounds: -10...10