Ruby/Rails convert string to a class attribute

Question

Suppose I have a class Article, such that:

class Article

  attr_accessor :title, :author

  def initialize(title, author)
    @title = title
    @author= author
  end

end

Also, variable atrib is a String containing the name of an attribute. How could I turn this string into a variable to use as a getter?

a = Article.new
atrib='title'
puts a.eval(atrib)     # <---- I want to do this

EXTENDED

Suppose I now have an Array of articles, and I want to sort them by title. Is there a way to do the compact version using & as in:

col = Article[0..10]
sorted_one = col.sort_by{|a| a.try('title') }   #This works
sorted_two = col.sort_by(&:try('title'))   #This does not work

to_sym is not actually required a string is acceptable as well so you could call a.send(atrib) — engineersmnky
– engineersmnky, Commented Oct 31, 2013 at 15:18
@ArupRakshit I reduced it to a simple example. The tricky thing is that I've got the attribute name stored into a string. — lllllll
– lllllll, Commented Oct 31, 2013 at 15:20

ichigolas · Accepted Answer · 2021-05-15 19:50:34Z

6

You can use either send or instance_variable_get:

a = Article.new 'Asdf', 'Coco'
a.pubic_send(:title) # (Recommended) Tries to call a public method named 'title'. Can raise NoMethodError
=> "Asdf"
# If at rails like your case:
a.try :title # Tries to call 'title' method, returns `nil` if the receiver is `nil` or it does not respond to method 'title'
=> "Asdf"
a.send(:title) # Same, but will work even if the method is private/protected
=> "Asdf"
a.instance_variable_get :@title # Looks for an instance variable, returns nil if one doesn't exist
=> "Asdf"

Shot answer to your extended question: no. The &:symbol shortcut for procs relies on Symbol#to_proc method. So to enable that behavior you'd need to redifine that method on the Symbol class:

class Symbol
  def to_proc  
    ->(x) { x.instance_eval(self.to_s) }    
  end  
end

[1,2,3].map(&:"to_s.to_i * 10")
=> [10, 20, 30]

edited May 15, 2021 at 19:50

answered Oct 31, 2013 at 15:13

ichigolas

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5 Comments

pdu Over a year ago

Or .try, which would be a more graceful way.

lllllll Over a year ago

@pduersteler great insight! only applying to Rails though

lllllll Over a year ago

@nicooga Please see updated post, for a subquestion of the initial question.

timpone Over a year ago

what is the difference of these three different methods?

lllllll Over a year ago

@nicooga thanks for the answer to the extended part! I see why now

Stefan · Accepted Answer · 2013-10-31 19:14:20Z

1

ActiveRecord instances have an attributes hash:

a = Article.new(title: 'foo')
#=> <#Article id: nil, title: "foo">

atrib = 'title'
a.attributes[atrib]
#=> "foo"

You can use order to get sorted objects from your database:

Article.order('title').first(10)
#=> array of first 10 articles ordered by title

answered Oct 31, 2013 at 19:14

Stefan

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