Please explain this ruby code (in rails)

Question

Basically question is about ruby syntax

class Person < ActiveRecord::Base
      validates :name, presence: true #no error
end

Code Source

My 2 Questions

1

To me this code validates :name, presence: true looks a like call to a method.

How can we call a method inside class body, outside any method? I think its not possible in oops.

2

Why I am getting error in these two variations

validates (:name ,  presence: true) #error

or

validates (:name ,  {presence: true}) #error

I have added parentheses to method call, as its allowed in ruby. First parameter is symbol and 2nd parameter is hash.

In first case I have not added curly braces to hash, as I read that if last parameter is hash you can leave curly braces, In second code I have explicitly added curly braces but still got error.

Can anyone explain this syntax.

Thanks

Edit 1

Errors

In first I am getting

product.rb:8: syntax error, unexpected ',', expecting ')' validates (:name , presence: true) # error ^

In Second I am getting

product.rb:10: syntax error, unexpected ',', expecting ')' validates (:title , {presence: true}) # error ^

What error do you get? You are aware thet you're using :name in the first example, and :title in the second. That hasn't got anything to do with it? — harald
– harald, Commented Nov 2, 2015 at 18:42
In first case i am getting this error. "product.rb:8: syntax error, unexpected ',', expecting ')' validates (:name , presence: true) # error ^" — Praveen Prasad
– Praveen Prasad, Commented Nov 2, 2015 at 18:46
Ruby is strongly influenced by Smalltalk which is perhaps the quintessential example of object-oriented programming. That you can call methods when defining a class does not take away from this. It's a powerful feature, though one that might seem impossible or strange if coming from a Java or C++ background. — tadman
– tadman, Commented Nov 2, 2015 at 19:03

harald · Accepted Answer · 2015-11-02 19:01:45Z

3

1: In ruby you can call a method when defining a class:

% irb
2.2.2 :001 > class Dummy
2.2.2 :002?>   puts "Making a class..."
2.2.2 :003?>   def hello
2.2.2 :004?>     puts "Hello"
2.2.2 :005?>   end
2.2.2 :006?> end
Making a class...
 => :hello 
2.2.2 :007 > d = Dummy.new
 => #<Dummy:0x000000009ebbf0> 
2.2.2 :008 > d.hello
Hello
 => nil

So that's exactly what's going on.

2: You get an error because you have a space between the method name and the argument list:

% irb
2.2.3 :001 > def func(*splat)
2.2.3 :002?>   puts splat.inspect
2.2.3 :003?> end
 => :func 
2.2.3 :004 > func(:test, :another => :test)
[:test, {:another=>:test}]
 => nil 
2.2.3 :005 > func (:test)
[:test]
 => nil 
2.2.3 :006 > func (:test, :another => :test)
SyntaxError: (irb):6: syntax error, unexpected ',', expecting ')'
func (:test, :another => :test)
            ^
        from /home/haraldei/.rvm/rubies/ruby-2.2.3/bin/irb:11:in `<main>'

The second example above, where I'm passing just one arg works because you can enclose any valid expression in parenthesis. This is not the same as an argument list. So the expression:

(:test, :another => :test)

is not a valid expression, but the parser tries to pass it as one parenthesized argument to the method.

So to summarize, both your argument lists are correct, if you remove the space between them and the function name.

edited Nov 2, 2015 at 19:01

answered Nov 2, 2015 at 18:43

harald

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1 Comment

tadman Over a year ago

More specifically: (:test, { :another => : test }) to make the fact that the second argument is a Hash.

sugaryourcoffee · Accepted Answer · 2015-11-04 19:27:43Z

2

The answer to your first question: "Yes it is a method" and this is also somehow the answer to your second question.

The answer to your second question is "remove the space between validates and (". When having validates (...) it will throw

syntax error, unexpected ',', expecting ')' (SyntaxError)
  validates (:name ,  presence: true)

validates is a method, and if using parentheses you mustn't use spaces.

edited Nov 4, 2015 at 19:27

answered Nov 2, 2015 at 18:56

sugaryourcoffee

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