Differences between Named Scopes, Lambdas and Procs

Question

What is the exact difference between Named Scopes(or Scopes in Rails 4), Lambdas and Procs?

Between Lambda and Proc, which one to choose?

Answer 1

1. Proc doesn't check the parameters passed, but lambda does

 proc1 = Proc.new { |a, b| a + 5 }

 proc1.call(2)      # call with only one parameter
 => 7               # no error, unless the block uses the 2nd parameter

 lambda1 = lambda { |a, b| a + 5 }

 lambda1.call(2)
 => ArgumentError: wrong number of arguments (1 for 2)

Proc will throw error only if the block uses the second param.

 proc2 = Proc.new { |a, b| a + b }

 proc2.call(2)      # call with only one parameter
 => TypeError: nil can't be coerced into Fixnum

2. Proc returns from the calling method, while lambda doesn't

 def meth1
   Proc.new { return 'return from proc' }.call
   return 'return from method'
 end

 puts meth1
 => return from proc

 def meth2
   lambda { return 'return from lambda' }.call
   return 'return from method'
 end

 puts meth2
 => return from method

If they are not called inside a method,

 proc1 = Proc.new { return "Thank you" } 

 proc1.call 
 => LocalJumpError: unexpected return

 lambda1 = lambda { return "Thank you" } 

 lambda1.call
 => "Thank you"

3. Scopes/Named scopes are a feature of Rails

It is used to specify commonly used queries which can be referenced as method calls on the association objects or models

eg, in user.rb:

scope :developers, -> { where(:role => 'developer') }

You can use it as

@developers = User.developers

Scopes are chainable, so you can do queries like

@developers = User.developers.where(:age => 40)

The scope defined in the example, is exactly same as defining a class method, like below.

def self.developers
  where(:role => 'developer')
end