conditional chaining in ruby

Question

Is there a good way to chain methods conditionally in Ruby?

What I want to do functionally is

if a && b && c
 my_object.some_method_because_of_a.some_method_because_of_b.some_method_because_of_c
elsif a && b && !c
 my_object.some_method_because_of_a.some_method_because_of_b
elsif a && !b && c
 my_object.some_method_because_of_a.some_method_because_of_c

etc...

So depending on a number of conditions I want to work out what methods to call in the method chain.

So far my best attempt to do this in a "good way" is to conditionally build the string of methods, and use eval, but surely there is a better, more ruby, way?

Answer 1

You could put your methods into an arry and then execute everything in this array

l= []
l << :method_a if a
l << :method_b if b
l << :method_c if c

l.inject(object) { |obj, method| obj.send(method) }

Object#send executes the method with the given name. Enumerable#inject iterates over the array, while giving the block the last returned value and the current array item.

If you want your method to take arguments you could also do it this way

l= []
l << [:method_a, arg_a1, arg_a2] if a
l << [:method_b, arg_b1] if b
l << [:method_c, arg_c1, arg_c2, arg_c3] if c

l.inject(object) { |obj, method_and_args| obj.send(*method_and_args) }

Answer 2

I use this pattern:

class A
  def some_method_because_of_a
     ...
     return self
  end

  def some_method_because_of_b
     ...
     return self
  end
end

a = A.new
a.some_method_because_of_a().some_method_because_of_b()

Answer 3

Maybe your situation is more complicated than this, but why not:

my_object.method_a if a
my_object.method_b if b
my_object.method_c if c

Answer 4

Although the inject method is perfectly valid, that kind of Enumerable use does confuse people and suffers from the limitation of not being able to pass arbitrary parameters.

A pattern like this may be better for this application:

object = my_object

if (a)
  object = object.method_a(:arg_a)
end

if (b)
  object = object.method_b
end

if (c)
  object = object.method_c('arg_c1', 'arg_c2')
end

I've found this to be useful when using named scopes. For instance:

scope = Person

if (params[:filter_by_age])
  scope = scope.in_age_group(params[:filter_by_age])
end

if (params[:country])
  scope = scope.in_country(params[:country])
end

# Usually a will_paginate-type call is made here, too
@people = scope.all

Answer 5

You can use tap:

my_object.tap{|o|o.method_a if a}.tap{|o|o.method_b if b}.tap{|o|o.method_c if c}