Circular Dependencies in Ruby

Question

Let\'s say we have two classes, Foo and Foo Sub, each in a different file, foo.rb and foo_sub.rb respectively.

foo.rb:

require \"foo_sub\"
class Foo
         


        

Answer 1

Sandi Metz explains one solution to this problem and how to solve it really nicely in her book Practical Object-Oriented Design in Ruby (POODR).

What she suggests (and I'm inclined to agree with as its worked the best for me so far), is to inject the sub-class FooSub into the master class Foo.

This would be done in foo.rb with:

1   class Foo
2     def initialize(foo_sub:)
3     end
4   end

to maintain clean code, and keep it easily changeable, you would then wrap the foo_sub in a wrapper method so your class now looks like this:

1   class Foo
2
3     attr_reader :foo_sub
4
5     def initialize(foo_sub:)
6       @foo_sub = foo_sub
7     end
8   end

(here, the attr_reader is setting up a method called foo_sub and then whatever is passed into the value of the initialize hash is an instance of foo_sub, therefore @foo_sub (line 6), can be set to the value of the method foo_sub).

You can now have your FooSub class with no requires, making it independent of anything:

1   class FooSub
2     SOME_CONSTANT = 1
3   end

and you can add a method to your Foo class that has access to #SOME_CONSTANT:

1   class Foo
2
3     attr_reader :foo_sub
4
5     def initialize(foo_sub:)
6       @foo_sub = foo_sub
7     end
8     
9     def foo
10      foo_sub.SOME_CONSTANT
11    end
12  end

In actuality, with this, you're setting up a method that returns the instance of foo_sub @foo_sub (that is injected at the initialize), with the method #SOME_CONSTANT appended onto it. Your class just expects whatever is injected in at the initialize to respond to #SOME_CONSTANT. SO for it to work you would have to inject you FooSub class when setting up Foo in a REPL (e.g IRB or PRY):

PRY
[1]>   require 'foo'
[2]>   => true
[3]>   require 'foo_sub'
[4]>   => true
[5]>   foo_sub = FooSub.new
[6]>   => #
[7]>   foo = Foo.new(foo_sub: foo_sub)
[8]>   => #
[9]>   foo.foo
[10]>  => 1

if, however, you injected something else, you'd end up with:

PRY
[1]>   require 'foo'
[2]>   => true
[3]>   require 'foo_sub'
[4]>   => true
[5]>   foo_sub = FooSub.new
[6]>   => #
[7]>   foo = Foo.new(foo_sub: 'something else as a string')
[8]>   => #
[9]>   foo.foo
[10]>  => UNDEFINED CONSTANT #SOME_CONSTANT ERROR MESSAGE

I don't know what the actual error message would read on line 10 but think along those lines. This error would of occurred because you'd have effectively tried to run the method #SOME_CONSTANT on the string 'something else as a string' or 'something else as a string'.SOME_CONSTANT which would obviously not work.