Ruby: calling methods on init - ruby

I switched to Ruby from PHP, and have yet to understand a curious Ruby class behavior where methods are executed outside of the class method definitions (see example below). In PHP, when we wanted to execute anything on class init, we would put it in the constructor method.
Ruby example (Rails):
class Comment < ActiveRecord::Base
belongs_to :post, :counter_cache => true
end
Am I correct in understanding that belongs_to will be executed on instantiation? And is belongs_to a class method inherited from ActiveRecord?
Thanks!

In Ruby, everything is executable code. Or, to put it another way: everything is a script. There is no such thing as a "class declaration" or something like that.
Any code that sits in a file, without being inside anything else like a method body, a class body, a module body or a block body, is executed when that file is loaded (or required or require_relatived). This is called a script body.
Any code that sits inside a class or module body is executed when that class or module is created. (This is the case you are referring to.)
The boring part: any code that sits inside a method body is executed when that method is called, or more precisely, when that method is invoked in response to receiving a message with the same name as the method. (Duh.)
Any code that sits inside a block body is executed when that block is yielded to.
Since a class definition is just a script, this means that it can contain any sort of code you want, including method calls:
class Foo
attr_accessor :bar # Yes, attr_accessor is just a method like any other
private # I *bet* you didn't know *that* was a method, too, did you?
end
or conditionals:
class Bar
if blah # e.g. check if the OS is Windows
def foo
# one way
end
else
def foo
# a different way
end
end
end

Yes, it's a class method from ActiveRecord. The method will execute when the class itself is created, not when an instance of it is created.

Yes, that's correct. See also this question.

Related

Ruby methods called at top of classes

I've seen this a lot in Ruby (started using Ruby for the first time a couple years ago):
class MyAwesomeClass
another_method
def initialize; end
end
And I can't get my head around what type of method another_method is...
Looking at examples, it's sometimes declared as a class method, but usually you call class methods in your code like this:
AnotherClass.another_method
And most importantly you call your methods regardless if they are from another class, module, or inherited class inside another method of your class you use them. For example:
module SomeModule
def some_method; end
end
class AnotherClass
private
def another_method; end
end
class YetAnotherClass
self.yet_another_method; end
end
class MyAwesomeClass < AnotherClass
include SomeModule
def initialize
some_method # method from my included module
another_method # method inherited from AnotherClass
YetAnotherClass.yet_another_method # class method called directly
end
end
So what are these methods that can be called at the top of a class and NOT called inside of another method as my examples above?
Are you able to just call class methods inside another class (that it inherits from) outside of your method calls?
I can't get my head around what type of method another_method is...
There is only one type of method in Ruby: every method in Ruby is an instance method of some module. There is no other type of method.
We may sometimes talk about "class methods" or "singleton methods", but that is just a convenient way of talking about a particular way to use methods. The methods themselves are no different.
Every method in Ruby is an instance method of some module. The question is simply: which module?
We are sometimes talking about class methods, but class methods don't really exist. Class methods are simply singleton methods of an object that happens to be an instance of the Class class.
There is absolutely no difference between
Foo = ''
def Foo.bar; end
and
class Foo; end
def Foo.bar; end
and
module Foo; end
def Foo.bar; end
We call the first one a "singleton method", the second one a "class method", and the third one a "module function", but there is actually no difference between the three. The only difference is that the first object is an instance of the String class, the second object is an instance of the Class class, and the third object is an instance of the Module class, that's it.
Actually, I must confess something: I lied. I wrote that class methods are simply singleton methods of an object that happens to be an instance of the Class class, but in reality … singleton methods don't exist either.
A singleton method is simply a boring old normal instance method of the singleton class.
There is no difference between
foo = ''
def foo.bar; end
and
foo = ''
class << foo
def bar; end
end
There you have it: singleton methods are actually just instance methods of the singleton class, class methods are actually just instance methods of the singleton class of an object that is an instance of the Class class, module functions are actually just instance methods of the singleton class of an object that is an instance of the Module class.
It's just that "instance method of the singleton class of an object that is an instance of the Class class" is annoying to say and write all the time, so we say "class method" instead, knowing full well that there is no such thing.
Looking at examples, it's sometimes declared as a class method, but usually you call class methods in your code like this:
AnotherClass.another_method
Again, there is no such thing as a class method. There is exactly one kind of method, and they are always called the same way:
receiver.method(args)
If the receiver is self, then you can leave it out, and if you have no arguments, you can leave them out, too.
And most importantly you call your methods regardless if they are from another class, module, or inherited class inside another method of your class you use them.
That is not true.
In fact, in your own example, you are calling two methods outside of a method body: Module#private and Module#include, and you seem to have no trouble with those. Other methods that I am sure you have already called outside of a method body are Module#attr_accessor, Kernel#require, or Kernel#puts. In fact, in simple scripts, there is often not a single method definition body at all.
So what are these methods that can be called at the top of a class and NOT called inside of another method as my examples above?
They are instance methods just like any other instance methods, there is absolutely nothing special about them.
Are you able to just call class methods inside another class (that it inherits from) outside of your method calls?
Since class methods don't exist, and these are simply instance methods just like every other instance method, the method lookup algorithm is of course also just the method lookup algorithm for instance methods:
Start with the private internal hidden __klass__ pointer of the receiver. If you can't find the method there …
Get the private internal hidden __superklass__ pointer and repeat.
And that's it. (Yes, okay, there is a tiny bit more to it: if you come to a point where there is no __superklass__ pointer, then you try again with method_missing and the name of the method as an argument, and if you also cannot find that, then you raise a NoMethodError.)
So, let's try that in your example:
class MyAwesomeClass
another_method
end
Okay, first off: what is the receiver? Well, if no receiver is given, then the receiver is always implicitly self. Now, what is self here?
A ha! That is actually the most important question in Ruby, always. At any point in Ruby, you must always know what self is.
Well, we know that inside of a method body, self is the receiver of the message send. So, we can guess: what would make the most sense for self inside of a class definition. The class itself, of course! Let's test that:
what_is_self = class MyAwesomeClass
self
end
p what_is_self
# MyAwesomeClass
Well, looks like we're right. Okay, we know the receiver of the message send is MyAwesomeClass, so what is the first step in our algorithm? We get the private internal hidden __klass__ pointer.
We cannot actually do that in Ruby (it is a private internal hidden implementation detail of the Ruby implementation). But let me assure you that is pretty much always going to be the singleton class. (There are some objects which cannot have singleton classes, and some Ruby implementations optimize the creation of singleton classes so that they are only created when needed, but for all intents and purposes, we can assume that the __klass__ pointer is always the singleton class.)
We look inside the singleton class, and we find no definition of the method another_method. So, we move to the second step of the algorithm: we get the __superklass__ of the singleton class.
The __superklass__ of the singleton class is usually going to be the class of the object. So, in this case, the object is MyAwesomeClass, which is a class, so the class of MyAwesomeClass and the __superklass__ of the singleton class is going to be Class.
Again, we look inside Class and we don't find another_method. Ergo, we get Class's __superklass__, which is Module. We also don't find another_method here, so we move on to Object, which also doesn't have another_method.
Now, it gets interesting again, because Object's __superklass__ pointer is actually Kernel and not BasicObject as you might have thought. (More precisely, it is an include class which shares its method table with the Kernel module, since Kernel is not a class at all and thus cannot be a superclass.)
Kernel also doesn't have a method named another_method, so we get Kernel's __superklass__ (again, technically we are talking about an include class that proxies Kernel and not Kernel itself), which is BasicObject. Finally, BasicObject does not have a __superklass__.
Which means we start aaaaaaaaaaaall the way back from step #1 again, but this time as-if you had written
class MyAwesomeClass
method_missing(:another_method)
end
We do the whole song-and-dance again, and we also never find a method until we get to the very top, to BasicObject, which has a method_missing that roughly looks like this:
class BasicObject
def method_missing(meth, *args)
raise NoMethodError, "undefined method `#{meth}' for #{inspect}", meth, args, receiver: self
end
end
So, if you want your call to another_method to not fail, it needs to be defined anywhere along the whole chain we just walked up:
In MyAwesomeClass's singleton class
In a module that MyAwesomeClass extends
In Class
In a module that Class includes
Or in a module included by that module
In Module
In a module that Module includes
Or in a module included by that module
In Object
In Kernel
Or another module that Object includes
Or in a module that Kernel includes
In BasicObject
In a module that BasicObject includes
Or in a module included by that module
Or maybe the Ruby implementation you are using has an implementation-specific superclass of BasicObject (e.g. MacRuby has Objective-C's NSObject as the superclass of BasicObject)
To illustrate difference between different type of methods, consider this example:
class Foo
puts 'method called outside of method'
def initialize
puts 'initialize called'
end
def instanse_method
puts 'instance method called'
end
def self.clazz_method
puts 'class method called'
end
end
Foo
foo = Foo.new
foo.instanse_method
Foo.clazz_method
What will be the output?
method called outside of method
initialize called
instance method called
class method called
So what are these methods that can be called at the top of a class and NOT called inside of another method as my examples above?
As you can see, any method can be called before initialize method. It's executed when class is loaded.
Just calling
Foo
would be sufficient enough to output:
method called outside of method
(but notice that it was not called multiple times)
Looking at examples, it's sometimes declared as a class method, but usually you call class methods in your code like this: AnotherClass.another_method
It's like static function in PHP. In my example, it's Foo#clazz_method.
And most importantly you call your methods regardless if they are from another class, module, or inherited class inside another method of your class you use them
Usually, you have to include a file that define another class before using its method. But most frameworks like Ruby on Rails, have already built-in autoloading system, so it looks like it's working magically without explicit require or include. But there are times when RoR does not know where your custom classes are. In this case, you have to require the file explicitly before you use it. Well, usually in the beginning of the file.
It'll be easier to understand by looking at it in action.
# Step 1
# Run this in IRB
class MyClass
def self.another_method
puts "Executed when class is loaded to memory"
end
end
# Step 2
# Now when you run the following in IRB after that, its going to execute the
# class method which was defined in its parent class (MyClass)
class MyAwesomeClass < MyClass
another_method # Executed ONCE when the class is loaded to memory for the first
def initialize; end
end
# Now if you instantiate MyAwesomeClass though, it will not print the same as of
# Step 2 as class method :another_method already has been executed when class
# was loaded
awesome1 = MyAwesomeClass.new
The body of a class will be interpreted and executed sequentially & behaves much like you'd expect it inside an instance method. Try putting a puts "Hello World" in the body of your class definition like so:
class MyClass
# Executes when the class is defined in IRB(Loaded to memory)
puts "Hello World"
end

How can I mock super in ruby using rspec?

I am extending an existing library by creating a child class which extends to the library class.
In the child class, I was able to test most of functionality in initialize method, but was not able to mock super call. The child class looks like something like below.
class Child < SomeLibrary
def initialize(arg)
validate_arg(arg)
do_something
super(arg)
end
def validate_arg(arg)
# do the validation
end
def do_something
#setup = true
end
end
How can I write rspec test (with mocha) such that I can mock super call? Note that I am testing functionality of initialize method in the Child class. Do I have to create separate code path which does not call super when it is provided with extra argument?
You can't mock super, and you shouldn't. When you mock something, you are verifying that a particular message is received, and super is not a message -- it's a keyword.
Instead, figure out what behavior of this class will change if the super call is missing, and write an example that exercises and verifies that behavior.
As #myron suggested you probably want to test the behavior happening in super.
But if you really want to do this, you could do:
expect_any_instance_of(A).to receive(:instance_method).and_call_original
Assuming
class B < A
def instance_method
super
end
end
class A
def instance_method
#
end
end
Disclaimer expect_any_instance_of are a mark of weak test (see):
This feature is sometimes useful when working with legacy code, though
in general we discourage its use for a number of reasons:
The rspec-mocks API is designed for individual object instances, but
this feature operates on entire classes of objects. As a result there
are some semantically confusing edge cases. For example, in
expect_any_instance_of(Widget).to receive(:name).twice it isn't clear
whether a specific instance is expected to receive name twice, or if
two receives total are expected. (It's the former.)
Using this feature is often a design smell. It may be that your test is trying to do too much or that the object under test is too
complex.
It is the most complicated feature of rspec-mocks, and has historically received the most bug reports. (None of the core team
actively use it, which doesn't help.)
A good way to test this is to set an expectation of some action taken by the superclass - example :
class Some::Thing < Some
def instance_method
super
end
end
and the super class:
class Some
def instance_method
another_method
end
def self.another_method # not private!
'does a thing'
end
end
now test :
describe '#instance_method' do
it 'appropriately triggers the super class method' do
sawm = Some::Thing.new
expect(sawm).to receive(:another_method)
sawm.instance_method
end
end
All This Determines Is That Super Was Called On the Superclass
This pattern's usefulness is dependent on how you structure your tests/what expectations you have of the child/derivative class' mutation by way of the super method being applied.
Also - pay close attention to class and instance methods, you will need to adjust allows and expects accordingly
YMMV
A bit late to this party, but what you can also do is forego using the super keyword and instead do
class Parent
def m(*args)
end
end
class Child < Parent
alias super_m m
def m(*args)
super_m(*args)
end
end
That way your super method is accessible like any other method and can e.g. be stubbed like any other method. The main downside is that you have to explicitly pass arguments to the call to the super method.

How to Intercept Class Definitions in Ruby?

Is it possible to code something that can tell me when a Ruby class is defined?
Yes!
class Object
def self.inherited(base)
puts "#{base} inherited from object"
end
end
class Animal
end
class Cat < Animal
end
Running the above code prints the following:
Animal inherited from object
Cat inherited from object
Basically, the self.inherited callback is triggered whenever a class is defined that inherits from the class it is defined on. Put it on Object and that's any class! (Although there may be some special case exceptions I can't think of just now).
I should probably add the disclaimer that, while it is possible to do this (because of just how awesome Ruby is as a language), whether it is advisable to do this, especially in code destined for production use, I'm not so sure. Well, actually, I am sure. It would be a bad idea.

Why use extend/include instead of simply defining method in main object?

RSpec adds a "describe" method do the top-level namespace. However, instead of simply defining the method outside of any classes/modules, they do this:
# code from rspec-core/lib/rspec/core/dsl.rb
module RSpec
module Core
# Adds the `describe` method to the top-level namespace.
module DSL
def describe(*args, &example_group_block)
RSpec::Core::ExampleGroup.describe(*args, &example_group_block).register
end
end
end
end
extend RSpec::Core::DSL
Module.send(:include, RSpec::Core::DSL)
What is the benefit of using this technique as opposed to simply defining describe outside any modules and classes? (From what I can tell, the DSL module isn't used anywhere else in rspec-core.)
I made this change a few months ago so that describe is no longer added to every object in the system. If you defined it at the top level:
def describe(*args)
end
...then every object in the system would have a private describe method. RSpec does not own every object in the system and should not be adding describe willy-nilly to every object. We only want the describe method available in two scopes:
describe MyClass do
end
(at the top-level, off of the main object)
module MyModule
describe MyClass do
end
end
(off of any module, so you nest your describes in a module scope)
Putting it in a module makes it easy to extend onto the main object (to add it to only that object, and not every object) and include it in Module (to add it to all modules).
Actually, if that's all there is in the code, I don't really believe it to be much better — if at all. A common argument is that you can easily check that RSpec is responsible for addinng this method in the global namespace by checking the method owner. Somehow it never felt this was needed, as the location of the method already stores that information.
Defining the method outside of any scope would have be equivalent to defining a private instance method in Object:
class Object
private
def double(arg)
arg * 2
end
end
double(3) # OK
3.double(3) # Error: double is private
self.double(3) # Error: double is private
I think privateness is a useful aspect, because it prevents from making certain method calls that have no meaning, that the code shown in the question lacks.
There's an advantge to defining the method in a module, though, but the RSpec code doesn't seem to make use of it: using module_function, not only do you preserve privateness of the instance method, but you also get a public class method. This means that if you have an instance method of the same name, you will still be able to refer to the one defined by the module, by using the class method version.
A common example of module_function is the Kernel module, which contains most function-like core methods like puts (another one is Math). If you're in a class that redefines puts, you can still use Kernel#puts explicitly if you need:
class LikeAnIO
def puts(string)
#output << string
end
def do_work
puts "foo" # inserts "foo" in #output
Kernel.puts "foo" # inserts "foo" in $stdout
end
end

How to add statements to an existing method definition in Ruby

I noticed for the class definition, if I open up the class MyClass, and add something in between without overwrite I still got the original method which defined earlier. The new statements added augment the existing one.
But as to the method definition, I still want the same behavior as the class definition, but it seems when I open up the def my_method, the exiting statements within the def and end is overwritten, I need to rewrite that again.
So is there any way to make the method definition behave the same as definition, something like super, but not necessarily is the sub-class?
I suppose you are looking for alias_method:
class A
alias_method :old_func, :func
def func
old_func # similar to calling 'super'
# do other stuff
end
end

Resources