Ruby on Rails instance vs class methods - ruby-on-rails

I have studied major difference between Ruby class ,instance method and the major difference I found is we don't need to create instance of that class we can directly call that method on class name directly.
class Notifier
def reminder_to_unconfirmed_user(user)
headers['X-SMTPAPI'] = '{"category": "confirmation_reminder"}'
#user = user
mail(:to => #user["email"], :subject => "confirmation instructions reminder")
end
end
So,here I defined instance method reminder_to_unconfirmed_user in my Notifier class to send email to unconfirmed users, and when I run Notifier.reminder_to_unconfirmed_user(User.last) it get called provided it's a instance method not a class method.

To define a class method, use the self keyword in the method's definition (or the class' name):
class Notifier
def self.this_is_a_class_method
end
def Notifier.this_a_class_method_too
end
def this_is_an_instance_method
end
end
In your case, reminder_to_unconfirmed_user should be defined as a class method:
class Notifier
def self.reminder_to_unconfirmed_user(user)
# ...
end
end
Then you can use it like this:
Notifier.reminder_to_unconfirmed_user(User.last)

I had the same question the OP did and after digging around I finally figured it out! The other answers just addressed when to use instance vs class methods in Ruby however Rails does some sneaky stuff behind the scences. The question wasn't when to use class vs instance methods but instead how come Rails allows you to call an instance method as if it's a class method as shown by his mailer example above. It's due to: AbstractController::Base and can be seen here: AbstractController::Base
Basically, in all controllers (whether they be your mailer or a standard controller), all defined methods are intercepted by "method_missing" and then returns an instance of that class! The defined methods are then also converted to public instance methods. Thus, because you never instantiate these classes (for example you never do Mailer.new.some_method) Rails automagically calls method_missing and returns an instance of that Mailer which then takes advantage of all the methods defined within that class.

In your case it must be :
class Notifier
def self.reminder_to_unconfirmed_user(user)
headers['X-SMTPAPI'] = '{"category": "confirmation_reminder"}'
#user = user
mail(:to => #user["email"], :subject => "confirmation instructions reminder")
end
end
As their name suggests:
Instance methods on a model should be used for logic/operations that relate to a specific instance of a model (the one on which the method is called.)
Class methods are for things which don't operate on an individual instance of a model or for cases where you don't have the instance available to you. Like in some cases you do want to apply changes on few group of objects.
If you want to update all users on a specific condition, Then you should go for class method.
They do have different way of calling :
class Test
def self.hi
puts 'class method'
end
def hello
puts 'instance method'
end
end
Foo.hi # => "class method"
Foo.hello # => NoMethodError: undefined method ‘hello’ for Test:Class
Foo.new.hello # => instance method
Foo.new.hi # => NoMethodError: undefined method ‘hi’ for #<Test:0x1e871>

Related

Reason for calling an instance method on a class in Ruby?

I would like to know is there any specific reason for doing this or is this a silly mistake done by someone (or is it something else that I am not understanding).
class SomeMailer < ActionMailer::Base
def first_method(user)
mail(to: user.email, subject: "Testing")
end
end
This method is called at some other place in the code as follows
SomeMailer.first_method(user).deliver
ActionMailer::Base classes are weird... Yes, you do indeed call instance methods on the class - which obviously won't work for 'normal' classes!
But there's some meta-programming magic under the hood:
module ActionMailer
class Base < AbstractController::Base
def method_missing(method_name, *args) # :nodoc:
if action_methods.include?(method_name.to_s)
MessageDelivery.new(self, method_name, *args)
else
super
end
end
end
end
If you look through the rails documentation, you'll see that calling instance methods on the class is, strangely, the normal thing to do for mailers.
This is how rails is intended to work.
It is also mention in rails guides that
You never instantiate your mailer class. Rather, you just call the method you defined on the class itself.
Rails do the internal processing by invoking method_missin.
Basically, any action method defined in mailer class will be intercepted by method_missing and will return an instance of MessageDelivery, otherwise it runs the default implementation. And where do action methods come from? ActionMailer::Base inherits from AbstractController::Base, so it works exactly the same as for controllers - it returns a set of public instance methods of a given class.
Rails itself encourage this behavior. For more information, you can refer this link
I will try to answer this since I have come across similar situations myself while working on existing code.
The instance methods like this in a class help when you do call backs on a class. For example, if you want to perform some action on a object that was created from that class.
Say you have another class User and you want to send an email to a user immediately after creating a new user. In that case you can call this method on that object by doing
after_save :method_name

call instance method with class instance variable - ruby on rails

I'm trying to call an instance method (defined in model) from the instance variable in controller but could not get that work and server logs saying
undefined method `active_users' for #< Admin
controller
#admin = Admin.first
#admin.active_users
Admin Model
def self.active_users
byebug
end
I know that we can call it through the class directly like Admin.active_users .
Why is it not accessible by instance of the class?
active_users method is defined for Admin object, not for its instances.
I do not know what you are trying to do, but message receiver matters.
To make #admin.active_users work, define a method an instance method:
def active_users
byebug
end
The thing is, that active_users (both with and without self) are instance methods. It is just that objects for which these methods defined are different.
The method with self is a "class instance method", while the one without self is an "instance method", e.g. method accessible by instances of the class Admin.
When you write def self.something_method, method will be available only for class, as in your example. If you want create method for object, you have to create method without self, like this: def something_method.
For example:
class Admin
def self.class_method
p 'class method'
end
def something_method
p 'method for object'
end
end
#admin = Admin.create(something_params)
# You can also use 'Admin.new', but it will not save your admin in database, or 'Admin.first', like in your example.
#admin.something_method
#=> 'method for object'
#admin.class_method
#=> not working :(
Admin.something_method
#=> not working :(
Admin.class_method
#=> 'class method'

Access a instance method from other model in ruby on rails?

I have a user model in my application. Now I want to replace some user model coding into 2 categories likely employ.rb and customer.rb under a module users, to avoid more number of codes in a single model. I want to access a method send_mail in customer.rb after a user created.
user.rb
after_create:send_msg_on_order
def send_msg_on_order
Users::Customer.send_mail
end
users/customer.rb
def send_mail
Mailer.send_mail_to_customer.deliver
end
And I am getting undefined method `send_mail' for Users::Customer:Module error.
You have defined send_mail method as instance method but calling it as a class method. Either make it a class method or create an instance of Customer model and call it.
Making the method a class method:
def self.send_mail
Mailer.send_mail_to_customer.deliver
end
If you wish to keep it an instance method, then call it like this:
after_create:send_msg_on_order
def send_msg_on_order
Users::Customer.new.send_mail
end
HTH
You can also call like this
def send_msg_on_order
Customer.send_mail
end

about ruby class method invoke

in rails3 i defined a class
#coding:utf-8
class CoreMail < ActionMailer::Base
def consulting_reply(email)
mail(:to => email, :subject => 'ssss')
end
end
i found i could invoke this method like this
CoreMail.consulting_reply(email)
but want i thought the right way is :
instance=CoreMail.new
instance.consulting_reply(email)
because the consulting_reply is the instance method,
did i missing something? hope someone could give me a help
ActionMailer::Base has a method_missing defined on it:
def method_missing(method, *args) #:nodoc:
return super unless respond_to?(method)
new(method, *args).message
end
This will call your instance method with the same arguments and then call the message method, returning the object of that mailer call. To deliver it, call deliver! on the end of your method call:
CoreMail.consulting_reply(email).deliver!
You're right, normally that would be an instance method and you would have to create an instance of the class before calling consulting_reply() (since it's not defined as self.consulting_reply). However, your class is inheriting from ActionMailer::Base, and there's a little ruby magic happening behind the scenes. Checkout the source for ActionMailer::Base to see it for yourself (https://github.com/rails/rails/blob/master/actionmailer/lib/action_mailer/base.rb), but to simplify for you what's happening, here's a quick example:
class Test
def self.method_missing(method, *args)
puts "Called by a non class method"
end
end
Test.fake_class_method
Which will output:
Called by a non class method
ActionMailer::Base does something similar to this, which dynamically looks for the method you called. I believe it also creates a instance of the class using:
new(method, *args).message
Anyway, I hope this sheds some light as to what's happening.

Can I define_method in rails models?

My rails model has code that is attempting to define_method(method_name) inside the model.
I keep getting:
NoMethodError: undefined method `define_method'
What am I doing wrong? Am I doing this in the wrong place. I need this method attached to this model. Where else can I define this method?
EDIT:
For those asking to see the code:
for field in rdev_fields
next if self.attributes.include?(field)
count = count + 1
rdev_hash[field.to_sym] = self.attributes["attribute#{count}"]
if !self.respond_to?(field) then
define_method("#{field}") do
self.send("attribute#{count}".to_sym)
end
end
end
There's nothing magical or about a rails model, it's just a normal class with a bunch of pre-existing methods,
So, the question is "can I define_method in a class"?
Part 1: Yes you can.
The important distinction is than you can define method in a class not in an instance method
For example:
class Cow
define_method "speak" do
"MOOOO"
end
end
Cow.new.speak
=> "MOOOO"
This should work fine. Note you're defining it on the class Cow, so any other Cows that you already have will automatically get that method added.
Part 2: What do you do if you want to define a method from within an instance method?
You can't define methods from an instance method, so you have to grab the class, and use that to define the method. Like this:
class Cow
def add_speak
self.class.send(:define_method, :speak) do
"MOOOO added"
end
end
end
Cow.new.speak
NoMethodError: undefined method 'speak' for #<Cow:0xb7c48530>
Cow.new.add_speak
Cow.new.speak
=> "MOOOO added"
Problem solved. Astute readers will note that in this example I'm using send(:define_method) - this is needed because define_method is private, and private methods are only accessible to the thing they're in. In this case, define_method is in the class, we are in the instance, so we can't directly access it.
As above though, we're adding the method directly to the class, so all other Cows which already exist will automatically also get the speak method added.
Part 3: What do you do if you want to define a method for only 1 object, not all objects of that class?
Example:
class Cow
def add_speak_just_me
class << self
define_method "speak" do
"MOOOO added for just me"
end
end
end
end
Cow.new.speak
NoMethodError: undefined method 'speak' for #<Cow:0xb7c72b78>
c = Cow.new
c.add_speak_just_me
c.speak
=> "MOOOO added for just me" # it works, hooray
Cow.new.speak # this new cow doesn't have the method, it hasn't been automatically added
NoMethodError: undefined method `speak' for #<Cow:0xb7c65b1c>
How does this work? Down the rabbithole you go!
Read this: http://dannytatom.me/metaid/ and good luck. It helps when you realise that 'adding a method' to an instance isn't actually adding it to the instance at all :-)
If you came here searching for how to dynamically define a CLASS method, because define_method wasn't working (because it defines INSTANCE methods), here is your answer:
Use define_singleton_method :)
was able to cobble this together. Very little understanding of what's actually going on though.
My instance method foo is opening the class and defining bar on it so that I can then call that on my instance. More experienced folks will let us know if this is opening a can of worms at the same time.
Would be useful to know your specific use for this though.
class User < ActiveRecord::Base
def foo
(class << self; self; end).class_eval do
define_method(:bar) {puts "bar"}
end
end
end
u = User.first
u.foo
u.bar #=> "bar"
The answer to your question is "yes, you can". As for why it's not working for you - it's impossible to say for sure why, if you don't provide some context for the code.

Resources