I need to know if I can include a module to an instantiated model.
What works today :
in the controller
#m = MyModel.create(params)
in the model
class Doc < ActiveRecord::Base
after_save :set_include
def set_include
if bool
self.class.send(:include, Module1)
else
self.class.send(:include, Module2)
end
end
end
and this works, but I'm afraid that self.class actually include the module for the class model an not the instantiated model
In this case, this will work.
The module methods are call after the object is saved.
But in many case, the controller will call some modules methods.
I thought of called the method set_include (up there) in a before_action of the controller.
But I really thinks that is not a good idea...
Any idea how I can really do that with in a good way ?
thanks !
Answer to your direct question is no. Your code only appears to be working and is actually not modifying instance of a class, but the class itself. So all instances of it will be getting this "benefit". Probably not what you wanted. Let me demonstrate with simple ruby example: https://repl.it/BnLO
What you can do instead is use extend with instance like: https://repl.it/BnLO/2
Applied to your code it would be:
class Doc < ActiveRecord::Base
after_save :set_include
def set_include
if bool
extend(Module1)
else
extend(Module2)
end
end
end
Also, self is not necessary. https://repl.it/BnLO/3
You need to use instance class (a.k.a eigenklass):
def set_include
singleton_class.instance_eval do
include bool ? Module1 : Module2
end
end
However the fact that you want to do this is suspicious and might lead to a disaster. So the question is: what are you really trying to achieve here - there surely is the better way of doing so.
Related
I'm trying my hands on metaprogramming after a long pause. I found a few questions but could not get an input to solve my problem so I hope someone can enlighten me.
In a rails 5 app, I am trying to write a concern that provides a class method to set configuration options. With those options, I want to define instance methods.
module Base64Attachable
extend ActiveSupport::Concern
class_methods do
attr_reader :base64_attachable_property
private
def base64_attachable(property)
#base64_attachable_property = property
end
end
included do
# ?
end
end
The concern above is used inside a User model:
class User < ApplicationRecord
include Base64Attachable
base64_attachable :image
end
In my understanding, the concern sets up the class method that is being called in the user model. However I do not seem to be able to get the base64_attachable_property inside the included block to define further methods based on the value of it. I thought I would find anything I need in self.class inside the included block, but that's not the case.
The aim in this case is to use define_method to define setters, getters and other methods for image in the user model.
What am I missing here?
The included block is run at the moment the concern is included on the class, the base64_attachable :image line is not run yet.
I'd suggest you follow what official gems does. Check ActiveStorage for example https://github.com/rails/rails/blob/530f7805ed5790af1d472a041bc74089dc183f47/activestorage/lib/active_storage/attached/model.rb#L35. It defines the methods that depends on that property right inside the class method (it uses class_eval, but I guess you can use define_method too):
def has_one_attached(name, dependent: :purge_later)
generated_association_methods.class_eval <<-CODE, __FILE__, __LINE__ + 1
def #{name}
#active_storage_attached_#{name} ||= ActiveStorage::Attached::One.new("#{name}", self)
end
def #{name}=(attachable)
attachment_changes["#{name}"] =
if attachable.nil?
ActiveStorage::Attached::Changes::DeleteOne.new("#{name}", self)
else
ActiveStorage::Attached::Changes::CreateOne.new("#{name}", self, attachable)
end
end
CODE
has_one :"#{name}_attachment", ......etc....
I have the following classes:
class AwardBase
class AwardOne < AwardBase
class Post < ActiveRecord::Base
The Post is an ActiveRecord, and the Award has a can_award? class method which takes a post object and checks to see if it meets some criteria. If yes, it updates post.owner.awards.
I know I can do this using an Observer pattern (I tested it and the code works fine). However, that requires me to add additional code to the model. I'd like not to touch the model at all if possible. What I'd like to do is run the Award checks like this (the trigger will be invoked at class load time):
class AwardOne < AwardBase
trigger :post, :after_save
def self.can_award?(post)
...
end
end
The intention with the above code is that it should automatically add AwardOne.can_award? to Post's after_save method
So essentially what I'm trying to do is to get the trigger call be equivalent to:
class Post < ActiveRecord::Base
after_save AwardOne.can_award?(self)
...
end
which is basically:
class Post < ActiveRecord::Base
after_save :check_award
def check_award
AwardOne.can_award?(self)
end
end
How can I do this without modifying the Post class?
Here's what I've done (which does not appear to work):
class AwardBase
def self.trigger (klass, active_record_event)
model_class = klass.to_class
this = self
model_class.instance_eval do
def award_callback
this.can_award?(self)
end
end
model_class.class_eval do
self.send(active_record_event, :award_callback)
end
end
def self.can_award? (model)
raise NotImplementedError
end
end
The above code fails with the error:
NameError (undefined local variable or method `award_callback' for #<Post:0x002b57c04d52e0>):
You should think about why you want to do it this way. I would argue it is even worse than using the observer pattern. You are violating the principle of least surprise (also called principle of least astonishment).
Imagine that this is a larger project and I come as a new developer to this project. I am debugging an issue where a Post does not save correctly.
Naturally, I will first go through the code of the model. I might even go through the code of the posts controller. Doing that there will be no indication that there is a second class involved in saving the Post. It would be much harder for me to figure out what the issue is since I would have no idea that the code from AwardOne is even involved.
In this case it would actually be most preferable to do this in the controller. It is the place that is easiest to debug and understand (since models have enough responsibilities already and are generally larger).
This is a common issue with metaprogramming. Most of the time it is better to avoid it precisely because of principle of least surprise. You will be glad you didn't use it a year from now when you get back to this code because of some issue you need to debug. You will forget what "clever" thing you have done. If you don't have a hell-of-a-good reason then just stick to the established conventions, they are there for a reason.
If nothing else then at least figure out a way to do this elegantly by declaring something in the Post model. For example by registering an awardable class method on ActiveRecord::Base. But the best approach would probably be doing it in the controller or via a service object. It is not the responsibility of AwardOne to handle how Post should be saved!
Because you are adding award_callback as class method. I bet it will be registered if you grep class methods.
So change your code like below. It should work fine.
model_class.class_eval do ## Changed to class_eval
def award_callback
this.can_award?(self)
end
end
Let me give a detailed example if it sounds confusing.
class Test
end
Test.instance_eval do
def class_fun
p "from class method "
end
end
Test.class_eval do
def instance_fun
p "from instance method "
end
end
Test.methods.grep /class_fun/
# => [:class_fun]
Test.instance_methods.grep /instance_fun/
# => [:instance_fun]
Test.class_fun
# => "from class method "
Test.new.instance_fun
# => "from instance method "
I've written a little helper method in my ApplicationController like this:
helper_method :dehumanize
def dehumanize (string)
string.parameterize.underscore
end
Now I would like to use it in one of my model files, but it seems not to be available there.
I tried also with:
ApplicationController.dehumanize(title)
in the model but it doesn't work.
any clue on how to make it work there?
thanks,
Models generally can't/don't/shouldn't access methods in controllers (MVC conventions), but the method you've written doesn't necessarily belong in a controller anyway - it would be better as an extension to the string class.
I would suggest you write an initializer to add dehumanize to String:
\config\initializers\string_dehumanize.rb
class String
def dehumanize
self.parameterize.underscore
end
end
You will need to restart your server/console but then you can call .dehumanize on any string:
some model:
def some_method
string1 = 'testing_the_method'
string1.dehumanize
end
Matt's answer is totally right, but to give you some clarification, you want to make sure that you're calling your methods on objects / instances, rather than classes themselves
For example, you mentioned you tried this:
ApplicationController.dehumanize(title)
This will never work because it's calling a method on a class which is not initialized, not to mention the class doesn't have that method. Basically, what will you expect if you called this method?
The way to do it is to use the method Matt recommended, or use a class method on your model itself, which will allow you to call the model's method directly:
#app/models/model.rb
class Model < ActiveRecord::Base
def self.dehumanize string
string.parameterize.underscore
end
end
# -> Model.dehumanize title
Let's say I have a model called Article:
class Article < ActiveRecord::Base
end
And then I have a class that is intended to add behavior to an article object (a decorator):
class ArticleDecorator
def format_title
end
end
If I wanted to extend behavior of an article object, I could make ArticleDecorator a module and then call article.extend(ArticleDecorator), but I'd prefer something like this:
article = ArticleDecorator.decorate(Article.top_articles.first) # for single object
or
articles = ArticleDecorator.decorate(Article.all) # for collection of objects
How would I go about implementing this decorate method?
What exactly do you want from decorate method? Should it simply add some new methods to passed objects or it should automatically wrap methods of these objects with corresponding format methods? And why do you want ArticleDecorator to be a class and not just a module?
Updated:
Seems like solution from nathanvda is what you need, but I'd suggest a bit cleaner version:
module ArticleDecorator
def format_title
"#{title} [decorated]"
end
def self.decorate(object_or_objects_to_decorate)
object_or_objects_to_decorate.tap do |objects|
Array(objects).each { |obj| obj.extend ArticleDecorator }
end
end
end
It does the same thing, but:
Avoids checking type of the arguments relying on Kernel#Array method.
Calls Object#extend directly (it's a public method so there's no need in invoking it through send).
Object#extend includes only instance methods so we can put them right in ArticleDecorator without wrapping them with another module.
May I propose a solution which is not using Module mixins and thereby granting you more flexibility. For example, using a solution a bit more like the traditional GoF decorator, you can unwrap your Article (you can't remove a mixin if it is applied once) and it even allows you to exchange the wrapped Article for another one in runtime.
Here is my code:
class ArticleDecorator < BasicObject
def self.[](instance_or_array)
if instance_or_array.respond_to?(:to_a)
instance_or_array.map {|instance| new(instance) }
else
new(instance_or_array)
end
end
attr_accessor :wrapped_article
def initialize(wrapped_article)
#wrapped_article = wrapped_article
end
def format_title
#wrapped_article.title.upcase
end
protected
def method_missing(method, *arguments)
#wrapped_article.method(method).call(*arguments)
end
end
You can now extend a single Article by calling
extended_article = ArticleDecorator[article]
or multiple articles by calling
articles = [article_a, article_b]
extended_articles = ArticleDecorator[articles]
You can regain the original Article by calling
extended_article.wrapped_article
Or you can exchange the wrapped Article inside like this
extended_article = ArticleDecorator[article_a]
extended_article.format_title
# => "FIRST"
extended_article.wrapped_article = article_b
extended_article.format_title
# => "SECOND"
Because the ArticleDecorator extends the BasicObject class, which has almost no methods already defined, even things like #class and #object_id stay the same for the wrapped item:
article.object_id
# => 123
extended_article = ArticleDecorator[article]
extended_article.object_id
# => 123
Notice though that BasicObject exists only in Ruby 1.9 and above.
You'd extend the article class instance, call alias_method, and point it at whatever method you want (although it sounds like a module, not a class, at least right now). The new version gets the return value and processes it like normal.
In your case, sounds like you want to match up things like "format_.*" to their respective property getters.
Which part is tripping you up?
module ArticleDecorator
def format_title
"Title: #{title}"
end
end
article = Article.top_articles.first.extend(ArticleDecorator) # for single object
Should work fine.
articles = Article.all.extend(ArticleDecorator)
May also work depending on ActiveRecord support for extending a set of objects.
You may also consider using ActiveSupport::Concern.
I am PHP dev and at the moment I am learning Rails (3) and of course - Ruby. I don't want to believe in magic and so I try to understand as much as I can about things that happen "behind" Rails. What I found interesting are the method calls like has_one or belongs_to in ActiveRecord models.
I tried to reproduce that, and came with naive example:
# has_one_test_1.rb
module Foo
class Base
def self.has_one
puts 'Will it work?'
end
end
end
class Model2 < Foo::Base
has_one
end
Just running this file will output "Will it work?", as I expected.
While searching through rails source I found responsible function: def has_one(association_id, options = {}).
How could this be, because it is obviously an instance (?) and not a class method, it should not work.
After some researching I found an example that could be an answer:
# has_one_test_2.rb
module Foo
module Bar
module Baz
def has_one stuff
puts "I CAN HAS #{stuff}?"
end
end
def self.included mod
mod.extend(Baz)
end
end
class Base
include Bar
end
end
class Model < Foo::Base
has_one 'CHEEZBURGER'
end
Now running has_one_test_2.rb file will output I CAN HAS CHEEZBURGER. If I understood this well - first thing that happens is that Base class tries to include Bar module. On the time of this inclusion the self.included method is invoked, which extends Bar module with Baz module (and its instance has_one method). So in the essence has_one method is included (mixed?) into Base class. But still, I don't fully get it. Object#extend adds the method from module but still, I am not sure how to reproduce this behaviour using extend. So my questions are:
What exactly happened here. I mean, still don't know how has_one method become class method? Which part exactly caused it?
This possibility to make this method calls (which looks like configuration) is really cool. Is there an alternative or simpler way to achieve this?
You can extend and include a module.
extend adds the methods from the module as class methods
A simpler implementation of your example:
module Bar
def has_one stuff
puts "I CAN HAS #{stuff}?"
end
end
class Model
extend Bar
has_one 'CHEEZBURGER'
end
include adds the methods from the module as instance methods
class Model
include Bar
end
Model.new.has_one 'CHEEZBURGER'
Rails uses this to dynamically add methods to your class.
For example you could use define_method:
module Bar
def has_one stuff
define_method(stuff) do
puts "I CAN HAS #{stuff}?"
end
end
end
class Model
extend Bar
has_one 'CHEEZBURGER'
end
Model.new.CHEEZBURGER # => I CAN HAS CHEEZBURGER?
I commend you for refusing to believe in the magic. I highly recommend you get the Metaprogramming Ruby book. I just recently got it and it was triggering epiphanies left and right in mah brainz. It goes over many of these things that people commonly refer to as 'magic'. Once it covers them all, it goes over Active Record as an example to show you that you now understand the topics. Best of all, the book reads very easily: it's very digestible and short.
Yehuda went through some alternatives on way to Rails3: http://yehudakatz.com/2009/11/12/better-ruby-idioms/
Moreover, you can use a (usually heavily abused, but sometimes quite useful) method_missing concept:
class Foo
def method_missing(method, *arg)
# Here you are if was some method that wasn't defined before called to an object
puts "method = #{method.inspect}"
puts "args = #{arg.inspect}"
return nil
end
end
Foo.new.abracadabra(1, 2, 'a')
yields
method = :abracadabra
args = [1, 2, "a"]
Generally, this mechanism is quite often used as
def method_missing(method, *arg)
case method
when :has_one
# implement has_one method
when :has_many
# ...
else
raise NoMethodError.new
end
end