Ruby/Rails: Circular dependency when including concern in ApplicationRecord - ruby-on-rails

I have a concern that creates a class macro that I want available for all the models in my Rails application. So I'm including it in ApplicationRecord. The code is as follows:
# application_record.rb
class ApplicationRecord < ActiveRecord::Base
include ::TestConcern
end
# app/concerns/test_concern.rb
module TestConcern
extend ActiveSupport::Concern
class_methods do
def some_class_macro_all_models_must_have
User.some_class_instance_variable << self
end
end
included do
User.include(UserModule)
end
module UserModule
def self.included(base)
base.class_eval do
def self.some_class_instance_variable
#some_class_instance_variable ||= Set.new
end
end
end
end
end
As you can see, the class macro will actually interact with a class instance variable in the model User.
So that's why, on the included hook of the concern, I'm trying to class_eval the User model to have that class instance variable initialized. The plan was to do it like this because otherwise any model can be invoking the class macro BEFORE the class instance variable is initialized in the User model.
However, this errors out with Circular dependency detected while autoloading constant User. As far as I can understand, ApplicationRecord loads, it includes the module, the module included hooks is called, it references the User model, and so the User model is loaded, which inherits from ApplicationRecord (which didn't finish loading yet), so it causes the circular dependency.
How to avoid this circular dependency paradox, knowing that many models will invoke this class macro, and those classes might be loaded before the User class itself, so I can't even count on defining the some_class_instance_variable class method in the User model itself?

After giving it some extra thought, I decided to simply store the some_class_instance_variable in the concern itself, and since the model User also called the some_class_macro_all_models_must_have, I decided to include the UserModule when it was invoked, effectively eliminating both the circular dependency and the load order issue.
The real code is much more complex than this contrived example, but the end result was something like this:
module TestConcern
def self.some_class_instance_variable
#some_class_instance_variable ||= Set.new
end
extend ActiveSupport::Concern
class_methods do
def some_class_macro_all_models_must_have
User.include(UserModule) if self == User
TestConcern.some_class_instance_variable << self
end
end
included do
end
module UserModule
def self.included(base)
base.class_eval do
# Class macro invocations, class method and instance method definitions
end
end
end
end

Related

Loading module into User Model in Rails

Trying to make available the methods I have stored in a Module which is located in app/models (side note: not sure if this is the correct place for modules?).
Module:
module MyModule
class MyClass
def some_method
# do something
end
end
end
User Model:
class User < ApplicationRecord
include MyModule
def another_method
some_method
end
end
I am getting a NoMethodError:
NoMethodError (undefined method 'some_method' for #<User:0x00007f6a3ce452c0>
You seem to have missunderstood what what modules and classes do in Ruby. In Ruby a module is simply an object that wraps a set of methods and constants.
A module can extend other modules, classes and objects and can be included in classes thus implementing multiple inheritance. Modules in Ruby fill the role that traits, namespaces and singletons do in other languages.
Classes are actually modules (Module is part of the ancestors chain of Class) with the key difference that you can make instances of a class and that class can inherit from a single other class and cannot extend other objects or be included.
The code example here actually doesn't make sense. If you want to declare a method that will be available to classes that include a module you want to declare it in the module itself:
module MyModule
def some_method
# do something
end
end
When you then call User#another_method it will look in the ancestors chain of the User class until it finds the method which is defined in MyModule.
module MyModule
class MyClass
def some_method
# do something
end
end
end
Will actually definte the class MyClass with an instance method that is only available to instances of MyClass. The only thing that the module does here is change the module nesting so that the class is defined in MyModule instead of the global namespace.
If you want to mix in a method from a method into your class then just put the methods directly in the module (without an intermediate class).
Module:
module MyModule
def some_method
# do something
end
end
User Model:
class User < ApplicationRecord
include MyModule
def another_method
some_method
end
end
Have a look at this answer, you need to instantiate your Class first. Or if you want to
class User < ApplicationRecord
include MyModule
def another_method
my_instance = MyClass.new
my_instance.some_method
end
end
As for a place to store your Module, have a look at this guide about service objects, it gave me some inspiration when it comes to different modules.

How to move logic out of a Rails model into a service module?

I want to move some functionality out of a Rails model into a service module.
A concern isn't the correct thing for this as it's only for one model, I just want to tidy up some code elsewhere.
I can't seem to get basic calls on the model to work, here's the set up I have:
/app/models/account.rb
class Account < ApplicationRecord
include SomeService
end
And in a differen't location:
app/services/some_service.rb
module SomeService
def test_code
"abc"
end
def self.test_code_2
"xyz"
end
end
In this case I would then expect Account.first.test_code to output abc and Account.test_code_2 to output zyx.
How do I move functionaity like this out of a model but not into a concern? I feel like I'm very close to this working.
This code doesn't actually define a class method:
module SomeService
def test_code
"abc"
end
def self.test_code_2
"xyz"
end
end
It declares a method on the module itself which you can verify by running SomeService.test_code_2. Thats because self is not a "class method keyword" like static in other languages - its just a reference to the current lexical scope. In this case the module itself.
When you declare methods in a class:
class Foo
def self.bar
"Hello world"
end
end
self is whats known as the singleton class - an instance of the Class class. So it defines a method on the Foo class.
When you include a module in a class you're adding the module to the ancestors chain of the class. It can thus call the instance methods of the module as if it where its own. You can contrast this with extend which imports the methods of the module into the class (test_code becomes a class method).
Thus the ClassMethods pattern which extends the class with an inner module when its included:
module SomeService
def self.included(base)
base.extend ClassMethods
end
def test_code
"abc"
end
module ClassMethods
def test_code_2
"xyz"
end
end
end
How do I move functionaity like this out of a model but not into a concern?
What you're doing is a concern. The term "concern" in Rails really just vaguely means something like "a module thats mixed into classes". The only real definition is that app/models/concerns and app/controllers/concerns are autoloading roots and ActiveSupport::Concern exists which just simplefies the boilerplate code needed when writing mixins. Like for example you can use its class_methods macro to shorten the above code.
There is no actual definition of what a concern should contain or what its role is in an application nor is there any requirement that it be mixed into multiple classes.
But...
Moving logic out of a model (or any class) and into a module actually accomplishes nothing if you then include it back into the model. You're just obscuring the code by shuffling it into multiple files.
The amount of responsibilites and complexity remains the same.
If you actually want to redestribute the responsibilites you want to create an object that can stand on its own and does a unit of work:
class SomeService
def initialize(thing)
#thing = thing
end
def perform
# do something awesome with #thing
end
def self.perform(thing)
new(thing).perform
end
end
This is commonly known as the service object pattern - service modules are AFAIK not a thing. This has a defined set of responsibilites and offloads the model. ActiveJob is an example of this pattern.
What you are doing is known as method extraction - basically just splitting a god like object into modules because modules are good and big classes are evil. Right? Nope. Its still a god class. This became really popular around the time that Rails introduced the concerns folders.
Another solution that should not be overlooked is to look at if the model is actually doing to much and should be split into multiple models with more clearly defined responsibilites.
You can define a ClassMethods module inside your module and include it in the base class. This way, the normal module methods will be available as instance methods and the methods defined inside ClassMethods will be available as class methods in the base class.
app/services/some_service.rb
module SomeService
def self.included(base)
base.extend ClassMethods
end
def test_code
"abc"
end
module ClassMethods
def test_code_2
"xyz"
end
end
end

Injecting custom callback in rails 3

I have a custom module which sets up a hash to be stored in my sql. As part of this it rolls a its own _changed accessor.
module MyAwesomeCustomModule
extend ActiveSupport::Concern
included do
after_save: wipe_preferences_changed
end
module ClassMethods
def blah
end
etc
end
end
and then in my model:
class MyModel < ActiveRecord::Base
include MyAwesomeCustomModule
after_save :something_that_expects_preferences_changed_to_be_available
blah
end
unfortunately, the after_save defined in the custom module runs before the one defined in the model. Is there a way to get the array of all callbacks and append to it? Is there a way to write a custom after_after_save callback? Is there a way to specify priority/ordering of after_save callbacks?
What would be a good way to resolve this race condition?
In spite of the order of model callbacks, the current design makes the module and the class very coupled.
To solve the current problem as well as improve design, you can define an expected callback in the module's method, and then the class who includes this module is free to respond it or not.
module MyAwesomeCustomModule
extend ActiveSupport::Concern
included do
after_save: wipe_preferences_changed
end
def wipe_preferences_changed
# previous logic to wipe
process_further if respond_to :process_further
end
end
class MyModel < ActiveRecord::Base
include MyAwesomeCustomModule
# Feel free to write this or not
# The content is the previous
# :something_that_expects_preferences_changed_to_be_available
def process_further
end
end
If you want to keep your original strategy (2 after_save callbacks) all you should need to do is move the include statement below the model after_save.
class MyModel < ActiveRecord::Base
after_save :something_that_expects_preferences_changed_to_be_available
include MyAwesomeCustomModule
blah
end
Callbacks are executed in the order they are defined. The include statement acts (very roughly) like you had copy and pasted the code from the module at that point, so by putting the include statement above the after_save in your model you were causing that callback to execute first.

Detect whether an acts_as_something line is present in a model or not?

Assuming the following module which allows to add acts_as_timeable functionality to an arbitrary model.
module Timeable
module ActsAsTimeable
extend ActiveSupport::Concern
module ClassMethods
def acts_as_timeable(options ={})
...
end
end
end
end
ActiveRecord::Base.send :include, Timeable::ActsAsTimeable
According to the last line, acts_as_timeable class method is made available in ActiveRecord::Base. So any model extending form ActiveRecord::Base will return true when calling Model.respond_to?(:acts_as_timeable) => true.
How can I detect whether a model actually acts_as_timeable based on whether a line starting with acts_as_timeable...
class Model < ActiveRecord::Base
acts_as_timeable
end
...(and maybe some options) has been added to the model or not?
I think that the simplest way to do this is to set this state on the model class itself.
In acts_as_timetable you can set a class variable, and expose it through an accessor, like this:
module ClassMethods
def timeable?
!!#timeable
end
def acts_as_timeable(options = {})
#timeable = true
# Rest of your code
end
end
Then you can check simply with MyModel.timeable?

Ruby on Rails: shared method between models

If a few of my models have a privacy column, is there a way I can write one method shared by all the models, lets call it is_public?
so, I'd like to be able to do object_var.is_public?
One possible way is to put shared methods in a module like this (RAILS_ROOT/lib/shared_methods.rb)
module SharedMethods
def is_public?
# your code
end
end
Then you need to include this module in every model that should have these methods (i.e. app/models/your_model.rb)
class YourModel < ActiveRecord::Base
include SharedMethods
end
UPDATE:
In Rails 4 there is a new way to do this. You should place shared Code like this in app/models/concerns instead of lib
Also you can add class methods and execute code on inclusion like this
module SharedMethods
extend ActiveSupport::Concern
included do
scope :public, -> { where(…) }
end
def is_public?
# your code
end
module ClassMethods
def find_all_public
where #some condition
end
end
end
You can also do this by inheriting the models from a common ancestor which includes the shared methods.
class BaseModel < ActiveRecord::Base
def is_public?
# blah blah
end
end
class ChildModel < BaseModel
end
In practice, jigfox's approach often works out better, so don't feel obligated to use inheritance merely out of love for OOP theory :)

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