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.
Related
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
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
I read this post: Ruby modules - included do end block – but still don't get when you would use the self.included do ... end block in a module.
The post says that code in the block will be ran when you include the module, but what's the point of that if a module's sole purpose is to be included? Wouldn't that code need to be run anyway? That block doesn't need to be there in order for that code to be run, right?
What would be the difference between the two below:
module M
def self.included(base)
base.extend ClassMethods
base.class_eval do
scope :disabled, -> { where(disabled: true) }
end
end
module ClassMethods
...
end
end
vs.
module M
def self.some_class_method
...
end
scope :disabled, -> { where(disabled: true) }
end
What's the difference between the two examples?
The first code block adds the class methods in ClassMethods to the including class and calls the scope method on it as well. The second one does neither of these things and will result in a NoMethodError because the module has no scope class method. self.some_class_method will not be available on the including class once the module is included.
For the full story on how module inclusion works in Ruby, read my answer here:
Inheriting class methods from modules / mixins in Ruby
What's the point of self.included if a module's sole purpose is to be included?
Inclusion is not the only purpose of modules. They are also used for other things such as namespacing or simply storing various class methods that are then callable on the module itself.
Why doesn't Ruby include class methods automatically?
Theoretically Ruby could automatically add all class methods defined in a module to the including class, but in practice that would be a bad idea, because you would not get to choose anymore whether you want to include class methods — all class methods would be included every time, whether or not they are intended to be included. Consider this example:
module M
def self.class_method
"foo"
end
def self.configure_module
# add configuration for this module
end
end
class C
include M
end
Here, the configure_module method is obviously not supposed to be added to C, as its purpose is to set the configuration for the module object. Yet, if we had auto-inclusion for class methods, you would not be able to prevent it from being included.
But all instance methods are already included! How is that okay then?
Instance methods in a module are only really useful if they are included into a class, since modules cannot have instances, only classes can. So in a module every instance method is expected to be included somewhere to work.
A "class" method on a module is different, because it can be called on the module itself, so it can be used just fine regardless of whether it's also added to the including class. That is why it is better that you have a choice there.
module M
# self.included is the hook which automatically runs when this module is included
def self.included(base)
puts 'this will be printed when this module will be included inside a class'
base.extend ClassMethods
base.class_eval do
scope :disabled, -> { where(disabled: true) }
end
end
def print_object_class
self.class.name # here self will be the object of class which includes this module
end
module ClassMethods
def print_class_name
self.name # Here self would be the class which is including this module
end
end
end
I tried to modify the above module to help you understand how a module(concern) can be helpful in code reusability
self.included is a hook which runs automatically when a module is included inside a class.
any method declare in the module ClassMethods will become class methods for the class including this module
any method declare outside the module ClassMethods will become instance methods for the class including this module
For Ex suppose there is a class Product and you have included the module in it
class Product < ActiveRecord::Base
include M
puts 'after including M'
end
If you try this example in you rails console you will notice that as soon as the module M gets included in class Product included hook of module run and
this will be printed when this module will be included inside a class this is printed on console
after that after including M this will be printed on the console.
Also you can try following commands
Product.disabled # a scope with name 'disabled' is avaialble because of including the module M
Product.print_class_name # Outputs => 'Product' This method is available to class with the help of module M
Product.new.print_object_class #Outputs => 'Product'
It also offers reusability, include this module M in any class and that class gets access to all those methods described in the module.
Whereas your second example is a mere example of basic module
module N
def self.abc
puts 'basic module'
end
end
Now abc method define in module can be accessible using only this module
N.abc # outputs 'basic module'
class Product < ActiveRecord::Base
include N
end
Product.abc #raises exception, No method found on class Product
Product.new.abc #raises exception, No method found on object of class Product
I hope this may help you understand the concept of module better.
Please let me know if you still have any doubts.
I wrote an upsert method for one of my models. I would like all my models to have this upsert method. It seemed to me that the logical solution was to define a model that inherits from ActiveRecord::Base and then have all my other models inherit from that. But if I do that, Rails complains that the new model I created doesn't have a table to go with it, which is true, but I don't care.
Since the way I tried is apparently not the right way to do it, what's the right way to do it?
You can extend ActiveRecord with a module. you only do it in one place and it will be accessible for all models that inherits from ActiveRecord.
module YourModule
def self.included(recipient)
recipient.extend(ModelClassMethods)
recipient.class_eval do
include ModelInstanceMethods
end
end # #included directives
# Class Methods
module ModelClassMethods
# A method accessible on model classes
def whatever
end
end
# Instance Methods
module ModelInstanceMethods
#A method accessible on model instances
def another_one
end
end
end
#This is where your module is being included into ActiveRecord
if Object.const_defined?("ActiveRecord")
ActiveRecord::Base.send(:include, YourModule)
end
There are two ways to do this.
1) To have a parent model, but not need to create a table for it (i.e. an abstract class) you should set
class YourAbstractClass < ActiveRecord::Base
self.abstract_class = true
# rest of class code
end
2) Put the method in a module, that you include from all your models that need it (as in #Mark's answer)
You can move that method to a module and include that module in all the models that require that method.
Like I have this Utils module in lib folder of my app
module Utils
...
def to_name(ref)
ref.gsub('_', ' ').split.collect { |w| w.capitalize }.join(' ')
end
...
end
Then in my model, I say
class MyModel < AR::Base
include Utils
...
end
Probably, if you are using Rails 3, you should load the files in the lib folder by configuring your application.rb
config.autoload_paths += %W(#{config.root}/lib)
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 :)