ActiveSupport::Concern should be included or extended - ruby-on-rails

I know include is used for access module methods as instance methods while extend is used to access module methods as class methods.
For ActiveSupport::Concern somewhere I see written as,
module Test
include ActiveSupport::Concern
end
while at some places written as,
module Test
extend ActiveSupport::Concern
end
Here my confusion is, ActiveSupport::Concern should be used with include or with extend?

You should use extend ActiveSupport::Concern, like shown in the examples in the documentation.
By using ActiveSupport::Concern the above module could instead be
written as:
require 'active_support/concern'
module M
extend ActiveSupport::Concern
included do
scope :disabled, -> { where(disabled: true) }
end
class_methods do
...
end
end
The reason for using extend is to make the methods defined in ActiveSupport::Concern available in module context. This allows you to use the methods included and class_methods within the module.
When using include those methods would not be available within the module and instead be available on instances of a class that includes M.
If you want to know the difference between the two I suggest taking a look at What is the difference between include and extend in Ruby?

You need to extend the module with ActiveSupport::Concern in order for the ActiveSupport::Concern#included and #class_methods methods to work properly.
These two methods are after all pretty much the only reason for its existance.
module A
extend ActiveSupport::Concern
# raises ArgumentError (wrong number of arguments (given 0, expected 1))
included do
puts "Hello World"
end
end
module B
extend ActiveSupport::Concern
included do
puts "Hello World"
end
end
class C
include B
end
# Outputs Hello World
Check out what happens if we inspect the included method:
module AB
include ActiveSupport::Concern
puts method(:included).source_location # nil
end
module ABC
extend ActiveSupport::Concern
puts method(:included).source_location # .../ruby/gems/2.7.0/gems/activesupport-6.0.2.1/lib/active_support/concern.rb
end
When we extend the module with ActiveSupport::Concern we are putting it on the ancestors chain of ABC, thus the methods of ActiveSupport::Concern are available as module methods of ABC. This does not happen when you use include and the included method called is actually Module#included from the Ruby core.

Related

self.included – including class methods from a module in Ruby

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.

ActiveSupport::Concern code placed inside the included block or in the module body

Suppose I have the following namespace with a Base module that defines some methods that can be reused
module MyNameSpace
module Magic
extend ActiveSupport::Concern
class_methods do
def magic_field(field_name)
# Defines methods and attributes based on field name
end
end
end
end
What is the difference (if there is any) between
module MyNameSpace
module Foo
extend ActiveSupport::Concern
include Magic
included do
magic_field(:foo)
end
end
end
and
module MyNameSpace
module Foo
extend ActiveSupport::Concern
included do
include Magic
magic_field(:foo)
end
end
end
(The question is about the difference of include Magic being either outside or inside the included block)
When the concern is included in a class, the include and class_methods allows for the receiving class to inherit those methods.
included adds instance methods while class_methods adds class methods.
Source: Rails Concerns Docs
On the other hand, if your question is the difference between the placement of include Magic, there is no difference in how the class would function.

How to get my model name within an ActiveSupport::Concern included block?

I'm using ActiveSupport::Concern to extend ActiveRecord funcionality, to DRY my modules. But in order to do so, I need the module name of my ActiveRecord inside included block.
For example, I have a Table where I keep my accesible attributes, And I want that each module will get his own accesible attributes
My module ActiveRecordExtension look like:
module ActiveRecordExtension
extend ActiveSupport::Concern
included do
AccessibleAttributes.where(name: "#{self.name.pluralize}").each do |e|
attr_accessible e.attribute.to_sym
end
end
end
# include the extension
ActiveRecord::Base.send(:include, ActiveRecordExtension)
A. How do I get my model name (dynamically) without getting nil:NilClass ?
B. And how do I check what included do returns ?

How to "nest" the inclusion of modules when using the Ruby on Rails ActiveSupport::Concern feature?

I am using Ruby 1.9.2 and the Ruby on Rails v3.2.2 gem. I would like to "nest" the inclusion of modules given I am using the RoR ActiveSupport::Concern feature, but I have a doubt where I should state the include method. That is, I have the following:
module MyModuleA
extend ActiveSupport::Concern
# include MyModuleB
included do
# include MyModuleB
end
end
Should I state include MyModuleB in the "body" / "context" / "scope" of MyModuleA or I should state that in the included do ... end block? What is the difference and what I should expect from that?
If you include MyModuleB in the "body" of MyModuleA, then it is the module itself that is extended with B's functionality. If you include it in the included block, then it is included on the class that mixes in MyModuleA.
That is:
module MyModuleA
extend ActiveSupport::Concern
include MyModuleB
end
produces something like:
MyModuleA.send :include, MyModuleB
class Foo
include MyModuleA
end
while
module MyModuleA
extend ActiveSupport::Concern
included do
include MyModuleB
end
end
produces something like:
class Foo
include MyModuleA
include MyModuleB
end
The reason for this is that ActiveSupport::Concern::included is analogous to:
def MyModuleA
def self.included(klass, &block)
klass.instance_eval(&block)
end
end
The code in the included block is run in the context of the including class, rather than the context of the module. Thus, if MyModuleB needs access to the class it's being mixed-in to, then you'd want to run it in the included block. Otherwise, it's effectively the same thing.
By means of demonstration:
module A
def self.included(other)
other.send :include, B
end
end
module B
def self.included(other)
puts "B was included on #{other.inspect}"
end
end
module C
include B
end
class Foo
include A
end
# Output:
# B was included on C
# B was included on Foo

Rails: Is that possible to define named scope in a module?

Say there are 3 models: A, B, and C. Each of these models has the x attribute.
Is that possible to define a named scope in a module and include this module in A, B, and C ?
I tried to do so and got an error message saying that scope is not recognized...
Yes it is
module Foo
def self.included(base)
base.class_eval do
scope :your_scope, lambda {}
end
end
end
As of Rails 3.1 the syntax is simplified a little by ActiveSupport::Concern:
Now you can do
require 'active_support/concern'
module M
extend ActiveSupport::Concern
included do
scope :disabled, where(:disabled => true)
end
module ClassMethods
...
end
end
ActiveSupport::Concern also sweeps in the dependencies of the included module, here is the documentation
[update, addressing aceofbassgreg's comment]
The Rails 3.1 and later ActiveSupport::Concern allows an include module's instance methods to be included directly, so that it's not necessary to create an InstanceMethods module inside the included module. Also it's no longer necessary in Rails 3.1 and later to include M::InstanceMethods and extend M::ClassMethods. So we can have simpler code like this:
require 'active_support/concern'
module M
extend ActiveSupport::Concern
# foo will be an instance method when M is "include"'d in another class
def foo
"bar"
end
module ClassMethods
# the baz method will be included as a class method on any class that "include"s M
def baz
"qux"
end
end
end
class Test
# this is all that is required! It's a beautiful thing!
include M
end
Test.new.foo # ->"bar"
Test.baz # -> "qux"
As for Rails 4.x you can use gem scopes_rails
It can generate scopes file and include it to your model.
Also, it can automatically generate scopes for state_machines states.

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