This is a fairly basic Ruby/Rails question but I'm not sure why it makes a difference if you call a class of an object in some circumstances vs calling an instance of that object in different places of the framework.
Say you have a model e.g. Product and you call Product.new you have a new instance of the class. But if you have certain methods that are defined in the model I only seem to be able to access these if I call the Class rather than an instance e.g. Product.where(param, param). But I cannot call product.where(param, param) - why is this?
There are two types of methods: Class methods, and instance methods. You must call the appropriate method on the right object.
class Product
def self.foo
# class method, only callable on Product
end
def name
# instance method, callable on an instance of Product.
end
end
If you attempt to call an instance method on a class, or vice versa, you'll see an undefined method error.
To use someone else's analogy, imagine a house and a blue print; the class is a blue print for an object, while a house would represent the instance. An instance of that class will have its own set of attributes (wall colour, window type, etc...).
What would this mean?
p = Product.find(1)
p.where('something == 2')
That doesn't make any sense, you have an instance, what are you querying for? Good API design results in methods defined where they make sense.
Related
I'm wondering if there is a way to force a subclass to override a method from its parent method in either Ruby or Rails (in Java you would do this with an abstract class/method).
Let's say I have the following classes:
class Pet
end
class Dog < Pet
def collar_color
"red"
end
end
class Cat < Pet
end
dog = Dog.new
dog.collar_color
==> red
cat = Cat.new
cat.collar_color
==> NoMethodError
In this example I would never instantiate a Pet object, but it exists to serve as a way to collect common methods to common classes. But let's say I want to ensure that all subclasses of Pet override the collar_color method. Is there a way to do that? Could I achieve it through testing in some way? Assume I don't want a default defined in the parent class.
My real-life use case is a collection of classes that all have polymorphic ownership of another class. If I have a display page of the owned class, then one of the owner classes not having a method could leave me with a NoMethodError problem.
No, there is no way to enforce this.
I can guarantee you, whatever idea you can come up with, it will break in some way.
First off: doing this statically is out of the question. Determining whether a method is overridden or not is known to be equivalent to solving the Halting Problem.
So, you have to do it dynamically. But even that is going to be problematic.
For example: you could implement the inherited hook and check whether every class that inherits from Pet implements the method. But, that will prevent someone from inheriting their own abstract class. (Also, there is no guarantee when the inherited hook will run – it could run when the class is opened, i.e. before the methods are defined.)
Also, even if you can check that the method exists at the point where a class inherits Pet, the method can still be removed again later, so you don't get any guarantees. And, of course, they can just provide a dummy method, in order to get around your protection.
You could create default implementations of the methods that just raise an Exception, but there is no need to do that: if you don't create a default implementation, that will already raise a NoMethodError exception anyway. (If you do go down this route, do not use NotImplementedError. Instead, use a custom exception that inherits from RuntimeError.)
There are examples of this in the core library: for example, the Enumerable mixin depends on an abstract method each that must be implemented by subclasses. And the way this is handled is by simply documenting that fact:
Usage
To use module Enumerable in a collection class:
Include it:
include Enumerable
Implement method #each which must yield successive elements of the collection. The method will be called by almost any Enumerable method.
That is actually the way any type-related issues have been dealt with in Ruby since the beginning. Since Ruby does not have types, typing only happens in the programmer's head and type information is only written down in documentation.
There always were informal third-party type annotation languages that were used by various IDEs. More recently, two type annotation languages have been introduced: RBI, a third-party type annotation language used by the Sorbet type checker, and RBS, a type annotation language that is part of Ruby proper.
As far as I know, RBS has no way of expressing abstract methods, but RBI does:
class Pet
extend T::Sig
extend T::Helpers
interface!
sig {abstract.returns(String)}
def collar_color; end
end
This will give you a type error if there is object instantiated from a subclass that does not at some point in the inheritance chain define the method. But, of course, only if the user of the code actually type-checks the code using a type-checker like Sorbet. If the user of the code does not type-check it, they will not get a type error.
Ruby has relatively few keywords but it provides the basic building blocks to implement something that vaguely resembles abstract classes or methods.
In its simplest form you just raise an error in the parent "abstract" method:
class AbstractMethodError < StandardError
def initialize(klass, m)
super("Expected #{klass} to implement #{m}")
end
end
class Pet
def collar_color
raise AbstractMethodError.new(self.class, __method__)
end
end
class Cat < Pet
end
Cat.new.collar_color # Expected Cat to implement collar_color (AbstractMethodError)
__method__ is a magic variable that contains the name of the current method.
You can make this a bit more elegant by creating a class method that defines the "abstract method":
module Abstractions
def abstract_method(name)
define_method(name) do
raise AbstractMethodError.new(self.class, __method__)
end
end
end
class Pet
extend Abstractions
abstract_method :collar_color
end
However Ruby is a dynamic langauge and you don't have a compile time check so this will only give a slightly more obvious error message when the method is called. It doesn't actually give any guarentees that subclasses implement the method.
That is down to testing or using type checkers like Sorbet or RBS. In general it might be helpful when learning to forget everything you think you know about Object Oriented Programming and learn the Ruby way. It has a very different design philophy compared to Java - instead of abstract methods and interfaces you use duck typing to see if the object responds to that method.
Just define the default method implementation in the abstract class by raising Not implemented error or something. By doing that you also clarifies in your class design that when others / you want to inherit the Pet class they need to override collar_color method. Clarity is a good think and there is no benefit in not defining a default method in the abstract class.
Or if you want to achieve that by testing you can create a test case for Pet class that check if its descendants is defining their own collar_color method or not. I think Rails / Ruby 3.1 have .descendants methods defined or you can just google them.
# Pet_spec.rb
describe "descendants must implement collar_color" do
it "should not throw error" do
descendants = Pet.descendants
descendants.each do |descendant|
expect { descendant.new.collar_color }.to.not raise_error
end
end
end
I have a class User that inherits from ApplicationRecord. When I check User.ancestors.include?(Class), the result is false but for User.class, the result is Class.
What's the use case for the information supplied by the class method in such cases? What does it really tell me to be of practical use? It doesn't seem to have anything to do with inheritance.
What's the use case for the information supplied by the class method in such cases? What does it really tell me to be of practical use? It doesn't seem to have anything to do with inheritance.
class works the same way for every object. Calling class on a class provides the same information as calling class on an instance. That's because in Ruby, classes are instances, too.
'foo'.class returns String because 'foo' is an instance of String. Likewise, User.class returns Class because User is an instance of Class.
In particular, User is not an instance of ApplicationRecord.
It might not be obvious that User is an instance of Class when creating it via the class keyword:
class User < ApplicationRecord; end
But it becomes very obvious when you create it explicitly via Class.new: (both examples produce the same result)
User = Class.new(ApplicationRecord)
User.class #=> Class
Because the above is just like: (using String.new for demonstration purposes)
foo = String.new('foo')
foo.class #=> String
User is a class. All classes are instances of the class Class (i.e., User.class #=> Class). Therefore, all instance methods defined on Class, or on Class's ancestors, can be invoked on User (i.e., every instance method of Class is a class method of User).
For example, one of Class's instance methods is instance_methods. One can therefore execute User.instance_methods to obtain an array of User's instance methods.
After reading another StackOverflow post, I realized that it can only be understood by keeping the following in mind:
User class has a double role. It plays Class as well as Object.
As Class, it lets other classes inherit from it.
As Object, it inherits the class method all the way up from the Object class.
If I wanted my User.class to behave on the traditional lines (that is, User.class shows ApplicationRecord), then I have to override
it as follows:
class User
def self.class
self.superclass
end
end
Classes in Ruby are first-class objects—each is an instance of class
Class
User is an object of class Class (which inherits from Object).
class is an instance method of Object. It returns the class of the object. Therefore, User.class returns Class. Every object will have this method, and it is useful to know what the object is (e.g., for debugging). Consider this code:
1.class
# => Fixnum
"q".class
# => String
The reason why User.ancestors does not return Class is because it returns the modules included in the module. For example, Class.ancestors will include Class, but not User because User is an object of Class, which includes its own modules.
I am watching Code School Rails testing course. There is an instance of the class zombie. The zombie model has a method:
def avatar_url
...
end
Within the test, .rb file has the following:
z.avatar_url
When I call a method like this, how does Rails distinguish if I'm calling a controller or model method? I hadn't thought of calling a model method from other than a controller, and only like Model.method and not object.method.
If both my controller and my model have a method with the same name, how would Rails know which one to call?
Update:
Lets take the class String as example, it is not a model, right?
So I could say:
s = String.new
s.capitalize
If this call doesn't go to a model and not to a controller, where does it go then? Where would a class like String be defined in the Rails directory?
A method inside a controller can only be called via URL.
Example:
/things/super_action
Should call the def super_action inside ThingsController.
As for Model methods, they can be accessed anywhere. Just note if they are instance or class methods:
Model.ultra_method
This is a class method call, it is probably defined as def self.ultra_method.
m = Model.new
m.instance_method
This is a instance method call, and it is probably defined as def ultra_method.
UPDATE
String is a core class of ruby language. As is Array, Number, etc. In your example you are creating an instance of String and calling an instance method of the String class.
It seems like you're new to Ruby as well as Rails. In Ruby, a class is sort of like a description of a type of object (although the class itself is an object, too). Whenever there is a class defined, you can create new instances of it, as with String.new. Note that classes always have capitalized names.
Class methods are methods that work on the class itself. You can tell when a method is a class method because it will be attached to the capitalized name of the class (just like String.new). On the other hand, instance methods only work on an instance of the class, not on the class itself (eg str = String.new; str.capitalize!). Usually there are more instance methods than class methods, because instances are the things that you're actually working with (new is the most common class method you'll see).
As others have mentioned here, String is not a Rails model; it's a basic Ruby class. When you're working in Rails, you have access to all the regular Ruby classes as well as other classes and methods that are defined within Rails' source code. So String is not defined in Rails itself, but Rails does provide some useful instance methods for strings (eg str.to_date).
A model in Rails is really just a Ruby class. To understand the workings of a model, you should make sure you understand how Ruby classes work. What makes Rails models special is that they inherit from a class defined in Rails' source code known as ActiveRecord (any class in Ruby can inherit from another class, this is just one example of that). ActiveRecord has a number of class and instance methods, which are also available to your models because they inherit from ActiveRecord. For example, if you have a class (model) called Person, you can automatically use the Person.find(id) class method to look up a particular instance of the Person class in the database. You also have the person.save instance method to save the instance to the database.
All of this was confusing to me when I first started, so my best advice is to familiarize yourself with Ruby as you learn Rails.
You can call Model class/instance methods from anywhere in Rails. Model are just the mapping to your database and acts as a proxy for your database. When you are calling
z.avatar_url
you are calling a method on "z" model instance. So not matter from where you call it will always call the model's method.
If you define a method with same name in both controller and model, you would always be calling a model's method with model instance or model class. Controller methods are simply action in Rails they are never referred directly from anywhere. They are used for Rails routing.
Hope I am clear.
I am getting confused as to how to properly set variables in a initializer, I want these to be class level variables, not instance.
And I also want to then create a single instance of another object (it is a connection object, which already has connection pooling built in, so I just need a single reference to it).
My initializer /initializers/my_class.rb
yml = YAML.load_file("#{Rails.root}/config/my_class.yml")
MYMODULE::MyClass.init(yml)
And here is my my_class.rb:
module MYMODULE
class MyClass
def self.init(yml)
#post_url = yml["defaults"]["post_url"]
end
def self.post_url
#post_url
end
# this should be a single instance
def connection_pool
# ???
end
end
end
These class level variables, how can I access them from both class methods and instance methods?
I'm getting wierd behaviour, and I'm confused as to how to reference the post_url from inside of either class methods and instance methods.
I have seen the following ways, unsure which is correct:
self.class.post_url
MyClass.post_url
#post_url
post_url
self.post_url
self.class.post_url or MyClass.post_url will work. The difference is how they work for subclasses (in the former case, subclasses will use their own version of this variable automatically, in the latter, they would share the variable in MyClass).
There is no way to directly access class instance variables from an instance: you have to call a class method which returns (or sets) them. See also: cattr_accessor.
That said, if this is really a singleton, it seems a little strange to me that you would configure part of it on the class, and then reference that info in the (single) instance. Wouldn't it make more sense just to configure this stuff on the instance? Or use a module as a singleton and not create an instance at all?
I am developing a Rails application and would like to understand when to use self.for.
Here is the code of a method that I would like to fully understand. If it is possible I would like to have an alternative to this code so it would make things more clear.
def self.for(facebook_id)
User.create_by_facebook_id(facebook_id)
end
self refers to the current object.
Within a class, self is used to define a class-level method.
class Foo
def self.for(facebook_id)
User.create_by_facebook_id(facebook_id)
end
end
defines a class method for in class Foo. It is invoked:
Foo.for(facebook_id)
You can google for class methods to learn more.
It could be that a part of Rails or a plugin/gem is expecting that some classes will have a class method "for" More context would be helpful in this regard.
What the method is doing
As is common for class methods, it is creating an instance of a class. For example, the ActiveRecord class has a class method "create" which attempts to create an instance of the model class that has been stored in the database. Thus User.create will return an instance of the User class that has been stored in the database.
In your example code, it is calling a class method "create_by_facebook_id" that is provided by the User class in the application.
Looks like the "for" method is being used for information hiding since all it's doing is making another method call (to User.create_by_facebook)
Added:
By the way, the default return value from Ruby methods is the value of the last statement. So your example method will return the user instance newly created from the supplied facebook_id.
It looks to me like self.for is just an alias for creating a user for a facebook id. I think self.for should actually be:
def self.for(facebook_id)
User.find_or_create_by_facebook_id(facebook_id)
end
That way it searches for the user with that facebook id, and if one isn't found, creates that record and returns it. Then, self.for means "return the user for this facebook id."
Short answer: self always refers to the current object. So within an instance method, self is the instance, within a class method, self is the class and within a class definition (like in your example), self is the class...
For more information on class methods and the code snippet you posted, see the answer by Larry K.
If that code is inside of a class named Foo, then the alternative could be:
def Foo.for(facebook_id)
User.create_by_facebook_id(facebook_id)
end
self in this context is necessary if you have some generic class level methods that you want to be able to use across multiple classes. You add them to a Module using self. to scope them (as you don't know the name of the actual class that they are going to a part of), then include that module as part of your class.