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Q: Please describe what polymorphic associations are in Rails and give an example of a scenario in which they should be used.

Answer:

Polymorphic associations allow a model to connect to multiple other models, imagine a scenario where you have two models, Employee and Product and you need to allow each one of those to have a picture image, without using polymorphic associations, you may be using two different models (EmployeePicture and ProductPicture) and tables (employee_pictures and product_pictures), by using polymorphic associations, you only need a single table to store the pictures since they can belong to more than one other model.

Q: Describe what inheritance, encapsulation, and polymorphism are in Rails, and provide an example for each.

Answer:

Inheritance allows the programmer to inherit the characteristics of one class into another class. Ruby supports only single class inheritance, it does not support multiple class inheritance but it supports mixins.

	
class Mammal  
  def breathe  
	puts "inhale and exhale"  
  end  
end  
 
class Cat < Mammal  
  def speak  
	puts "Meow"  
  end  
end
	cat = Cat.new
	cat.breathe # => “inhale and exhale”
	cat.speak # => “Meow”
	

Encapsulation means that the internal representation of an object is hidden from the outside. Only the object can interact with its internal data. Public methods can be created to open a defined way to access the logic inside an object.

	
class Document
  attr_accessor :name

  def initialize(name)
    @name = name
  end

  def set_name(name)
    @name = name
  end
end
d = Document.new('name1')
d.set_name('name1')
	

Polymorphism is the provision of a single interface to entities of different types.

	
class Document
  def initialize
  end

  def print
	raise NotImplementedError, 'You must implement the print method'
  end
end

class XmlDocument < Document
  def print
	p 'Print from XmlDocument'
  end
end

class HtmlDocument < Document
  def print
	p 'Print from HtmlDocument'
  end
end

XmDocument.new.print # Print from XmlDocument
HtmlDocument.new.print # Print from HtmlDocument
	

Q: What is the difference between a module and a class in Ruby?

Answer:

They may look similar, both can hold method definitions or constants, but there are some differences:

• you can instantiate objects from a class, but you cannot do it from a module
• you can create subclasses from a class, but you cannot create submodules from a module
• Modules are intended to be included on classes
  

Q: What is the difference between 'include ModuleName' and 'extends ModuleName?' How can class methods and variables be used when 'extended' versus 'included'? Please provide an example(s).

Answer:

include ModuleName is used to append methods from the module as instance methods of the class, while extend ModuleName is used to append methods as class methods. When using include the methods can be accessed directly from the instance

	
my = MyClass.new
my.method_defined_in_module
	

When using extends, the methods can be accessed from the class itself, like:

	
MyClass.method_defined_in_module
	

Q: What are the benefits of, or scenarios in which, you would use ActiveSupport::Concern in Rails?

Answer:

ActiveSupport::Concern provides a simpler way to create modules that can be included/shared in other classes, it also handles constant dependencies automatically. One pretty common scenario for concerns is extracting pieces of code that can be shared between different models.

Q: What is the difference between a Block, Proc and Lambda?

Answer:

Procs are objects, so we can create objects from a Proc and treat them as any other class instance. Lambdas are a kind of Proc that validates its parameters, procs do not validate them, however both can be used to create a block and store it in a variable to later pass it to a method. Blocks are like anonymous functions that can be passed into methods, they are called when using the yield keyword in a method definition.

Q: What is the purpose of HashWithIndifferentAccess class into ActiveSupport?

Answer:

Implements a hash where keys :foo and "foo" are considered to be the same. Internally symbols are mapped to strings when used as keys in the entire writing interface (calling []=, merge, etc).

Q: Describe at least two Ruby hook methods and explain when they are called.

Answer:

Methods

• respond_to_missing?: a way to make sure your dynamic methods defined with method_missing also handle respond_to? correctly
• method_missing: called when a method cannot be found, potentially to allow dynamically defining one instead
• method_added: called whenever a method is added… which can be used to modify the method
• method_removed: called whenever a method is removed
• singleton_method_added: method added to the singleton class of the object, to be callable just on this one instance
• singleton_method_removed: method removed from singleton class
• method_undefined: a method has been undefined, with undef_method. Undef_method is different from remove_method because remove_method may still allow superclasses to define the method – undef_method means it’s gone entirely.
• singleton_method_undefined: called when a singleton method is undefined entirely
• initialize_copy: an optional callback when cloning any Object
  

Classes

• inherited: a Ruby class is subclassed
  

Modules

• append_features: a Module is included, and its constants, methods and variables used
• included: a Module is included, which usually obsoletes “append_features”
• extend_object: a Module extends an Object
• extended: an Object is extended by a module, which mostly obsoletes extend_object
• const_missing: a constant isn’t already present
  

Q: Describe what Rails does on the client, and the format of the url that gets called, when `remote: :true` is added to a link.

Answer:

Rails using its UJS library listens for click events to links with the data-remote=true set, then it captures it preventing the default behaviour of the browser, then calling it via an AJAX request. The format of the URL usually ends in .js, ie, /users.js.

Q: What should be the response from the server for `remote: :true` call?

Answer:

Usually, response from the server for a remote call is javascript response, this means that the browser will evaluate the content of the response with Javascript, in Rails the response comes from a view partial with extension .js.erb allowing the evaluation of Ruby code inside before the Javascript code is sent back to the browser.

Q: What's the use of the db/seed.rb file? How do you handle multiple seed files?

Answer:

Seeds file is used as a mechanism of populating the database with test or default data. Handling multiple seeds files requires them to be manually loaded in the db/seeds.rb file

Q: What's the relative path inside of the project directory to a controller called Admin::ManagersController?

Answer:

app/controllers/admin/managers_controller.rb

Q: What is the difference between ActiveRecord and AREL?

Answer:

ActiveRecord is a high-level interface to construct SQL queries using Ruby and associate the result from database to ActiveRecord models (when possible), ActiveRecord queries are translated to SQL when sending to the database, it is simple and easy to learn and use, but it has limitations when it comes to complex SQL queries. Arel on the other side is the library used under the hood by ActiveRecord to construct the SQL queries, both of them allow you to build SQL queries using Ruby, but Arel is more flexible, it allows to dynamically compose SQL queries using a Ruby interface, it is strong and well designed.

Q: What ORM or query interface would you most want to use other than Active Record?

Answer:

Sequel, pretty powerful and flexible

Q: Who is your favorite Rubyist and why?

Answer:

This answer is based on opinion, focus on the why. Example, Aaron Patterson, he is working hard on improving the performance of Ruby towards the Ruby 3x3 (version 3 will be 3 times faster) goal.

Q: What are your top three sources for Ruby / Rails news and content?

Answer:

This answer is based on opinion. Official Rails Blog, the Slack Ruby Community, and the Ruby subreddit.