####1. What’s the difference between rendering and redirecting? What’s the impact with regards to instance variables, view templates?

  • rendering: render a template HTML code, like partial.
  • redirecting: make a new request to controller.

Regards to instance variables for showing view templates, instance variables only survive at the same request, so if we need some information like errors store in instance variables, we need to use render for this condition.

####2. If I need to display a message on the view template, and I’m redirecting, what’s the easiest way to accomplish this?

Use flash to store error message. This is the easiest way to store one message.

####3. If I need to display a message on the view template, and I’m rendering, what’s the easiest way to accomplish this?

Use instance variable to store errors, then render.

* or use flash.now and render it.

####4. Explain how we should save passwords to the database.

Use one way hash. Since we can’t promise that data in database never be stolen, the way to protect our password is using a one way hash to store the digest. If someone get this digest, it’s hard for him to decyphor this digest to a plain text password.

####5. What should we do if we have a method that is used in both controllers and views?

Put the methods into controllers, and make them become helpers. Like:

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helper_method :current_user

Now, method current_user can be used in both controllers and views.

####6. What is memoization? How is it a performance optimization?

Some repeatedly query action for database will reduce our app performance, using memoization to overcome this.

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@comment ||= User.find(session[:user_id]) if session[:user_id]

Syntax ||= do a OR combined action, if @comment is nil, then User.find invokes; if @comment exist, just return @comment without invoke User.find. This prevent to invoke database query again.

####7. If we want to prevent unauthenticated users from creating a new comment on a post, what should we do?

Add a helper method required_login in before_action for actions create and ‘’. Since this method is used in both post and comment, we can put this method into ApplicationController.rb.

####8. Suppose we have the following table for tracking “likes” in our application. How can we make this table polymorphic? Note that the “user_id” foreign key is tracking who created the like.

id user_id photo_id video_id post_id
1 4 12
2 7 3
3 2 6

According to this table, there should be 5 tables in database:

  • User
  • Photo
  • Video
  • Post
  • Like

And here are they might be in models:

User.rb
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class User ActiveRecord::Base
  has_many :likes, as: likeable
end
Photo.rb
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class Photo ActiveRecord::Base
  has_many :likes, as: likeable
end
Video.rb
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class Video ActiveRecord::Base
  has_many :likes, as: likeable
end
Post.rb
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class Post ActiveRecord::Base
  has_many :likes, as: likeable
end
Like.rb
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class Like ActiveRecord::Base
  belongs_to :user_id
  belongs_to :likeable, polymorphic: true
end

####9. How do we set up polymorphic associations at the model layer? Give example for the polymorphic model (eg, Vote) as well as an example parent model (the model on the 1 side, eg, Post).

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class Vote < ActiveRecord::Base
  belongs_to :voteable, polymorphic: true
end

class Post < ActiveRecord::Base
  has_many :votes, as: voteable
end

####10. What is an ERD diagram, and why do we need it?

Entity Relationship Dirgram(ERD) stands for tables in database relationship between each others. It’s a convinient way to communicate with others what the tables relationship, even for ourselves under developming.