Ruby On Rails 面试题与答案

问题 21. What deployment tools do you use for Ruby on Rails?

Capistrano is a popular deployment tool.

问题 22. What deployment tools do you use for Ruby on Rails?

Capistrano is a popular deployment tool.

问题 23. How can you migrate your database schema one level down?

The rake tool does most of the migrations. 

It has this nifty syntax to go back one step:

rake db:rollback

If you want to rollback all the way to the beginning you would use:

rake db:reset

This would drop the database, recreate the Database and load the current schema into it

If you want to rollback multiple steps at the same time you would use:

rake db:rollback STEP=3

To rollback all the way and if you are not worried about losing the data then you can drop the database completely with purge like this:

rake db:purge

问题 24. What is Sweeper in Ruby on Rails?

Sometimes you want to have control over how often and when the cache expires. 

Sometimes it is a good idea to have the system determine that on a logical basis. Say you have a list of product on your site and you want to reload the cache each time a new product is added/updated/deleted, then you can achieve this by using the sweeper. 

class ProductSweeper < ActionController::Caching::Sweeper
  OBSERVE PRODUCT# This sweeper is going to keep an eye on the Product model 
  # If our sweeper detects that a Product was created call this
  def after_create(product)
    expire_cache_for(product)
  end
  # If our sweeper detects that a Product was updated call this
  def after_update(product)
    expire_cache_for(product)
  end
  # If our sweeper detects that a Product was deleted call this
  def after_destroy(product)
    expire_cache_for(product)
  end
  private
  def expire_cache_for(product)
    # Expire the index page now that we added a new product
    expire_page(:controller => 'products', :action => 'index')
    # Expire a fragment
    expire_fragment('all_available_products')
  end
end

问题 25. How can you implement Caching in Ruby on Rails?

Rails offers multiple ways to cache content.

Fragment caching is my favorite because it gives you the choice to fragment to pull a portion from the cache and the remaining from a real-time DB call. 

Say you wanted to show all the orders placed on your website in real time and didn’t want to cache that part of the page, but did want to cache the part of the page which lists all products available, you could use this piece of code:

<% Order.find_recent.each do |o| %>
  <%= o.buyer.name %> bought <%= o.product.name %>
<% end %>
<% CACHE DO %>  All available products:
  <% Product.all.each do |p| %>
    <%= link_to p.name, product_url(p) %>
  <% end %>
<% end %>

Another technique that works well for static pages is page caching. This technique is often used for home pages and is super fast.

class ProductsController < ActionController
   CACHES_PAGE:index
  def index
    @products = Products.all
  end
end