2: A Demo App in Java

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2: A Demo App
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Error messages for a failed micropost creation
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Listing 210 A user has many microposts
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app/models/userrb
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class User < ActiveRecord::Base has_many :microposts end
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Listing 211 A micropost belongs to a user
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app/models/micropostrb
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class Micropost < ActiveRecord::Base belongs_to :user validates :content, :length => { :maximum => 140 } end
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The Microposts Resource
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users name Michael Hartl Foo Bar
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id 1 2 3
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microposts content user_id First post! 1 Second post 1 Another post 2
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email mhartl@examplecom foo@barcom
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The association between microposts and users
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We can visualize the result of this association in Figure 215 Because of the user_id column in the microposts table, Rails (using Active Record) can infer the microposts associated with each user In 11 and 12, we will use the association of users and microposts both to display all a user s microposts and to construct a Twitter-like micropost feed For now, we can examine the implications of the user-micropost association by using the console, which is a useful tool for interacting with Rails applications We first invoke the console with rails console at the command line, and then retrieve the first user from the database using Userfirst (putting the results in the variable first_user):14
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$ rails console >> first_user = Userfirst => #<User id: 1, name: "Michael Hartl", email: "michael@exampleorg", created_at: "2010-04-03 02:01:31", updated_at: "2010-04-03 02:01:31"> >> first_usermicroposts => [#<Micropost id: 1, content: "First micropost!", user_id: 1, created_at: "2010-04-03 02:37:37", updated_at: "2010-04-03 02:37:37">, #<Micropost id: 2, content: "Second micropost", user_id: 1, created_at: "2010-04-03 02:38:54", updated_at: "2010-04-03 02:38:54">]
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Here we have accessed the user s microposts using the code first_usermicroposts; with this code, Active Record automatically returns all the microposts with user_id equal to the id of first_user (in this case, 1) We ll learn much more about the association facilities in Active Record in 11 and 12
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14 Your console prompt will probably be something like ruby-192-head >, but I ll use >> so that the prompt isn t tied to a specific Ruby version
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2: A Demo App
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234 Inheritance Hierarchies
We end our discussion of the demo application with a brief description of the controller and model class hierarchies in Rails This discussion will only make much sense if you have some experience with object-oriented programming (OOP); if you haven t studied OOP, feel free to skip this section In particular, if you are unfamiliar with classes (discussed in Section 44), I suggest looping back to this section at a later time We start with the inheritance structure for models Comparing Listing 212 and Listing 213, we see that both the User model and the Micropost model inherit (via the left angle bracket <) from ActiveRecord::Base, which is the base class for models provided by ActiveRecord; a diagram summarizing this relationship appears in Figure 216 It is by inheriting from ActiveRecord::Base that our model objects gain the ability to communicate with the database, treat the database columns as Ruby attributes, and so on
Listing 212 The User class, with inheritance
app/models/userrb
class User < ActiveRecord::Base end
Listing 213 The Micropost class, with inheritance
app/models/micropostrb
class Micropost < ActiveRecord::Base end
The inheritance structure for controllers is only slightly more complicated Comparing Listing 214 and Listing 215, we see that both the Users controller and the Microposts controller inherit from the Application controller Examining Listing 216, we see that ApplicationController itself inherits from ActionController::Base; this is the base class for controllers provided by the Rails library Action Pack The relationships between these classes is illustrated in Figure 217
The Microposts Resource
ActiveRecord::Base
User
Micropost
The inheritance hierarchy for the User and Micropost models
Listing 214 The UsersController class, with inheritance app/controllers/users_controllerrb
class UsersController < ApplicationController end
Listing 215 The MicropostsController class, with inheritance
app/controllers/microposts_controllerrb
class MicropostsController < ApplicationController end
Listing 216 The ApplicationController class, with inheritance
app/controllers/application_controllerrb
class ApplicationController < ActionController::Base end
2: A Demo App
ActionController::Base
ApplicationController
UsersController
MicropostsController
The inheritance hierarchy for the Users and Microposts controllers
As with model inheritance, by inheriting ultimately from ActionController::
Base both the Users and Microposts controllers gain a large amount of functionality, such
as the ability to manipulate model objects, filter inbound HTTP requests, and render views as HTML Since all Rails controllers inherit from ApplicationController, rules defined in the Application controller automatically apply to every action in the application For example, in Section 824 we ll see how defining a rule in the Application controller allows us to filter passwords from all the Rails log files, thereby avoiding a serious potential security breach
235 Deploying the Demo App
With the completion of the Microposts resource, now is a good time to push the repository up to GitHub:15
$ git add $ git commit -a -m "Done with the demo app" $ git push
15 Ordinarily, you should make smaller, more frequent commits, but for the purposes of this chapter a single big commit at the end is just fine