w3d6-w3d7 Flashcards
What is a callback?
Code that is run at a certain moment of an object’s life cycle (i.e. at creation, updating, deletion, etc.)
In an association, what happens to objects that belong to another object when the object they belong to is destroyed (read: the object the foreign key points to is destroyed)?
The objects are considered widowed.
Explain the following code:
class User < ActiveRecord::Base
has_many :posts, :dependent => :destroy
end
When a User is destroyed, the posts that belong to it are destroyed as well.
Where is :dependent added?
In has_many relations.
What are some other options for :dependent?
:nullify
Sets the foreign key to null when its target is destroyed.
How do we guarantee referential integrity?
Add a FOREIGN KEY constraint at the db level.
Explain the following code:
class User true before_validation :ensure_random_code
protected
def ensure_random_code
# assign a random code
self.random_code ||= SecureRandom.hex(8)
end
end
This is an example of callback registration: the random code attribute is added in case it is forgotten as part of the object’s validation.
What is an important property of callback methods?
They should be protected or private.
What are some useful ActiveRecord callbacks?
before_validation (handy as a last chance to set forgotten fields upon creation, not saving/updating)
• after_create (handy to do some post-create logic, like send a confirmation email)
• after_destroy (handy to perform post-destroy clean-up logic)
Explain the following code:
before_validation(:on => :create) do
self.number = number.gsub(/[^0-9]/, “”) if attribute_present?(“number”)
end
Strip everything but digits, so the user can specify “555 234 34” or
“5552-3434” or both will mean “55523434”
What is wrong with the following ERB:
The Law of Demeter is being violated; objects are talking to objects besides their neighbors/more than one dot is being used.
What does #delegate do and what options does it take?
delegate allows getter methods to be generated for other classes in associations, so that multiple dots are no longer needed. prefix: true can be specified so that sensible prefixes can be given to these getter methods.
Assuming we have an office, doctor, and patient, what code would make the following ERB viable:
class Office < ActiveRecord::Base belongs_to :doctor end
class Doctor < ActiveRecord::Base
has_one :office
has_many :patients
delegate :number,
:address,
to: :office,
prefix: true
end
class Patient < ActiveRecord::Base
belongs_to :doctor
delegate :name,
:specialty,
:office_number,
:office_address,
to: :doctor,
prefix: true
end
T/F: It does not matter what kind of SQL db you’re using; Rails migrations are db agnostic and will work with all of them.
T
What is the first thing that happens when a server running Rails gets a request from a web browser?
It first goes to the Rails router.
What does the Rails router do?
The router decides what the request is trying to do based on its address and other HTTP parameters.
What is the command to see all available routes in a Rails app?
rake routes
Explain the output of rake routes:
Prefix Verb URI Pattern Controller#Actionarticles GET /articles(.:format) articles#index
The prefix, articles, provides articles_path (the relative path) and articles_url (the full http address) as Rails methods.
The verb, GET, is the HTTP verb.
The URI pattern is like a regex in that is specifies what is given to the controller with that name (articles can take /articles/, /articles.json, etc.)
The final column is where the route maps to the controller method.
What is special about instance variables in Rails controllers?
They are automatically transferred from the controller to the object which renders the view template.
What are the arguments given to the #link_to helper?
The displayed text of the link, and the URL of the link (typically _path or _url macro)
What is a synonym for a controller action?
A controller method.
What action is typically used to render a view?
show
Where are URL parameters stored?
In the ‘params’ hash
What is the basic reason to add forms to a page?
To create a viable method for users to interact with the web and, by proxy, the db.
What does #form_for do?
This Rails helper takes an iv param, an object, and that object can be put in a do loop for each element the user needs to input.
Views can reference instance variables. Where are these instance variables declared?
In the corresponding controller method.
What is one way to inspect the params hash?
Put a fail/raise statement into the controller action which is called ath the point where you want to inspect.
What is the point of having strong parameters?
They prevent us from blindly accepting the entirety of the params hash as a parameter to creating an object. What if it included malicious code?
How can we use #require to to enforce strong parameters?
Use it in a helper to declare, along with #permit, which parts of the params hash you want to use:
def article_params
params.require(:article).permit(:title, :body)
end
What is the naming convention for partials?
Begin the filename with an underscore:
app/views/articles/_form.html.erb
What is a flash and how can we add one to an action?
A flash is a message to let a user know that something has taken place:
flash.notice = “Article ‘#{@article.title}’ Updated!”
