9/18/12 Notes

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IT 231 -- 9/18/12

Review Questions

What is the difference betweeen these pairs of Rails delimiters:
Ans: ERB delimiters are used to call Rails helper functions or insert variable values into a view or layout. Use <%= ... %> when calling a Rails method that generates HTML code. Use <% ... %> when calling an ERB control method.
Write Rails helper functions to
Identify the parts of this URL:
Ans: protocol; server name; port number; controller; action; id
You enter the following URL in a browser, but the corresponding page does not display properly. List some things that might be wrong?
Ans:
1. You might have saved the view or layout properly.
2. You might not have started the server.
3. You didn't wait long enough for the server to startup before viewing the page.
4. You didn't convert the controller name to underscore notation before trying to use it as a URL.
5. The controller name includes an uppercase letter.
6. You don't have a proper end title tag.
7. You are missing an ending ERB delimiter.
What does this ERB statment do?
Ans: It marks where the view is to be inserted into a layout page.
Add a new view named contact_us to the Info controller of the Library project. This involves these three tasks:
1. Add the new view contact_us.html.erb file to the folder app/views/info.
2. Add a new method named contact_us in the the contact_us_controller.rb file in app/controllers.
3. Add a new get route in the routes.rb file in app/config.

The Controller Class

The controller class contains methods called actions that are executed on behalf of the views.
Controller files are stored in the app/controllers file.
Look at the controller file for the Library Example, named info-controller-rb
The name of the controller class is InfoController. It inherits most of its functionality from the ApplicationController class.
The InfoController class contains the three actions index, staff, and policies. Before a view is displayed its corresponding action is executed.
@title is an instance variable. Instance variables always have the @ prefix and can be used anywhere in a Rails class or in an ERB file that is associated with that controller class.
A local variable does not have the @ prefix. A local variable can only be used within a method.

Alternative Forms for Helper Methods

Look at the generated source code for each view of the Library project.
For the following Examples, the Rails helper method invocation is shown on the first line of each pair.
The generated HTML source code or path is shown indented on the second line of each pair.

image_tag method

<%= image_tag 'car.jpg' %>
        <img src="/images/car.jpg" alt="Car" /> 

<%= image_tag 'dogs/image.jpg' %>
        <img src="/images/dogs/collie.jpg" 
            alt="Collie" /> 

<%= image_tag 'car.jpg', :alt => 'Ford Taurus' %>
        <img src="/images/car.jpg" 
            alt="Ford Taurus" /> 

<%= image_tag 'car.jpg', :size => '250x200' %>
        <img src="/images/car.jpg" height="250" 
            width="200" alt="Ford Taurus" /> 

<%= image_tag 'car.jpg', :width => '250' %>
        <img src="/images/car.jpg" height="250" 
            alt="Ford Taurus" /> 

<%= image_tag 'car.jpg', :class => 'carimg' %>
        <img src="/images/car.jpg" class="carimg" 
            alt="Ford Taurus" />

Reference: api.rubyonrails.org/classes/ActionView/Helpers/AssetTagHelper.html

stylesheet_link_tag method

<%= stylesheet_link_tag 'styles.css' %>
        <link rel="stylesheet" type="text/css" 
            media="screen" 
            href="stylesheets/styles.css" >

<%= stylesheet_link_tag 'styles' %>
        <link rel="stylesheet" type="text/css" 
            media="screen" 
            href="stylesheets/styles.css" >

<%= stylesheet_link_tag 'dogs/styles' %>
        <link rel="stylesheet" type="text/css" 
            media="screen" 
            href="stylesheets/dogs/styles.css" >

<%= stylesheet_link_tag 'styles', 'dogs/canines' %>
        <link rel="stylesheet" type="text/css" 
            media="screen" 
            href="stylesheets/styles.css" >
        <link rel="stylesheet" type="text/css" 
            media="screen" 
            href="stylesheets/dogs/canines.css" >

<%= stylesheet_link_tag 'styles', :media => 'print' %>
        <link rel="stylesheet" type="text/css" 
            media="print" 
            href="stylesheets/styles.css" >

<%= stylesheet_link_tag :all, :media => 'print' %>
        Link to all stylesheets in stylesheets folder.

Reference: api.rubyonrails.org/classes/ActionView/Helpers/AssetTagHelper.html

link_to method

<%= link_to 'To Profiles/Show Page', profiles_show_path %>
        <a href="/profiles/show">To Profile Page</a>

<%= link_to 'To Profiles/Show Page', profiles_show_path,
    :class => 'link' %>
        <a href="/profiles/show" class="link">
            To Profile Page</a>

<%= link_to 'To Profiles/Show Page', profiles_show_path,
    :class => 'link' :id => 'linka' %>
        <a href="/profiles/show" class="link" id="linka">
            To Profile Page</a>

<%= link_to 'To Profile/Show Page', profiles_show_path, 
    :confirm => 'Are you sure?' %>
        <a href="/profiles/show" ">To Profile Page</a>
        Need JavaScript code to show an alert box.

Reference: api.rubyonrails.org/classes/ActionView/Helpers/UrlHelper.html

Datatypes Used by Rails Scaffold-based Projects

The Rails scaffold software can generate applications complete with a back end database to store information.

The allowed datatypes for database fields that can be specified in a scaffold statement:

:string	A string of characters.
:text	A string of characters, generally longer than a string, which can contain more than one line.
:integer	A positive, negative, or zero number without a fractional part; for example 238372, -3273, 0. Integers are represented in binary.
:float	A positive, negative, or zero floating point number, which can contain a decimal point; for example 345.39, -438.86, 0.00. Floats are also represented in binary.
:decimal	A positive, negative, or zero floating point number, which can contain a decimal point. Decimal numbers are represented in base 10 and are accurate for financial calculations.
:datetime	A combination type that represents a date and time to the nearest millisecond.
:timestamp	Used to mark when a row is entered in a database.
:time	The hour, minute, and second of the time recorded in the database.
:date	The year, month, and day of the date recorded in the database.
:binary	Unstructured binary data (sometimes called a BLOB for binary large object). This may be compressed data that is uncompressed when it is retrieved from the database, or it may be image or sound data.
:boolean	Contains a 0 or 1 that represents false or true.

Some Rails datatype mappings to databases.

Projects Created by the Rails Scaffold Software

Enter a Rails scaffold statement with the model name MovieReview:

rails g scaffold MovieReview movie_name:string \
   reviewer_name:string review:text review:integer \ 
   view_date:date straight_to_video:boolean

This statement does the following:
- creates a controller,
- creates these views: index, show, new, edit,
- creates the specs for the database fields,
Now enter the rake statement to create the actual database and database table:

The following names are created:

Resource Name	movie_reviews
Database Table Name	movie_reviews
Model Name	MovieReview
Controller Name	MovieReviewController
View Folder Name	movie_reviews
URL	http://localhost:3000/movie_reviews

Note which names are written upper camel notation and which others are written in underscore notation.
The Rails scaffolding software knows how to pluralize the name that represents table rows. In this case, Person represents the name of a single row and People represents the entire collection of rows in the table.
Here are the names generated by scaffold if the model name is Person:

Resource Name person

Database Table Name people

Model Name Person

Controller Name PersonController

View Folder Name people

URL http://localhost:3000/people
Warning: Rails does not know how to pluralize every noun with an irregular plural. For Rails, the plural of sheep is sheeps. The plural of goose is gooses.

Project 2

Look at the Project 2 Description.

Routing

Routes are specified in the file app/config/routes.rb.
So far, the Rails applications we have created have used named routes.
For example, if the controller name is Info and the view name is staff, then the named route used to view this page is
A project created using a scaffold uses a resource based route:

Modifying the Dropdown Menus for Selecting a Date

By default, the year dropdown menu for selecting a date only displays five years on either side of the current year.
To change the displayed years, go into the app/views/new.html.erb. Change the method call
to
Caution: 1983 and 2015 must not be in quotes because they are integers, not strings.

Here are some other options for f.date_select:

# Use month numbers instead of names.
:use_month_numbers => true

# Add month numbers in addition to names.
:add_month_numbers => true

# Use short form of months: Jan, Feb, Mar, ...
:use_short_month => true

# Don't include a dropdown menu for days.
:discard_day => true

# Don't include a dropdown menu for months.
:discard_month => true

# Don't include a dropdown menu for years.
:discard_year => true

Validations

When a value of the wrong datatype is placed into a model, no error message is given.
For example, if a non-numeric string value is placed into an integer field, it will silently converted to 0.
Validations allow a Rails project to show error messages when illegal values are entered into fields.

Validations are placed into the body of the model in app/model.rb. Here are some examples of older Rails validations:

validates_presence_of :first_name

validates_length_of :student_id,
    :minimum => 4, :maximum => 4

validates_numericality_of :gpa,
    :greater_than_or_equal_to => 0.0,
    :less_than_or_equal_to => 4.0,
    :only_integer

Here are some examples using the modern Rails validations:

validates :title, :presence => true

validates :email, :uniqueness => true

validates :username, :length => { :within => 8..20 }

validates :age, :numericality => true

validates :age, :numericality => { :only_integer => true }

validates :age, :numericality => { :greater_than => 0 }

Some Rails documentation recommends using the older forms validates_presence_of, validates_length_of, and validates_numericality_of instead of the modern form validates.

Rails Databases and Migrations

Create a Student scaffold with this rails statement:

rails g scaffold Student name:string year:integer birth_date:date gpa:float

student is the name of the model.

Remember that the datatypes in a generate scaffold statement are case sensitive.

Caution: stay away from Rails reserved words, both for the model name and for the field names within the model. Here is a website that lists Rails reserved words:

oldwiki.rubyonrails.org/rails/pages/ReservedWords
Look at the file with a name similar to
This is the migration file that contains the directions for creating the database.
Now run the migration file with
This actually creates the database and schema in
and
respectively.
Look at the views that have been created by the scaffold software for the the School project.
Sites created with the scaffold software implement the CRUD applications. CRUD means Create, Read, Update, Delete.
Start the Rails server and enter some students into the students database table.

Resource Name	person
Database Table Name	people
Model Name	Person
Controller Name	PersonController
View Folder Name	people
URL	http://localhost:3000/people