ActiveRecord DB Mechanics

Rails environments

• Rails server “modes” (technically they are environments)
• rails server -e production (for Rails)
• Note that on Heroku deployment the environment is automatically set to production

What does the choice of environment affect?

• WHich of the actual databases are used (see config/database.yml)
• development - working on your own computer, developing
• test - running tests
• production - working on Heroku for example
• Gemfile - which gems are installed
• Various caching settings
• And more

Going Deeper on ActiveRecord

• Object Relational Mapper
• Model class
• ORM means we have to keep the database and the Objects “in sync”
• Library from Ruby on Rails which is the connection between the rails application and the underlying database
• For advanced designs, there are options other than ActiveRecord
• ActiveRecord is also often used by itself, especially in Sinatra

ActiveRecord model classes and instances

• Attributes
  • Rails will automagically create the methods to access each field (column) of the record in the database
• Two copies!
  • Instance of the class, in memory
  • Corresponding record in database
  • One or the other or both may exist!
  • Need to save, need to query, etc.
• Datatypes supported by ActiveRecord
  • <%= link_to_topic :ar_db_types %>
  • <%= link_to_topic :ar_migrations %>

Associations

• While this is basic Rails it is also some of the more “magical” parts of Rails which can sometimes be confusing
• You know that to create a one to many relationship you need a foreign key in the “many” table
• That is a basic relational database concept
• You access this in an OO way by also telling ActiveRecord
• It seems redundant but they are separate
• The Rails Guide To Associations is a good place to start

Updating the Model source files

* `has_many :comments`
* `belongs_to :answer`

class Answer < ApplicationRecord
  belongs_to :question
  has_many :comments
end

# allows the following
q = Question.create(question: "What is the meaning of life?")
a = q.answers.create(body: "42")
a.comments.create(body: "I agree")

CRUD: Reading and Writing Data

• NB: memory vs. persisted copy of the object
• Create: create record and persist it
• New: create model object without persisting it
• Queries and other ways to access the database
  • Rails ActiveRecord Queries
  • ActiveRecord Guide for Queries
• Updating
  • Retrieve the object (.where etc), modify the attribute, and save it
• Deleting
  • Retrieve the object and .destroy (also class method .destroy_all)
• Validations
  • Rails Guide on ActiveRecord Validations

Different database ‘servers’

• SQLite - local database, no need for a server
• MySQL - Standard industrial strength database, free
• PostgreSQL - Becoming very popular with Rails community

Tools and Notes

• Check out MesaSQLite
• Also check out SQLPro for Postgres
• On Linux, checkout SqliteMan
• References: ActiveRecord Basics, ActiveRecord Query Interface