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MVC Sinatra (Active Record)

Opening

We have used Sinatra to create simple applications. However, we can apply the design pattern of MVC to make more complex applications.


I Do: What is a model?

When we designed classes, we said we were 'modelling' them. Essentially, thats what models are. They allow us to deal with objects in our code.


I Do: Intro to ORM

ORM stands for:

  • Object
  • Relational
  • Mapping

ORM is a technique that connects the rich objects of an application to tables in a relational database management system.

E.g. (Draw on board)

A user object in application ----> Users table in the db

ORM generates SQL

Using ORM, the properties and relationships of the objects in an application can be easily stored and retrieved from a database without writing SQL statements directly and with less overall database access code.


I Do: Intro Active Record

Active Record, as an ORM Framework, gives us several mechanisms, the most important being the ability to:

  • Represent models and their data.
  • Represent associations between these models.
  • Represent inheritance hierarchies through related models.
  • Validate models before they get persisted to the database.
  • Perform database operations in an object-oriented fashion.

Active Record is the M in MVC - the model - which is the layer of the system responsible for representing business data and logic.


We Do: Look at the starter code

Send across the starter code.

Controllers

There is a controller file in the controllers directory with 7 actions:

These are just exactly the sames is if they were written in the main.rb file.

Take a look at some of the controller actions:

get '/artists' do
  @artists = Artist.all
  erb(:"artists/index")
end

We can see that ActiveRecord lets us write things like:

Artist.all
Artist.create(params[:artist])
Artist.find(params[:id])

This will generate SQL code for us!

Models

Have a look at the code in the artist model:

class Artist < ActiveRecord::Base
end

This is the same as a normal Ruby class except it inherits from ActiveRecord::Base.


We Do: Create a database in postgresql

We need a database to connect to:

$ createdb tunr_db
$ psql -d tunr_db -f db/schema.sql

Launch app

Run the code with:

$ ruby app.rb

And view from http://localhost:4567.


We Do: Adding Active Record to Sinatra

As you can seem We can use 'ActiveRecord' to connect to the database.

First, we need to require active record:

require 'activerecord'

This will allow use access to the ActiveRecord's methods. Then in the main sinatra file:

 # Load Active Record and connect to the DB
ActiveRecord::Base.establish_connection({
  database: 'tunr_db',
  adapter: 'postgresql'
})

b# Fix an issue with sinatra and Active Record where connections are left open
after do
  ActiveRecord::Base.connection.close
end

This enables ActiveRecord to connect to the database.


Assess

Your task is to add songs to this app. A song should have:

  • id SERIAL PRIMARY KEY,
  • title VARCHAR(255),
  • album VARCHAR(255),
  • preview_url VARCHAR(512),
  • artist_id INTEGER REFERENCES artists(id),
  • price INTEGER

The songs should have already been created by the sql file.


##Closure

Summary and presentation of the solutions.