Converting Database Records to Ruby Objects

Learning Goals

Build methods that read from a database table
Build a Song.all class method that returns all songs from the database
Build a Song.find_by_name class method that accepts one argument, a name, and searches the database for a song with that name and returns the matching song entry if one is found
Convert what the database gives you into a Ruby object

Introduction

In this lesson, we'll cover the basics of reading from a database table that is mapped to a Ruby object.

Our Ruby program gets most interesting when we add data. To do this, we use a database. When we want our Ruby program to store things, we send them off to a database. When we want to retrieve those things, we ask the database to send them back to our program. This works very well, but there is one small problem to overcome — our Ruby program and the database don't speak the same language.

Ruby understands objects. The database understands raw data.

We don't store Ruby objects in the database, and we don't get Ruby objects back from the database. We store raw data describing a given Ruby object in a table row, and when we want to reconstruct a Ruby object from the stored data, we select that same row in the table.

When we query the database, it is up to us to write the code that takes that data and turns it back into an instance of the appropriate class. We, the programmers, will be responsible for translating the raw data that the database sends into Ruby objects that are instances of a particular class.

Code Along

Let's continue building out the Song class and its object-relational mapping methods from the previous lesson. We can use our code to make new songs and persist them to the database, but what if we want to access existing songs from the database?

We need to build three methods to access all of those songs and convert them to Ruby objects.

To start, review the code from the Song class. Then take a look at this code in the bin/run file:

#!/usr/bin/env ruby

require 'pry'
require_relative '../config/environment'

def reset_database
  Song.drop_table
  Song.create_table
  Song.create(name: "Hello", album: "25")
  Song.create(name: "99 Problems", album: "The Black Album")
end

reset_database

binding.pry
"pls"

This file is set up so that you can explore the database using the Song class from a Pry session. We'll use this code later on during this code along.

`Song.new_from_db`

The first thing we need to do is convert what the database gives us into a Ruby object. We will use this method to create all the Ruby objects in our next two methods.

One thing to know is that the database, SQLite in our case, will return an array of data for each row. For example, a row for Michael Jackson's "Billie Jean" from the album "Thriller" that has an id of 1 would look like this: [1, "Billie Jean", "Thriller"].

class Song

  # ... rest of methods

  def self.new_from_db(row)
    # self.new is equivalent to Song.new
    self.new(id: row[0], name: row[1], album: row[2])
  end

end

Now, you may notice something — since we're retrieving data from a database, we are using new. We don't need to create records. With this method, we're reading data from SQLite and temporarily representing that data in Ruby.

`Song.all`

Recall that in previous lessons with Ruby classes, we used the Class.all method along with the @@all class variable to return an array of all instances of our class. In those examples, @@all was the single source of truth for instances in a particular class.

That approach showed some limitations, however. Using that method meant that our Ruby objects were only persisted in memory as long as our Ruby program was running. If we exited the program and re-ran our code, we'd lose access to that data.

Now that we have a SQL database, our classes have a new way to persist data: using the database!

To return all the songs in the database, we need to execute the following SQL query: SELECT * FROM songs. Let's store that in a variable called sql using a heredoc (<<-) since our string will go onto multiple lines:

sql = <<-SQL
  SELECT *
  FROM songs
SQL

Next, we will make a call to our database using DB[:conn]. This DB hash is located in the config/environment.rb file:

DB = { conn: SQLite3::Database.new("db/music.db") }

Notice that the value of the connection in this hash is actually a new instance of the SQLite3::Database class. This is how we will connect to our database. Our database instance responds to a method called execute that accepts raw SQL as a string. Let's pass in that SQL we stored above:

class Song

  # ... rest of methods

  def self.all
    sql = <<-SQL
      SELECT *
      FROM songs
    SQL

    DB[:conn].execute(sql)
  end

end

This will return an array of rows from the database that matches our query. Now, all we have to do is iterate over each row and use the self.map method to create a new Ruby object for each row:

class Song
  def self.all
    sql = <<-SQL
      SELECT *
      FROM songs
    SQL

    DB[:conn].execute(sql).map do |row|
      self.new_from_db(row)
    end
  end
end

With this method in place, let's try using the Song.all method from Pry to access all the songs in the database. Run ruby bin/run, and then follow along in the Pry terminal:

Song.all
# => [#<Song:0x00007ffc7a093098 @album="25", @id=1, @name="Hello">,
 #<Song:0x00007ffc7a093048 @album="The Black Album", @id=2, @name="99 Problems">]

Success! We can see both songs in the database as an array of song instances. We can interact with them just like any other Ruby objects:

Song.all.first
# => #<Song:0x00007ffc7a0b1480 @album="25", @id=1, @name="Hello">
Song.all.last
# => #<Song:0x00007ffc7a0c4a08 @album="The Black Album", @id=2, @name="99 Problems">
Song.all.last.name
# => "99 Problems"
Song.all.last.name.reverse
# => "smelborP 99"

`Song.find_by_name`

This one is similar to Song.all, with the small exception being that we have to include a name in our SQL statement. To do this, we use a question mark where we want the name parameter to be passed in, and we include name as the second argument to the execute method:

class Song

  # ... rest of methods

  def self.find_by_name(name)
    sql = <<-SQL
      SELECT *
      FROM songs
      WHERE name = ?
      LIMIT 1
    SQL

    DB[:conn].execute(sql, name).map do |row|
      self.new_from_db(row)
    end.first
  end
end

Don't be freaked out by that #first method chained to the end of the DB[:conn].execute(sql, name).map block. The return value of the #map method is an array, and we're simply grabbing the #first element from the returned array. Chaining is cool!

Let's try out this new method. Exit Pry, and run ruby bin/run again:

Song.find_by_name("Hello")
# => #<Song:0x00007f7f579ae6c8 @album="25", @id=1, @name="Hello">

Success!

wamwangi-mathenge/phase-3-orms-mapping-database-records-to-ruby-objects