billyott / orms-mapping-database-records-to-ruby-objects

Mapping Tables to Objects

Objectives

• 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, 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.

Example

Let's use a song domain as an example. Imagine we have a Song class that is responsible for making songs. Every song will come with two attributes, a title and a length. We could make a bunch of new songs, but first, we want to look at all the songs that have already been created.

Imagine we already have a database with 1 million songs. We need to build three methods to access all of those songs and convert them to Ruby objects.

.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.

The first 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 "Thriller" (356 seconds long) that has a db id of 1 would look like this: [1, "Thriller", 356].

def self.new_from_db(row)
  new_song = self.new  # self.new is the same as running Song.new
  new_song.id = row[0]
  new_song.name =  row[1]
  new_song.length = row[2]
  new_song  # return the newly created instance
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

Now we can start writing our methods to retrieve the data. 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/songs.db")}. Notice that the value of the 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
  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.new_from_db 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

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
  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!