How to use ActiveRecord to map classes to database tables

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Recently I have been learning ActiveRecord and I can’t help wowing because I really love it! It is a great tool when dealing with tables in a database and models in ruby. So I decided to write down what I have learned so far. I believe that will help solidify my knowledge about AR and hopefully, it will help you too.

In the beginning, I want to briefly talk about ORM (Object Relational Mapping), which is a way for us “to manage database data by “mapping” database tables to classes and instances of classes to rows in those tables” (From Flatiron School tutorial).

In my opinion, tables in the database and classes in OO language are two parallel worlds because we set them up to have the same data and structure. However, after establishment, data will change and ORM is an efficient way to connect between Class in ruby and table in a database so they can reflect each other’s change. When using ORM, we usually build methods in the following pattern.

Set up a database connection in the environment document:

require 'sqlite3'
DB = {:conn =>"db/music.db")}

DB refers to the database all the time since then and we can build the method like this:

Class Song
def save
sql = <<-SQL
INSERT INTO songs (name, album)
VALUES (?, ?)
DB[:conn].execute(sql,, self.album)

As you can see, we write some raw SQL statements and then tell the connection to execute those SQL statements with passed arguments. With this method, when a new song instance is created, we can call saveon it so it can be saved into the database.

In the process of learning ORM, I wrote a lot of methods with which we can use to create, read, update and delete(CRUD) data in the database, like update,find_by, create… And active record write all these for you!

So instead of writing every single method by ourselves, we can use all of those methods as we like as long as we tell classes to inherit from ActiveRecord::Base.And one more reason I love about AR, we don’t have to worry about what kind of grammar the SQL software uses. AR will do everything for you!

Great, now let’s see how can we use AR.

Set up the connection

Do you remember we set up a connection to database when we work on ORM? Same here when using ActiveRecord.

:adapter => "sqlite3",
:database => "db/students.sqlite"

adapter tells ActiveRecord which SQL software is used and then it will take care of the SQL language for us. database tells ActiveRecord where the database is.

Set up tables

As we mentioned before, what we are trying to do is mapping database tables to classes. So we will have to create a table and assign the specific class’s attributes to that table as columns.

Like if we have a Cat class with name,breed,age attributes then we will have a table called cats with name,breed,age columns. To create a table, we will need to use migration.

Usually, I will create a db directory on the top level, and then create a new directory called migrate under db, to store all the migrations we have. To name a migration file, we name it by putting time (including seconds) or if our application is very small, simply putting numbers like 01 (to represent it is the first migration) before the name of your file.

Because when it runs, AR will run the file in alpha-numerical order. Like 02xxxxx. rb will be run earlier than 03xxxxxx.rb, add_column.rb will run faster than delete_column, so to control the order of files being run, we better name file with a prefix number. After that, we can start to create tables or change tables.

Within the table we just create, define a class after our file name but without the number. For example, if our file is called 01_create_songs_table.rb, then name our class as CreateSongsTable, and make that class inherit from ActiveRecord::Migration[4.2]. [4.2] is the version of ActiveRecord::Migration, we have to point out which version we use.

Now, we can start to write what we want to do inside the class. Define a method called change first:

def change
create_table :artists do |t|


Pass a method called create_table in, put the name as Symbol, and then define the column information like the type and name as the following example:

class CreateSongsTable < ActiveRecord::Migration[4.2]
def change
create_table :songs do |t|
t.string :name
t.string :genre
t.integer :year

After all this, we can run rake db:migrate in the terminal to tell AR to implement this migration. Easy right? Migration is basically for tables, so if we want to remove a table, remove a column, or add a column, we can create new migration files and pass methods to change method.

To know more, here is a very comprehensive reference.

Now we have a table that is called songs, how do we relate that with the class Song? Simple, we only have to tell Song class to inherit from AR, like this:

class Song < ActiveRecord: Base

Then AR will know to look for a table called songs and relate both of them. And that’s why we name the table as songs–to lowercase and pluralize the class name so that AR can find it. That is a patter for AR!

Simulate relations between classes

Now we successfully relate one table to one class. What if we have a few classes that are related to each other. Can we simulate relations between classes too? Yes!

First, make sure we make every class inherit from ActiveRecord: Base, and then, we have to decide which of the following association we should use to describe the relationship between classes. There are four associations in AR: belongs_to and has_one to describe one-to-one relationship, has_many to describe one-to-many relatioship and has_and_belongs_to_many to describe many-to-many relationship.

To use them, you can write statement right after the class definition, for example:

class Song < ActiveRecord: Base
belongs_to :artist

On the contrary,

class Artist < ActiveRecord: Base
has_many :songs

And then, AR will do the rest and help us connect two classes. But make sure in a one-to-many relationship, the class that has belongs_to should have a foreign key in the table. For example, in the songs table, we should add a column called artist_id, so that AR can look up the songs table to see which artist the song “belongs to”.

Other than that, AR will also automatically add new methods to class once the connection is done. Like if we make a new song by and make a new artist by, it is ok for us to do this: s.artist=a. When we ask s : who is your artist by putting: s.artist, it will tell us it is the Artist object a. And we can also do like s.build_artist to build an artist for s. To know more handy methods you can use, you can go HERE. It is really worth to have a look, very good explanation.

However, just one thing to notice, we have to put the singular form of the class name after belongs_to, while the pluralized form after has_many, like belongs_to :artist and has_many :songs.that is a must because that is how AR accordingly find tables, and set up foreign ids and so on.

Perform CRUD actions

After we set up all the relationships among tables and classes, we should be able to use it. I am not going to cover all of that here, but you can go to this link to find out how to make queries.

Instead of writing a long query statement, AR offers a lot of methods for us to manage data in the database through models, which makes queries neater and easier!

Thanks for finishing the blog!

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