Sinatra Activerecord Associations


  1. Understand primary and foreign key.
  2. Understand the has many/ belongs to relationship.


Previously, we learned about foreign and primary keys in SQL and how they relate tables to one another. In this lesson, we're going to learn how to do this using ActiveRecord. This lab has pre-written code that you can follow along with - feel free to fork and clone the repo locally.

Primary Keys

The end goal of this readme is to create a relationship in our app that mimics the real life cat-owner relationship: owners can have many cats and cats belong to an owner. Let's assume we have two tables in our database: cats and owners, which we created from the command line using rake.

Review: Creating a table with ActiveRecord

First, we create a cats table from the command line: rake db:create_migration NAME="create_cats"

This will give us an empty migration in our db/migrate/ folder. Now lets give our cats table attributes: name, age and breed. This will go into our change method.

class CreateCats < ActiveRecord::Migration
  def change
    create_table :cats do |t|
      t.string :name
      t.integer :age
      t.string :breed

Review: Primary Keys

A primary key uniquely identifies each record in a table. It must be unique and cannot have NULL values. Luckily, ActiveRecord will create the primary key for us and will also auto-increment it every time we save a new row in our table.

Go ahead and use Tux to create three instances of the Cat class:

Cat.create(:name => "Maru", :age => 3, :breed => "Scottish Fold")
Cat.create(:name => "Hannah", :age => 2, :breed => "Tabby")
Cat.create(:name => "Patches", :age => 2, :breed => "Calico")

And two instances of our Owner class:

Owner.create(:name => "Sophie")
Owner.create(:name => "Ann")

Our cats table looks like this:

id name age breed
1 Maru 3 Scottish Fold
2 Hannah 2 Tabby
3 Patches 2 Calico

Our owners table looks like this:

id name
1 Sophie
2 Ann

Now, we need to tell our tables how to relate to each other. This is where we'll use a foreign key.

Using Foreign Keys

A foreign key points to a primary key in another table. In ActiveRecord we will use the tablename_id convention. To add the foreign key to our cats table, we will create another migration.

The foreign key always sits on the table of the object that belongs to. In this case, because cats belong to an owner, the owner_id becomes a column in the cats table.

class AddColumnToCats < ActiveRecord::Migration
  def change
    add_column :cats, :owner_id, :integer

Our cats table should look like this:

id name age breed owner_id
1 Maru 3 Scottish Fold 1
2 Hannah 2 Tabby 2
3 Patches 2 Calico 1

We now know what our table should look like. However, we haven't told our application how to relate the models to each other.

belongs_to and has_many

Before we write our association let's think about our table structure: A cat belongs to an owner, and an owner can have many cats.

This translates into ruby like this:

class Cat
  belongs_to :owner
class Owner
  has_many :cats

Whenever we use a has_many we also have to use the belongs_to (and vice-versa) in the other model. Keep in mind: The model with the belongs_to association also has the foreign key.

Creating objects

After setting our associations, we can create a cat and an owner and save them to our database. Try using Tux to play around. Create objects, view them, edit them, delete them!

sophie = Owner.create(name: "Sophie")
maru = "Maru", age: 3, breed: "Scottish Fold")
maru.owner = sophie

We used the .create method to instantiate and save the owner to our database. To instantiate the cat object we used the .new method, after that we set "Maru's" owner to the owner we created. Because the .new method did not save the cat object to our database the last line will persist the cat object to our database.

The has_many/belongs_to relationship is the most used association, but there are others as well. You can read more about ActiveRecord Associations here.

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