Previous Articles and tutorials about Ruby:

1. Howto: create a blog using Ruby on Rails
2. Installing Ruby on Rails on Mac is a pain

In the previous article we created a post entity and a comment entity. If you remember, we did not create the one-to-many relationship between these two objects of the model. Let's see how to create them.

In real world applications there are three types of relations:

• one-to-one
• one-to-many
• many-to-many

I have ~stolen~ acquired a photo on how to organize relations inside ruby objects:

Let's take as example the comment entity. It has a post_id attribute that obviously will contain the Id of the post linked. So, as we see in the previous table, our relation is of type Many-To-One (since many comments can belong to only one post). Comment is the model with the foreign key so we will add this line to the Comment.rb class:

belongs_to :post

While in the post class we will write

has_many :comments

Note that Ruby will handle by itself the pluralism of the entity, that's why we write :comments and not :comment.

Another modification that we can do is to say that, if we delete a post, we want to delete all the relative comments too. We can achieve this by writing

has_many :comments, dependent: :destroy

As of now we have linked the two models of our application.

How to quickly test it? We can open the Rails Console by executing

rails console

In a terminal.

Here we have a full Ruby console but with all Rails classes preloaded, so we can work on the entities and explore a little bit.

For example, let's write:

Comments.all

To get all the comments in the database (if you don't have any comments start your rails application and write some comments at http://localhost:3000/comments).

Let's write

Posts.all

To get a reference of all posts. Now we can treat all posts like an array, so If we write

p[1].destroy

You will see that the second post will be deleted (you know that we start counting from zero, don't you?). And since we have used the :destroy option, we will see also that all relative comments will be deleted too.

Check with Comments.all.

Let's digit quit to exit the console and let's have a look at the routes, writing rake routes.

What we see now is routes for all the comments and routes for all the posts. But the ideal would be to have comments under the Posts route (something like /posts/1/comments).

How to do this?

Let's open the /config/routes.rb file. All the routes are shown just using the commands resource :posts and resources :comments. If we want to show the comments route under the posts, we have to write:

resources :posts do
   resources :comments
end

Launch again rake routes in the console and now you'll see new routes, exactly as we wanted.

If we start the Webserver (Rails server) we can try these new urls, however they actually break something in the view, something that we will fix soon.

In the next lesson we will talk about validation. This is a good moment to save our work!