Rendering Collections

Objectives

1. Use the collection keyword with partials
2. Pass a collection to the render method
3. Handle empty collections

Overview

Up until now our only way to render collections was somewhat manually. We could iterate over an array and render the partial for each object in the array. Let's see how Rails can abstract this into a nicer syntax.

Lesson

Make sure you run bin/rails db:migrate and bin/rails db:seed before testing the app in your browser. This lesson focuses on using the collection keyword with partials. The connection between authors and posts is hard-coded. In the posts controller create action, the newly created post is linked with the first author in the database.

Currently, the posts#index view manually renders the partial in a loop:

<% @posts.each do |post| %>
  <%= render partial: "post", locals: { post: post } %>
<% end %>
```erb

Rails provides a cleaner way to render a collection using a partial:

```erb
<%= render partial: "post", collection: @posts %>

This approach is more concise and clear. You can also pass an array directly to the render method:

<%= render @posts %>

This approach is a bit more abstract. Under the hood Rails uses the convention that you will have a partial with the name of the models in the collection. Rails will even render a collection of heterogeneous models ([customer, order, customer]) calling the correct partial for each one.

Empty Collections

What happens if the collection you pass to your render call is empty? If you don't handle this exception the render method will return nil and nothing will appear on the screen. A useful trick is to use the || operator to print something to the screen to alert the user to this.

<%= render(@posts) || "There are no blog posts!" %>

Note: When dealing with an empty collection, you'll need to use () to wrap that collection.

Conclusion

As always, Rails has tried to abstract commonly used functionality into more terse and implicit code. Experiment with these tricks in the upcoming lab.