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Craig Petterson

Nested Forms

Nested resources are a powerful tool and Rails has made it easy to deal with doing this directly in your usual form, well, format…

A typical Rails form is made using the form_for tag. When dealing with a normal form, you would just pass in your controller object and add in the relevant input fields.

The following example is using Devise, so #new and #create for a User is handle differently. The form I’m showing is User#edit.

Say you have a user that just needs a name and a bio section, that would look as follows;

<%= form_for @user do |f| %>
  <%= f.label :name %>
  <%= f.text_field :name %>
  <%= f.label :bio %>
  <%= f.text_area :bio %>
<% end %>

For a basic form this might do the job, but what if you know you want to create other things at the same time? It’s not always reasonable to force someone to take multiple steps when one step will do.

Say we’re building an application to record information for event organisers to find speakers (who knows why that might be useful?), then you might want to let them record what talks they have done in the past too.

This sounds like the perfect opportunity for nested forms. We know that a user would have many talks;

has_many :talks

…and a talk belongs to a user;

belongs_to :user

So, we want to add their talk titles and a link as an option into the form when they are updating their name and bio too. Our old form needs a nested form;

<%= form_for @user do |form| %>
  <%= form.label :name %>
  <%= form.text_field :name %>
  <%= form.label :bio %>
  <%= form.text_area :bio %>

  <%= form.fields_for :talks do |talks_form| %>
    <%= talks_form.label :title %>
    <%= talks_form.text_field :title %>
    <%= talks_form.label :link %>
    <%= talks_form.text_field :link %>
  <% end %>
<% end %>

It’s important to remember that the form needs a new name. You might notice that our original form was named |form| but out new form is called |talks_form|.

We’re not out of the woods just yet. We still need to update a couple of things;

With a nested form like this, we need to build the object that will be used - much in the same way we do for a #new action (eg; @talk = Talk.new)

To do this in UserController, we would update the #edit action to:

def edit
  @user.talks.build
end

Without this, our talk fields would not be displayed correctly in the form.

Additionally, we need to whitelist these params. Otherwise they will be ignored and thrown away by our controller;

def user_params
  params.fetch(:user, {}).permit \
    :name, :bio,
    talks_attributes: [:id, :title, :link, :user_id]
end

Now, the params passed through from the form are being permitted and will be used to create the relevant records in our database. Note how we’ve included id and user_id in our talks_attributes!

Lastly, we need to add accepts_nested_attributes_for :talks to our User model to enable the attribute writer for the nested attributes.

With these records saved, we’re free to use a user’s talks to display anywhere we like in the usual manner. We might want to show their own talks on their profile, which we can access with @user.talks thanks to our earlier association.

Now, it’s also possible to handle nested resources, not just using nested forms. A nested resource would still use it’s own controller to performs it’s actions, but would also be available at a new route. The Rails Guides shows this in great detail.

In our User / Talk example, you might show all talks as a nested resource for users. In config/routes.rb, that would be written with a do/end;

resources :users do
  resources :talks
end

This would generate the CRUD routes for a user’s talks. They would still use the TalksController, but effectively pre-set the :user_id as a parameter. This will also generate additional route helper methods, such as new_user_talk_path, as follows;

Helper HTTP Path Controller#Action
user_talks_path GET /users/:user_id/talks(.:format) talks#index
  POST /users/:user_id/talks(.:format) talks#create
new_user_talk_path GET /users/:user_id/talks/new(.:format) talks#new
edit_user_talk_path GET /users/:user_id/talks/:id/edit(.:format) talks#edit
user_talk_path GET /users/:user_id/talks/:id(.:format) talks#show
  PATCH /users/:user_id/talks/:id(.:format) talks#update
  PUT /users/:user_id/talks/:id(.:format) talks#update
  DELETE /users/:user_id/talks/:id(.:format) talks#destroy

Nested forms and nested resources can get complicated. It’s important to think about their usage before going crazy. Before you know it, you might have a nested resource within another nested resource and, suddenly, you’re bogged down in a routing nightmare. Perhaps shallow nesting or routing concerns are what you’re looking for?