Nested forms in Rails

Jun 09, 2024 #tutorial

What I’m trying to achieve

I have two models:

• Training Steps
• Training Session which has many Training Steps

When a user creates a Training Session, I want a form that contains fields for Training Steps inside the Training Session form. When I submit the form, I will create a Training Session object along with the related Training Steps.

1. Accepting nested attributes

First, I added accepts_nested_attributes_for to my Training Session model.

class TrainingStep < ApplicationRecord
  belongs_to :training_session
end

class TrainingSession < ApplicationRecord
  has_many :training_steps
  accepts_nested_attributes_for :training_steps, reject_if: lambda { |attributes| attributes['description'].blank? }
end

By default, nested attribute updating is turned off. With accepts_nested_attributes_for I’m able to create a Training Session and the Training Steps in one go. With this method, TrainingSession will now accept a key training_steps_attributes as a value. With reject_if, it will also silently ignore any records that don’t pass the lambda.

2. Adding the controller new method

class TrainingSessionsController < ApplicationController
  def new
    @training_session = TrainingSession.new
    2.times { @training_session.training_steps.build }
  end
end

In my new method, I’ll initialise a new Training Session model assigned to @training_session, and build two training_steps models along with it. @training_session will be used in the view.

3. Adding a nested form in the view

<%= form_with model: @training_session do |form| %>
  Session Steps:
  <ul>
    <%= form.fields_for :training_steps do |training_steps_form| %>
      <li>
        <%= training_steps_form.label :description %>
        <%= training_steps_form.text_field :description %>
      </li>
    <% end %>
  </ul>
  <%= form.submit "Create New Session" %>
<% end %>

In the view, new.html.erb, we’re using form_with to build a form that routes to the correct create path. Rails will infer the correct routes based on the resource, in this case @training_session. fields_for is then used to refer to the nested training_steps objects. This method is used to refer to a specific model object, training_steps in this case, but it yields to the form_with element. In other words, you would use fields_for inside a form_with.

TIL - form_with is the new way to build forms. I used form_for in the past but that’s now obselete (here’s the PR with the change to form_with).

4. Creating a nested object

class TrainingSessionsController < ApplicationController
  def new
    @training_session = TrainingSession.new
    2.times { @training_session.training_steps.build }
  end

  def create
    training_session = TrainingSession.new(training_session_params)
    if training_session.save
      training_session.save
      redirect_to training_sessions_path
    else
      render "new"
    end
  end

  private

  def training_session_params
    params.require(:training_session).permit(training_steps_attributes: [:id, :description])
  end
end

In the controller, we can add the create method to handle creating the Training Session object. Notice that we use the training_steps_attributes in training_session_params. We’ll pass these parameters directly into TrainingSession.new and when we save Rails will create both the TrainingSession object and the associated Training Steps objects.