Agent Skills: Rails Form Helpers Expert

Rails Form Helpers Expert

Build correct, modern forms in Rails 8 using form_with and associated helpers.

When To Use This Skill

• Building any form in a Rails view (form_with, nested forms, selects, checkboxes)
• Adding nested attributes with fields_for and accepts_nested_attributes_for
• Choosing the right form helper (text_field, select, collection_select, etc.)
• Building custom form builders
• Integrating forms with Stimulus controllers

The One Rule

form_with is the ONLY form helper you use. Period.

• ❌ form_for — deprecated since Rails 5.1. Do not use.
• ❌ form_tag — deprecated since Rails 5.1. Do not use.
• ✅ form_with — the one true form helper.

If you see form_for or form_tag in existing code, migrate it to form_with when touching that file. If you're writing new code, there is zero reason to use anything else.

Two Modes of form_with

Model-backed forms (most common)

<%= form_with model: @article do |form| %>
  <%= form.text_field :title %>
  <%= form.submit %>
<% end %>

Rails infers everything: action URL, HTTP method (POST for new, PATCH for persisted), field name scoping (article[title]), submit button text ("Create Article" vs "Update Article").

URL-based forms (no model)

<%= form_with url: search_path, method: :get do |form| %>
  <%= form.search_field :query %>
  <%= form.submit "Search" %>
<% end %>

Use for search forms, external APIs, or any form not tied to a model.

Instructions

Step 1: Determine Form Type

| Scenario | Use | |----------|-----| | Creating/editing a model record | form_with model: @record | | Search form | form_with url: path, method: :get | | Form posting to external URL | form_with url: "https://...", authenticity_token: false | | Namespaced resource (e.g. admin) | form_with model: [:admin, @article] | | Nested resource | form_with model: [@parent, @child] |

Step 2: Check Existing Patterns

ALWAYS check the project's existing forms first:

# Find existing form patterns
rg "form_with" app/views/ --type erb

# Check for custom form builders
rg "FormBuilder" app/ --type ruby

# Check for any deprecated form helpers (migration candidates)
rg "form_for\|form_tag" app/views/ --type erb

Match existing project conventions. If the app uses a custom form builder, use it.

Step 3: Build the Form

Use the appropriate input helpers on the form builder object. Every helper takes the attribute name as its first argument.

Common input helpers:

<%= form_with model: @user do |form| %>
  <%= form.text_field :name %>
  <%= form.email_field :email %>
  <%= form.password_field :password %>
  <%= form.telephone_field :phone %>
  <%= form.url_field :website %>
  <%= form.textarea :bio, size: "70x5" %>
  <%= form.number_field :age, in: 18..120 %>
  <%= form.range_field :satisfaction, in: 1..10 %>
  <%= form.date_field :birthday %>
  <%= form.time_field :preferred_time %>
  <%= form.datetime_local_field :available_at %>
  <%= form.color_field :favorite_color %>
  <%= form.hidden_field :referrer, value: "homepage" %>
  <%= form.checkbox :terms_accepted %>
  <%= form.submit %>
<% end %>

Step 4: Use Correct Select Helpers

Selects are where agents mess up most. Read the reference for full patterns.

Static options:

<%= form.select :status, ["Draft", "Published", "Archived"] %>
<%= form.select :status, [["Draft", "draft"], ["Published", "published"]] %>

From a collection (belongs_to):

<%= form.collection_select :city_id, City.order(:name), :id, :name %>

With option groups:

<%= form.grouped_collection_select :city_id, Country.order(:name), :cities, :name, :id, :name %>

With prompt/include_blank:

<%= form.select :category_id, categories, prompt: "Select a category" %>
<%= form.collection_select :author_id, Author.all, :id, :name, include_blank: "None" %>

Step 5: Handle Nested Attributes Correctly

This is the hardest part of Rails forms. Follow this pattern exactly.

1. Configure the model:

class Person < ApplicationRecord
  has_many :addresses, inverse_of: :person, dependent: :destroy
  accepts_nested_attributes_for :addresses, allow_destroy: true,
    reject_if: :all_blank
end

2. Build empty children in controller:

def new
  @person = Person.new
  @person.addresses.build  # at least one empty set of fields
end

def edit
  @person = Person.find(params[:id])
  @person.addresses.build if @person.addresses.empty?
end

3. Build the nested form:

<%= form_with model: @person do |form| %>
  <%= form.text_field :name %>

  <h3>Addresses</h3>
  <%= form.fields_for :addresses do |address_form| %>
    <div class="nested-fields">
      <%= address_form.hidden_field :id %>
      <%= address_form.text_field :street %>
      <%= address_form.text_field :city %>
      <%= address_form.label :_destroy, "Remove" %>
      <%= address_form.checkbox :_destroy %>
    </div>
  <% end %>

  <%= form.submit %>
<% end %>

4. Permit nested params:

def person_params
  params.expect(person: [
    :name,
    addresses_attributes: [[:id, :street, :city, :_destroy]]
  ])
end

Critical notes on nested forms:

• fields_for renders NOTHING if the association is empty — you MUST build at least one child
• allow_destroy: true requires the _destroy field AND permitting :_destroy in strong params
• reject_if: :all_blank prevents saving empty nested records
• Always include the hidden :id field for existing records (fields_for does this automatically)
• The double array [[:id, :street, ...]] in params.expect is intentional — it means "array of hashes with these keys"

Step 6: Handle File Uploads

<%= form_with model: @user do |form| %>
  <%= form.file_field :avatar %>
  <%= form.submit %>
<% end %>

• form_with automatically sets enctype="multipart/form-data" when a file_field is present
• In the controller, params[:user][:avatar] is an ActionDispatch::Http::UploadedFile
• For production file handling, use Active Storage — don't roll your own

Multiple files:

<%= form.file_field :documents, multiple: true %>

Permit as array: params.expect(user: [documents: []])

Step 7: Strong Parameters for Complex Forms

Simple model:

params.expect(article: [:title, :body, :published])

With nested attributes:

params.expect(person: [
  :name, :email,
  addresses_attributes: [[:id, :street, :city, :state, :zip, :_destroy]]
])

With arrays (checkboxes, multi-select):

params.expect(article: [:title, tag_ids: []])

With rich text (Action Text):

params.expect(article: [:title, :body])  # :body is the rich text attribute name

Key Concepts

CSRF Protection

Every non-GET form automatically includes an authenticity_token hidden field. This is Rails' CSRF protection. Don't disable it unless posting to an external service:

<%# External API - disable token %>
<%= form_with url: "https://external.api/webhook", authenticity_token: false do |form| %>

HTTP Method Emulation

HTML forms only support GET and POST. Rails emulates PATCH, PUT, DELETE via a hidden _method field:

<%# This generates method="post" with a hidden _method="patch" %>
<%= form_with model: @article, method: :patch do |form| %>

For form_with model: @article on a persisted record, Rails sets PATCH automatically.

Record Identification

form_with model: @article figures out:

• New record? → POST to /articles (create)
• Persisted record? → PATCH to /articles/:id (update)
• Submit text → "Create Article" or "Update Article"

This requires resources :articles in your routes.

Namespaced & Nested Resources

# Admin namespace
form_with model: [:admin, @article]
# → POST /admin/articles or PATCH /admin/articles/:id

# Nested resource
form_with model: [@magazine, @article]
# → POST /magazines/:magazine_id/articles

# Deep nesting
form_with model: [:admin, @magazine, @article]

Custom Form Builders

When you repeat the same form patterns, create a custom builder:

# app/form_builders/application_form_builder.rb
class ApplicationFormBuilder < ActionView::Helpers::FormBuilder
  def text_field(attribute, options = {})
    label(attribute) + super(attribute, options.merge(class: "form-input"))
  end
end

Use it:

<%= form_with model: @user, builder: ApplicationFormBuilder do |form| %>
  <%= form.text_field :name %>  <%# Automatically includes label + CSS class %>
<% end %>

Or create a helper to use it by default:

# app/helpers/application_helper.rb
def app_form_with(**options, &block)
  options[:builder] = ApplicationFormBuilder
  form_with(**options, &block)
end

Rich Text (Action Text)

<%= form.rich_text_area :body %>

Requires Action Text to be installed (rails action_text:install). The attribute is declared on the model:

class Article < ApplicationRecord
  has_rich_text :body
end

Radio Buttons and Checkboxes

Radio buttons — user picks one:

<%= form.radio_button :flavor, "vanilla" %>
<%= form.label :flavor_vanilla, "Vanilla" %>
<%= form.radio_button :flavor, "chocolate" %>
<%= form.label :flavor_chocolate, "Chocolate" %>

Collection radio buttons — from a collection:

<%= form.collection_radio_buttons :city_id, City.all, :id, :name %>

Checkboxes — single boolean:

<%= form.checkbox :terms_accepted %>
<%= form.label :terms_accepted, "I accept the terms" %>

Collection checkboxes — has_many or HABTM:

<%= form.collection_checkboxes :interest_ids, Interest.all, :id, :name %>

Note: checkbox (not check_box) generates a hidden field with value "0" so unchecked boxes still submit a value.

Date and Time Helpers

Prefer HTML5 native inputs (modern, mobile-friendly):

<%= form.date_field :born_on %>
<%= form.time_field :starts_at %>
<%= form.datetime_local_field :event_at %>

Select-based date/time (legacy, multi-select dropdowns):

<%= form.date_select :born_on %>
<%= form.time_select :starts_at %>
<%= form.datetime_select :event_at %>

Date selects produce multi-parameter attributes like born_on(1i), born_on(2i), born_on(3i). Active Record knows how to reassemble these.

Anti-Patterns

1. Using form_for or form_tag — always form_with
2. Forgetting to build nested children — fields_for renders nothing for empty associations
3. Missing _destroy in strong params — checkbox exists but destroy silently fails
4. Using select for belongs_to without _id suffix — the field name must be the foreign key
5. Hardcoding form action URLs — let model: infer them, or use route helpers
6. Forgetting allow_destroy: true on accepts_nested_attributes_for
7. Not adding reject_if: :all_blank — empty nested forms create blank records
8. Manually setting enctype for file uploads — form_with + file_field handles this
9. Using _tag helpers inside a form builder block — use the form builder methods instead
10. Not permitting id in nested attributes — needed to update (not duplicate) existing records

Dynamically Adding/Removing Nested Fields

For adding nested fields dynamically (without page reload), use Stimulus:

<%# Render a hidden template for new fields %>
<template data-nested-form-target="template">
  <%= form.fields_for :addresses, Address.new, child_index: "NEW_RECORD" do |af| %>
    <div class="nested-fields" data-new-record>
      <%= af.text_field :street %>
      <%= af.text_field :city %>
      <%= af.hidden_field :_destroy, value: false, data: { nested_form_target: "destroy" } %>
      <button type="button" data-action="nested-form#remove">Remove</button>
    </div>
  <% end %>
</template>

<div data-nested-form-target="container">
  <%= form.fields_for :addresses do |af| %>
    <div class="nested-fields">
      <%= af.text_field :street %>
      <%= af.text_field :city %>
      <%= af.hidden_field :_destroy, value: false, data: { nested_form_target: "destroy" } %>
      <button type="button" data-action="nested-form#remove">Remove</button>
    </div>
  <% end %>
</div>

<button type="button" data-action="nested-form#add">Add Address</button>

The Stimulus controller replaces NEW_RECORD in the template with a unique timestamp to ensure unique parameter names. See reference.md for the full Stimulus controller code.

Quick Reference: Input Helper → HTML Type

| Helper | HTML type | |--------|------------| | text_field | text | | email_field | email | | password_field | password | | telephone_field | tel | | url_field | url | | search_field | search | | number_field | number | | range_field | range | | date_field | date | | time_field | time | | datetime_local_field | datetime-local | | month_field | month | | week_field | week | | color_field | color | | hidden_field | hidden | | file_field | file | | textarea | <textarea> | | checkbox | checkbox | | radio_button | radio |

Debugging Forms

# Check what params your form submits
# In controller:
Rails.logger.debug params.inspect

# Check generated HTML
# In browser dev tools, inspect the <form> element for:
# - action (correct URL?)
# - method (post?)
# - hidden _method field (patch/delete?)
# - authenticity_token present?
# - field names correct (model[attribute]?)

# Common param issues with nested forms:
# - addresses_attributes vs addresses (must be _attributes)
# - Missing id field (creates duplicates instead of updating)
# - _destroy not permitted (silently ignored)