Frontend Patterns

Build Rails frontend using Slim templates, Simple Form, Stimulus controllers, and Optics CSS.

Tech Stack: Slim (HTML) • Simple Form (forms) • Stimulus (JavaScript) • Optics (CSS)

See references/EXAMPLES.md for detailed code examples.

Slim Templates

Core Conventions

• Use Ruby 3+ syntax ( e.g. keyword arguments with :)
• Keep view logic minimal - extract to helpers/partials
• Always add policy checks around actions (e.g. edit/delete links)
• Never use inline styles
• Extract repeated markup into partials (DRY principle)
• Always use locals with keyword arguments: render 'partial', user:, active: true

Helpers vs Partials

Use Helpers for:

• Single elements with conditional text/classes
• Data formatting (dates, currency)
• Stateless logic
• Example: status_badge(status)

Use Partials for:

• Multi-element structures
• Reusable UI components
• Collection rendering
• Example: _form.html.slim, _user_card.html.slim

Rule: Single element = helper. Structure/layout = partial.

Partial Organization

app/views/
  resource_name/
    index.html.slim           # Main views
    _form.html.slim           # Forms (shared by new/edit)
    _resource_name.html.slim  # Individual item
  shared/
    _status_badge.html.slim   # Cross-feature components

Common Patterns

-# Conditional classes
.card class=class_names('card--active': active, 'card--urgent': urgent)

-# Partial with locals
= render 'user_card', user:, show_actions: true

-# Collection rendering
= render partial: 'item', collection: @items

-# Conditional rendering
- if policy(@resource).update?
  = link_to 'Edit', edit_path(@resource)

Simple Form

Always use Simple Form. Never use form_with or form_for.

Basic Forms

Model form:

= simple_form_for @user do |f|
  = f.input :name
  = f.input :email, required: true
  
  .form__actions
    = link_to 'Cancel', :back, class: 'btn btn--outline'
    = f.submit 'Save', class: 'btn btn--primary'

Non-model form (search, filters, bulk actions):

= simple_form_for :search, url: search_path, method: :get do |f|
  = f.input :query
  = f.submit 'Search', class: 'btn btn--primary'

Important: :symbol forms nest params: params.dig(:search, :query)

Common Input Types

Type	Example
Text	= f.input :name
Textarea	= f.input :description, as: :text, input_html: { rows: 4 }
Association	= f.association :project, collection: @projects, prompt: 'Select...'
Select	= f.input :type, collection: @project_types, prompt: 'Select...'
Boolean	= f.input :active, as: :boolean
Date	= f.input :start_date, as: :date
Hidden	= f.hidden_field :organization_id, value: current_user.organization_id

Input Options

• placeholder: - Placeholder text
• label: - Custom label
• hint: - Help text below input
• required: true - Mark as required
• disabled: true - Disable input
• input_html: {} - HTML attributes for input element
• wrapper_html: {} - HTML attributes for wrapper div

Collections

/ Basic collection
= f.input :category_id, collection: @categories

/ Custom label/value methods
= f.input :project_id, 
  collection: @projects,
  label_method: :name,
  value_method: :id,
  prompt: 'Select project...'

Special Form Patterns

Bulk action form:

= simple_form_for :bulk_action, url: bulk_path, method: :post, html: { id: 'bulk-form' } do |f|
  = f.button 'Approve Selected', class: 'btn btn--primary'

-# Checkboxes reference form
= check_box_tag 'entry_ids[]', entry.id, false, form: 'bulk-form'

Modal form with external submit:

= simple_form_for @record, html: { id: 'modal-form' }, data: { turbo_frame: '_top' } do |f|
  = f.input :reason, as: :text, input_html: { rows: 3, required: true }

-# In modal footer
= button_tag 'Submit', type: 'submit', form: 'modal-form', class: 'btn btn--primary'

Form Options

• html: {} - HTML attributes for form element
• data: {} - Data attributes (e.g., Turbo, Stimulus)
• url: - Form submission URL (required for :symbol forms)
• method: - HTTP method (:get, :post, :patch, :delete)

See references/EXAMPLES.md for complex form examples.

Stimulus Controllers

JavaScript interactions using Stimulus framework.

Controller Structure

import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
  static targets = ["output", "input"]
  static values = { url: String, delay: Number }
  static classes = ["hidden", "active"]

  connect() {
    // Initialization when controller connects to DOM
  }

  disconnect() {
    // Cleanup when controller disconnects
  }

  action(event) {
    // Action methods called from HTML
  }
}

Usage in Slim

.component data-controller="example" data-example-url-value="/api/endpoint"
  input data-example-target="input" data-action="input->example#search"
  .results data-example-target="output"

Best Practices

• One controller per behavior (focused, composable)
• Use data attributes for configuration
• Name controllers in kebab-case in HTML
• Keep controllers simple and testable
• Clean up in disconnect() (timers, listeners)

See references/EXAMPLES.md for complete controller examples.

CSS & Optics

Guidelines

• Keep custom CSS minimal and component-scoped
• Never use inline styles
• Use of utility classes is strongly discouraged. BEM (Block Element Modifier) structure should be used instead.
• Use class_names helper for conditional classes

Common Patterns

/ Custom component with BEM
.time-entry.time-entry--running
  .time-entry__header
    h3.time-entry__title = entry.description
  .time-entry__body
    span.time-entry__duration = entry.duration

Conditional Classes

/ Using class_names helper
.card class=class_names(
  'card--active': @record.active?,
  'card--featured': @record.featured?
)

Quick Reference

Form actions pattern:

.form__actions
  = link_to 'Cancel', :back, class: 'btn btn--outline'
  = f.submit 'Save', class: 'btn btn--primary'

Empty state:

- if @items.empty?
  = render 'shared/empty_state', title: 'No items', message: 'Create your first item.'

Authorization check:

- if policy(@resource).update?
  = link_to 'Edit', edit_path(@resource), class: 'btn'

Turbo confirmation:

= button_to 'Delete', path, method: :delete, data: { turbo_confirm: 'Are you sure?' }