Views & Templates

ERB conventions for vanilla Rails applications.

Partial Organization

Lowest common ancestor - place partials at the highest shared directory:

code

# Shared by cards/show and cards/index
app/views/cards/_card.html.erb

# Shared across controllers
app/views/application/_flash.html.erb

# Display variants of same model
app/views/cards/display/_compact.html.erb
app/views/cards/display/_full.html.erb

Never create deeply nested partials only used in one place.

Optional Locals

Use local_assigns.fetch with explicit defaults:

erb

<%# Good - explicit default, raises if required %>
<% pinned = local_assigns.fetch(:pinned, false) %>
<% card = local_assigns.fetch(:card) %>

<%# Bad - silent nil, ambiguous intent %>
<% pinned = local_assigns[:pinned] || false %>

CSS Class Helper

Build classes with array + compact + join:

erb

<%# Good %>
<div class="<%= [
  'card',
  ('card--pinned' if card.pinned?),
  ('card--closed' if card.closed?)
].compact.join(' ') %>">

<%# Bad - string interpolation %>
<div class="card <%= 'card--pinned' if card.pinned? %>">

For complex cases, use token_list helper:

erb

<div class="<%= token_list('card', 'card--pinned': card.pinned?) %>">

Collection Rendering

Always cache, always specify as::

erb

<%= render partial: 'cards/card',
           collection: @cards,
           as: :card,
           cached: true %>

Turbo Streams

Prepend/append with update for empty states:

erb

<%# Add item and update counter/empty state %>
<%= turbo_stream.before :cards, @card %>
<%= turbo_stream.update :cards_count, @cards.count %>

<%# Remove and handle empty %>
<%= turbo_stream.remove @card %>
<%= turbo_stream.update :cards_empty, partial: 'empty' if @cards.none? %>

Stimulus Integration

Layer controllers on existing elements:

erb

<%# Good - multiple controllers on body %>
<body data-controller="keyboard shortcuts dropdown">

<%# Bad - wrapper div just for controller %>
<div data-controller="card-actions">
  <%= render @card %>
</div>

Keyboard shortcuts via body controller:

erb

<body data-controller="keyboard"
      data-action="keydown->keyboard#handle">

Quick Reference

Pattern	Use
`local_assigns.fetch(:x, default)`	Optional locals
`[...].compact.join(' ')`	Conditional CSS classes
`cached: true, as: :item`	Collection rendering
`turbo_stream.before` + `.update`	Add + refresh related
`data-controller="a b c"`	Multiple controllers

Common Mistakes

Wrong	Right
`local_assigns[:x] \|\| default`	`local_assigns.fetch(:x, default)`
Wrapper div for Stimulus	Add controller to existing element
`render @cards` without cache	`render collection:, cached: true`
Partial in deep nested path	Lowest common ancestor