Rails Presenter

TDD Workflow

Step 1: Create Presenter Test (RED)

# test/presenters/event_presenter_test.rb
require "test_helper"

class EventPresenterTest < ActiveSupport::TestCase
  setup do
    @event = events(:one)
    @presenter = EventPresenter.new(@event)
  end

  test "delegates to the model" do
    assert_equal @event.name, @presenter.name
  end

  test "responds to model methods" do
    assert_respond_to @presenter, :name
    assert_respond_to @presenter, :status
    assert_respond_to @presenter, :created_at
  end

  test "exposes the underlying model" do
    assert_equal @event, @presenter.model
  end

  test "#display_name returns the formatted name" do
    assert_equal @event.name, @presenter.display_name
  end

  test "#formatted_date returns formatted date when present" do
    @event.update(event_date: Date.new(2026, 7, 15))
    result = @presenter.formatted_date
    assert_includes result, "2026"
  end

  test "#formatted_date returns placeholder when nil" do
    @event.update(event_date: nil)
    result = @presenter.formatted_date
    assert_includes result, "text-slate-400"
    assert_includes result, "italic"
  end

  test "#status_badge returns HTML-safe string" do
    assert_predicate @presenter.status_badge, :html_safe?
  end

  test "#status_badge includes status text" do
    assert_includes @presenter.status_badge, @event.status.humanize
  end

  test "#status_badge uses correct color for active" do
    @event.update(status: :active)
    presenter = EventPresenter.new(@event)
    assert_includes presenter.status_badge, "bg-green-100"
  end

  test "#status_badge uses correct color for inactive" do
    @event.update(status: :inactive)
    presenter = EventPresenter.new(@event)
    assert_includes presenter.status_badge, "bg-red-100"
  end

  test "#formatted_amount formats cents as currency" do
    @event.update(amount_cents: 15000)
    assert_equal "150,00 EUR", @presenter.formatted_amount
  end
end

Step 2: Run Test (Confirm RED)

bin/rails test test/presenters/event_presenter_test.rb

Step 3: Implement Presenter (GREEN)

# app/presenters/event_presenter.rb
class EventPresenter < BasePresenter
  STATUS_COLORS = {
    active: "bg-green-100 text-green-800",
    inactive: "bg-red-100 text-red-800",
    pending: "bg-yellow-100 text-yellow-800"
  }.freeze

  DEFAULT_COLOR = "bg-slate-100 text-slate-800"

  def display_name
    name
  end

  def formatted_date
    return not_specified_span if event_date.nil?
    I18n.l(event_date, format: :long)
  end

  def status_badge
    tag.span(
      status_text,
      class: "inline-flex items-center px-2.5 py-0.5 rounded-full text-xs font-medium #{status_color}"
    )
  end

  def formatted_amount
    return "0,00 EUR" if amount_cents.nil? || amount_cents.zero?
    number_to_currency(
      amount_cents / 100.0,
      unit: "EUR",
      separator: ",",
      delimiter: " ",
      format: "%n %u"
    )
  end

  private

  def status_text
    I18n.t("activerecord.attributes.event.statuses.#{status}", default: status.to_s.humanize)
  end

  def status_color
    STATUS_COLORS.fetch(status.to_sym, DEFAULT_COLOR)
  end

  def not_specified_span
    tag.span(
      I18n.t("presenters.common.not_specified"),
      class: "text-slate-400 italic"
    )
  end
end

Step 4: Run Test (Confirm GREEN)

bin/rails test test/presenters/event_presenter_test.rb

Common Presenter Methods

Date Formatting

def formatted_event_date
  return not_specified_span if event_date.nil?
  I18n.l(event_date, format: :long)
end

def short_date
  return "\u2014" if event_date.nil?
  event_date.strftime("%d/%m/%Y")
end

Currency Formatting

def formatted_budget
  return not_specified_span if budget_cents.nil?
  number_to_currency(
    budget_cents / 100.0,
    unit: "EUR",
    separator: ",",
    delimiter: " ",
    format: "%n %u",
    precision: 0
  )
end

Badge/Tag Generation

def type_badge
  tag.span(
    display_type,
    class: "inline-flex items-center px-2 py-1 rounded text-xs font-medium #{type_color}"
  )
end

Contact Links

def display_email
  return not_specified_span if email.blank?
  mail_to(email, email, class: "text-blue-600 hover:underline")
end

def display_phone
  return not_specified_span if phone.blank?
  link_to(phone, "tel:#{phone}", class: "text-blue-600 hover:underline")
end

Usage in Controllers

# Single resource
@event = EventPresenter.new(@event)

# Collection
@events = events.map { |e| EventPresenter.new(e) }

# With view context (for route helpers)
@event = EventPresenter.new(@event, view_context)

Checklist

• Test written first (RED)
• Extends BasePresenter
• Delegation tested
• HTML output is html_safe
• Uses I18n for all text
• Currency stored in cents, displayed formatted
• Color mappings use constants (Open/Closed)
• not_specified_span for nil values
• All tests GREEN