Presenters — how values render

Source: app/presenters/cafe_car/presenter.rb in the gem, plus per-type presenters alongside it (CurrencyPresenter, DatePresenter, RecordPresenter, ActiveRecord::RelationPresenter, ActiveStorage::AttachmentPresenter, …).

present(value) (aliased p in views) wraps anything in a presenter and returns an html-safe object. Records render as a linked preview (logo + title); dates, currency, rich text, attachments, and collections each render through their type’s presenter.

present works in any view — customer-facing pages included, not just CafeCar-rendered ones. It reaches every view through the installer’s include CafeCar::Controller in ApplicationController; a host that skips that include can expose formatting alone with helper CafeCar::Formatting (the scalar as: path renders through Rails’ own number/date helpers — no CafeCar CSS or partials).

How a presenter is chosen

The value’s ancestor chain is walked and the first <Ancestor>Presenter constant found wins — a host InvoicePresenter beats the gem’s RecordPresenter by existing; nothing is registered. Force a type with as::

present(@invoice)                    # InvoicePresenter, else RecordPresenter
present(record.total, as: :currency) # CurrencyPresenter

Status-ish attributes are the one convention on top: an ActiveRecord enum, or a string column named status/state, renders through BadgePresenter as a colored Badge pill wherever the attribute appears — tables, show pages, cards. as: :badge forces it for anything else; per-value styles live in the locale under badge.styles (unlisted values render the neutral badge).

Note: constants the gem defines under CafeCar:: (e.g. CafeCar::StringPresenter) resolve engine-first — a same-named top-level constant does not shadow them. Override per model (ProductPresenter) or per ancestor the gem doesn’t claim.

Writing one

# app/presenters/invoice_presenter.rb
class InvoicePresenter < CafeCar::Presenter
  show :total, as: :currency          # render this attribute through a type
  show :number, -> { "#%03d" % _1 }   # or through a lambda

  def title = show(:number)           # what links/cards/breadcrumbs call this record
end

show class macros set per-attribute rendering defaults used everywhere the attribute appears — tables, show pages, cards.

The instance API (used inside overridden views)

In view code, present(@article) (or the object local in _grid_item) gives:

p = present(@article)
p.title                    # policy.title_attribute, rendered
p.logo(size: :icon)        # policy.logo_attribute as an image
p.show(:author)            # one attribute's value, presented
p.attribute(:author)       # labeled Field (label + value)
p.attributes(:a, :b)       # several labeled fields; no args = all displayable
p.remaining_attributes     # displayable minus already-shown
p.associations             # labeled fields for displayable associations
p.timestamps               # created_at/updated_at etc.
p.controls                 # the show/edit/delete control links
p.href                     # canonical path, nil if not routable
p.object                   # the underlying record (for url helpers etc.)

show/preview of an associated record is already a link to it. For explicit links use the helpers:

href_for(record)          # path to the record in the current namespace
link(record).show         # LinkBuilder: policy-aware link (disabled when denied)
link(record).edit
link(record).destroy      # turbo DELETE with locale-driven confirm
link(model).index
link(model.new).new

link(...) renders disabled-with-tooltip when the policy denies the action — prefer it over hand-written link_to so authorization stays visible in the UI.