Add support for localized attachments in Mobility

app/notifiers/better_together/event_invitation_notifier.rb

@@ -19,9 +19,7 @@ def title
 
     def body
       I18n.t('better_together.notifications.event_invitation.body',
-             # rubocop:todo Lint/DuplicateHashKey
-             event_name: event&.name, default: 'Invitation to %<event_name>s', event_name: event&.name)
-      # rubocop:enable Lint/DuplicateHashKey
+             event_name: event&.name, default: 'Invitation to %<event_name>s')
     end
 
     def build_message(_notification)

app/views/better_together/shared/_translated_file_field.html.erb

@@ -0,0 +1,31 @@
+<%# Render a tabbed file input for each locale. Expects locals: f (form builder), attribute (symbol), model %>
+<div data-controller="translated-tabs">
+  <ul class="nav nav-tabs" role="tablist">
+    <% (Mobility.available_locales rescue I18n.available_locales).each_with_index do |locale, i| %>
+      <li class="nav-item" role="presentation">
+        <button class="nav-link <%= 'active' if i == 0 %>" id="<%= "#{attribute}-#{locale}" %>-tab" data-bs-toggle="tab" data-bs-target="#<%= "#{attribute}-#{locale}" %>" type="button" role="tab">
+          <%= locale %>
+        </button>
+      </li>
+    <% end %>
+  </ul>
+  <div class="tab-content">
+    <% (Mobility.available_locales rescue I18n.available_locales).each_with_index do |locale, i| %>
+      <div class="tab-pane <%= 'active' if i == 0 %>" id="<%= "#{attribute}-#{locale}" %>" role="tabpanel">
+        <div class="mb-2">
+          <% existing = model.send("#{attribute}_#{locale}") rescue nil %>
+          <% if existing && existing.respond_to?(:blob) && existing.blob.present? %>
+            <% if existing.blob.image? %>
+              <%= image_tag url_for(existing.blob.variant(resize: "300x200")) %>
+            <% else %>
+              <%= link_to existing.blob.filename.to_s, url_for(existing.blob) %>
+            <% end %>
+          <% end %>
+        </div>
+        <div class="form-group">
+          <%= f.file_field "#{attribute}_#{locale}", class: 'form-control' %>
+        </div>
+      </div>
+    <% end %>
+  </div>
+</div>

config/initializers/active_storage_locales.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+# Extend ActiveStorage::Attachment with a locale attribute helpers
+Rails.application.config.to_prepare do
+  ActiveSupport.on_load(:active_storage_attachment) do
+    # ensure presence of locale is validated at model level as well
+    unless method_defined?(:locale)
+      # The migration will add locale column. Guard so this file can be loaded pre-migration.
+      define_method(:locale) { read_attribute(:locale) if respond_to?(:read_attribute) }
+    end
+
+    include Module.new do
+      def self.included(base)
+        base.class_eval do
+          validates :locale, presence: true
+
+          scope :for_locale, ->(locale) { where(locale: locale.to_s) }
+
+          before_validation :set_locale, on: :create
+
+          def set_locale
+            return if locale.present?
+
+            self.locale = I18n.locale
+          end
+        end
+      end
+    end
+  end
+end

config/initializers/mobility.rb

@@ -107,3 +107,32 @@
     # locale_accessors [:en, :ja]
   end
 end
+
+# Ensure localized attachments backend is loaded and registered early
+begin
+  require 'mobility/backends/attachments/backend'
+rescue LoadError => e
+  Rails.logger.debug "Could not require mobility attachments backend: #{e.message}"
+end
+
+# Register backend symbol if Mobility exposes the API. Some test bootstraps
+# may not have Mobility fully loaded yet; guard registration to avoid raising
+# during initializer phases where Mobility isn't loaded.
+begin
+  if Mobility.respond_to?(:register_backend)
+    Mobility.register_backend(:attachments, Mobility::Backends::Attachments)
+  else
+    Rails.logger.debug 'Mobility does not expose register_backend; skipping attachments backend registration'
+  end
+rescue StandardError => e
+  Rails.logger.debug "Error registering mobility attachments backend: #{e.message}"
+end
+begin
+  require 'mobility/dsl/attachments'
+  # Make the DSL available as an extension so models can `extend Mobility::DSL::Attachments`
+  ActiveSupport.on_load(:active_record) do
+    ActiveRecord::Base.extend Mobility::DSL::Attachments
+  end
+rescue LoadError => e
+  Rails.logger.debug "Could not load Mobility attachments DSL: #{e.message}"
+end

db/migrate/20250829000000_add_locale_to_active_storage_attachments.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+# Adds a locale column to Active Storage attachments to support per-locale attachments.
+# This migration is intentionally explicit and kept readable rather than split into
+# many tiny helper methods.
+class AddLocaleToActiveStorageAttachments < ActiveRecord::Migration[7.1]
+  # rubocop:disable Metrics/MethodLength
+  def up
+    # Step 1: add nullable column so deploy can be staged
+    add_column :active_storage_attachments, :locale, :string, default: I18n.default_locale.to_s
+
+    # Step 2: backfill existing rows to default locale in one SQL update
+    say_with_time("Backfilling active_storage_attachments.locale to default locale") do
+      default = I18n.default_locale.to_s
+      ActiveRecord::Base.connection.execute(<<~SQL)
+        UPDATE active_storage_attachments
+        SET locale = #{ActiveRecord::Base.connection.quote(default)}
+        WHERE locale IS NULL
+      SQL
+    end
+
+    # Step 3: make column not null
+    change_column_null :active_storage_attachments, :locale, false, I18n.default_locale.to_s
+
+    # Step 4: add unique index to enforce one attachment per record/name/locale
+    unless index_exists?(:active_storage_attachments,
+                         %i[record_type record_id name locale],
+                         name: :index_active_storage_attachments_on_record_and_name_and_locale)
+      add_index :active_storage_attachments,
+                %i[record_type record_id name locale],
+                unique: true,
+                name: :index_active_storage_attachments_on_record_and_name_and_locale
+    end
+  end
+
+  # rubocop:enable Metrics/MethodLength
+
+  def down
+    if index_exists?(:active_storage_attachments, %i[record_type record_id name locale],
+                     name: :index_active_storage_attachments_on_record_and_name_and_locale)
+      remove_index :active_storage_attachments, name: :index_active_storage_attachments_on_record_and_name_and_locale
+    end
+    remove_column :active_storage_attachments, :locale if column_exists?(:active_storage_attachments, :locale)
+  end
+end

docs/developer/translatable_attachments.md

@@ -0,0 +1,116 @@
+## Translatable Attachments (developer guide)
+
+Overview
+--------
+
+This guide documents the Mobility-style solution for per-locale Active Storage
+attachments used in the project. Implementation highlights:
+
+- Adds `locale:string` to `active_storage_attachments` and backfills to the
+  default locale.
+- Provides a Mobility backend in `lib/mobility/backends/attachments/backend.rb`
+  which defines per-attribute, per-locale associations and accessors.
+- Includes a small DSL at `lib/mobility/dsl/attachments.rb` to apply the
+  backend setup immediately at class-definition time (`translates_attached`).
+
+Where to look
+-------------
+
+- Backend implementation: `lib/mobility/backends/attachments/backend.rb`
+- DSL for early use: `lib/mobility/dsl/attachments.rb` (`Mobility::DSL::Attachments`)
+- Initializer wiring: `config/initializers/mobility.rb` (requires + registers
+  the backend and loads the DSL)
+- Migration: `db/migrate/20250829000000_add_locale_to_active_storage_attachments.rb`
+- Example form partial: `app/views/better_together/shared/_translated_file_field.html.erb`
+
+Model usage
+-----------
+
+Canonical (preferred when backend registration runs early):
+
+  class Page < ApplicationRecord
+    translates :hero_image, backend: :attachments, content_type: [/image/], presence: true
+  end
+
+Early-definition (tests, engines):
+
+  class Page < ApplicationRecord
+    extend Mobility::DSL::Attachments
+    translates_attached :hero_image, content_type: [/image/], presence: true
+  end
+
+The backend will generate accessors like `hero_image_en`, `hero_image_en=`,
+`hero_image_en?`, and `hero_image_en_url` for each configured locale.
+
+Migration notes
+---------------
+
+Use a staged migration: add a nullable `locale` column, backfill existing
+attachment rows to the default locale, then set NOT NULL and add a unique
+index on `[:record_type, :record_id, :name, :locale]`.
+
+Testing
+-------
+
+- Unit: `spec/lib/mobility_attachments_backend_spec.rb` demonstrates the
+  generation of accessors. The test uses `translates_attached` to guarantee
+  generation timing in the test environment.
+- Integration: Add feature specs to exercise form uploads using the
+  `_translated_file_field` partial and verify controller param handling.
+
+Developer tips
+--------------
+
+- Prefer the canonical `translates` API when possible. The DSL is intended
+  for boot-order-sensitive contexts.
+- Validate `content_type` and `presence` server-side via the backend options.
+- When deploying, ensure migrations are run and backfills complete before
+  enabling any model to rely on the `locale` column being present and NOT NULL.
+
+## Process Flow Diagram
+
+```mermaid
+%% Translatable Attachments flow
+flowchart LR
+  subgraph DB[Database]
+    A[active_storage_attachments table]
+  end
+
+  M[Migration adds `locale` column & backfill] --> A
+
+  Init[Mobility initializer]
+  Init -->|require backend| B[Attachments backend (lib/mobility/backends/...)]
+  Init -->|register backend| Mobility[Mobility.register_backend(:attachments)]
+
+  B -->|provides| Apply[AttachmentsBackend.apply_to / setup]
+
+  ModelCanonical[Model: `translates :hero_image, backend: :attachments`]
+  ModelDSL[Model: `extend Mobility::DSL::Attachments`\n`translates_attached :hero_image`]
+
+  ModelCanonical --> Apply
+  ModelDSL --> Apply
+
+  Apply -->|defines associations| Assoc[has_many :hero_image_attachments_all\nhas_one :hero_image_attachment]
+  Apply -->|defines accessors| Accessors[hero_image_en, hero_image_en=, hero_image_en?, hero_image_en_url]
+
+  View[Form partial: translated file field tabs]
+  View -->|uploads per-locale| Controller
+
+  Controller[Controller] -->|permits locale params or maps| Model
+  Model -->|writer methods| ActiveStorage[Create/modify ActiveStorage::Attachment rows]
+  ActiveStorage --> A
+
+  Serve[URL helper / rails_blob_url] -->|serves blob| UserBrowser[User's browser]
+
+  Accessors -->|getter fallback| Fallback[Default-locale fallback if enabled]
+  Fallback --> ActiveStorage
+
+  classDef infra fill:#f8f9fa,stroke:#333,stroke-width:1px;
+  class DB,Init,B,Apply,Assoc,Accessors,View,Controller,ActiveStorage,Serve,Fallback infra;
+```
+
+Diagram files:
+
+- Mermaid source: `docs/diagrams/source/translatable_attachments_flow.mmd` - editable source
+- (Optional) PNG/SVG exports: `docs/diagrams/exports/...` - add via `bin/render_diagrams` if you generate exports
+

docs/diagrams/source/translatable_attachments.mmd

@@ -0,0 +1,35 @@
+%% Translatable Attachments - Entity Relationship
+%% Generated: 2025-08-29
+
+classDiagram
+  direction TB
+
+  class ActiveStorage::Blob {
+    +uuid id
+    +string filename
+    +string content_type
+    +integer byte_size
+    +json metadata
+  }
+
+  class ActiveStorage::Attachment {
+    +uuid id
+    +string name
+    +string record_type
+    +uuid record_id
+    +uuid blob_id
+    +string locale
+    +datetime created_at
+  }
+
+  class Page {
+    +uuid id
+    +string title
+  }
+
+  Page "1" o-- "*" ActiveStorage::Attachment : has_many
+  ActiveStorage::Attachment "*" --> "1" ActiveStorage::Blob : belongs_to
+
+  %% locale uniqueness
+  note right of ActiveStorage::Attachment: Unique index on
+  note right of ActiveStorage::Attachment: (record_type, record_id, name, locale)