ERB → ViewModel: Extract Ruby logic out of an ERB template

When the user types /erb-to-view-model, migrate an ERB template away from instance variables / helper calls by precomputing all required values in a Ruby ViewModel under app/view_models/, following existing repo conventions.

1) Collect required inputs (ask before doing anything)

Before proceeding, ask the user for these parameters (do not proceed until all are provided):

• ERB template path: <erb-path> (example: app/views/groups/index.html.erb)
• Controller + action: <controller>#<action> (example: GroupsController#index)
• Target ViewModel constant: <view-model-constant> (example: Groups::IndexViewModel)
• Target ViewModel file path: <view-model-path> (example: app/view_models/groups/index_view_model.rb)

This command always uses the template local variable name view_model (it is not configurable).

Also ask for these parameters (strongly recommended; do not proceed without tests):

• Characterization spec path: <spec-path> (example: spec/controllers/groups_controller_characterization_spec.rb)
• Feature flag constraint: <feature-flag> (example: groups_index_view_model_migration)

This command assumes layout variants are always web-only.

2) Load the current project rules (required)

Read ALL rules in:

@.cursor/rules/

In addition, these constraints are part of this command's prompt and must be followed:

• ERB → ViewModel migration constraints

  • Replace all instance variables (@something) with view_model.something
  • Do not use can?(:...) in ERB; replace with precomputed view_model.can_* flags
  • Do not use current_user.* in ERB; replace with precomputed view_model.current_user_* or derived flags
  • Do not use polymorphic_path / polymorphic links directly in ERB; precompute paths/urls on the ViewModel

• ViewModels must be "precompute-first"

  • Use T::Struct with const values for anything the template reads
  • Prefer self.init(...) that computes everything once, then new(...)
  • Prefer booleans like has_attachments (not has_attachments?) for conditionals
  • If something can be missing, type it as T.nilable(...) and handle nil explicitly

• No direct model references inside ViewModels

  • Do not store AR models (e.g., User, Group, ChatThread) as const values in new/modified ViewModels
  • Store primitives (String/Integer/T::Boolean), precomputed hashes/arrays, or nested ViewModels instead
  • If serialization is needed, precompute the serialized form during init (do not serialize at render time)

• How views should consume ViewModels

  • Treat ViewModel values as precomputed data, not "call-to-compute" methods
  • Preserve safe navigation (&.) in templates for nilable ViewModel properties
  • Pass required parameters explicitly through initializers; do not implicitly derive them in views
  • Prefer constants like has_translation (not has_translation?)

• Controller integration must be thin

  • Controllers should pass only basic dependencies and delegate business logic (auth, data fetching, derivations) to the ViewModel
  • Avoid service layers for ViewModel creation; move that logic into ViewModels instead

• Refactor bottom-up

  • Start at the deepest partial(s), convert them to ViewModels, then backpropagate required parameters up the render tree
  • Goal state: no direct model access in ERB; only ViewModel reads + simple presentation

• Characterization test golden rule

  • Characterization specs should be at the controller level
  • Must use render_views
  • Assertions should be against final HTML (response.body)
  • Avoid partial-level view specs and avoid assigns(...)-style assertions

• Ruby method signatures

  • Do not use default parameters (no param = nil, no param: nil)
  • Make nilability explicit via T.nilable(...) and require callers to pass values explicitly

• Avoid OpenStruct

  • Do not use OpenStruct in production code or specs for this work; use typed structs, hashes, or verified doubles as appropriate

3) Identify existing ViewModel patterns to follow (required)

Use existing converted ViewModels as the primary examples (read before writing new code):

• @app/view_models/groups/index_view_model.rb (nested VMs, helpers, permissions)
• @app/view_models/chats/message_show_view_model.rb (large precompute init, URLs, flags)
• If your template's domain differs, scan @app/view_models/ for the closest match.

If the ViewModel needs Rails helpers (paths, formatting), prefer the typed adapter pattern used in Groups:

• @app/view_models/groups/helper_interface.rb
• @app/view_models/groups/helper_adapter.rb

4) Baseline safety: characterization test first (required)

Follow @.cursor/rules/rspec/controller-characterization-test-golden-rule.mdc:

• Heavily prefer a controller-level characterization spec to exist for <controller>#<action> before changing any ERB/ViewModel code.

• If you cannot find an existing characterization spec for this controller/action:

  • Create one first using @prompts/erb_characterization_tests.md
  • Put it at <spec-path> (or the repo's established characterization spec location for that controller)
  • Run it and confirm it passes
  • Do not proceed with the ERB/ViewModel migration until this safety net exists and is green

• Add or extend a controller-level characterization spec at <spec-path> (preferred: reuse an existing one)

• Ensure it uses render_views

• Assert against response.body HTML (not instance variables, not partial-level specs)

• Cover the key states that are likely to regress during migration:

  • Empty state vs populated state
  • Permission-gated UI sections
  • Web-only rendering behavior (this command assumes no app layout variants)

Run the spec and confirm it passes before changing the ERB or ViewModel code.

5) Perform a bottom-up migration (required)

Follow @.cursor/rules/view_models/hierarchy-refactoring.mdc:

1. Trace the render tree

   • From <erb-path>, list every rendered partial and the locals it expects.
   • Start migrating the deepest partial(s) first, then work upwards.

2. Build or extend ViewModels

   • Create/modify <view-model-path> so it is a T::Struct with precomputed const values
   • Add self.init(...) with explicit parameters (no default args)
   • If a value can be nil, type it T.nilable(...) and pass nil explicitly from the caller
   • Do not use OpenStruct

3. Do not add direct model references to new ViewModels

   • Prefer primitives (String/Integer/T::Boolean), precomputed hashes, and child view models.
   • If an existing ViewModel already exposes models for backward compatibility, do not expand that pattern. Keep model exposure minimal and isolated.

4. Update controller creation

   • Make the controller thin: it should gather only the basic dependencies and call <view-model-constant>.init(...).
   • Prefer rendering with a local named view_model:

render :index, locals: { view_model: @view_model }

If the controller has multiple render branches (app vs web), keep them consistent in how they pass the ViewModel.

6) Refactor the ERB and partials to consume the ViewModel (required)

In <erb-path> and its partials:

• Replace instance variables: @something → view_model.something
• Replace helper calls with precomputed values:
  • Paths/URLs: polymorphic_path(...) → view_model.some_path
  • Formatting: simple_format, auto_link, pluralization text, etc. → view_model.some_html / view_model.some_text
• Replace authorization and current-user access:
  • can?(...) → view_model.can_*
  • current_user.* → view_model.current_user_* or other precomputed flags
• Replace complex conditions with booleans:
  • if thing.present? → if view_model.has_thing
• Collections:
  • render partial: ..., collection: models → collection: view_model.child_view_models, as: :view_model
• Safe navigation:
  • Preserve &. in templates for nilable ViewModel properties (don't "tighten" callsites).

7) Verification and cleanup (required)

• Run the characterization spec again; it should pass unchanged.
• Run any directly related controller specs touched by <controller>#<action>.
• Ensure the ERB contains no @variable access and no direct can? / current_user.* / polymorphic_path calls per erb/template-viewmodel-migration.mdc.
• For any Ruby files changed, ensure no new default parameters were introduced (see ruby-no-default-params.mdc).

8) Output format (required)

Structure the response as:

## ERB → ViewModel migration

### Inputs
- ERB: <erb-path>
- Controller/action: <controller>#<action>
- ViewModel: <view-model-constant> (<view-model-path>)
- Template var: view_model

### Render tree
- [List partials, top to bottom]

### Changes made
- [File list + 1-line summary each]

### Rule compliance notes
- [Call out any tricky spots and which rule/pattern you followed]

### Test plan
- [Exact rspec commands run and outcomes]