Polymorphic Parent Resources

Overview

Apps accumulate child resources that hang off many different parents — comments, generated_reports, duplications, attachments. The naive approaches both scale badly:

• A namespaced controller per parent (Estimates::CommentsController, Widgets::CommentsController, Accounts::CommentsController) — identical code, N copies, N sets of specs.
• Non-RESTful actions bolted onto each parent controller (EstimatesController#add_comment) — breaks REST and spreads comment logic across the app.

This pattern uses one route concern plus one controller to serve every parent. The parent's class name travels as a route default, and resource_for turns it back into the record.

When to Use

• A child resource belongs to two or more parent models (or will soon)
• About to duplicate a controller whose only difference is which parent it loads
• Consolidating existing per-parent controllers — see references/retrofit.md

When not to use: a child with exactly one parent and no plans for more. A plain nested controller with Parent.find(params[:parent_id]) is clearer. Reach for this pattern at the second parent, not in anticipation of one.