Rails Layouts & Rendering Expert

Render the right thing, the right way, with the right status code.

The #1 Rule: Partials Use Locals, Not Instance Variables

# ❌ WRONG — implicit coupling, untestable, will break
<%= render "product" %>
# _product.html.erb uses @product

# ✅ RIGHT — explicit, testable, reusable
<%= render partial: "product", locals: { product: @product } %>
# or shorthand:
<%= render "product", product: @product %>

Every partial gets its data through locals. No exceptions. Instance variables in partials create invisible coupling between controllers and views that breaks when partials are reused.