Strict local variables
Rails partials let you do strict local variables with a magic comment:
<%# app/views/customers/_customer.html.erb %>
<%# locals: (customer:) -%>
<%= customer.name %>
If you don't pass a customer to the parital, it will raise an error:
<%= render "customer", not_customer: 42 %>
You must pass the customer:
<%= render "customer", customer: %>