Components

Use this rule when the architecture decision is “this kind of code should not exist here at all.”

component(:services, in: "app/services/**/*.rb")
  .must_be_empty(because: "behavior belongs on models")

Rule id: components.empty

Every file that lands in the component fails the check:

[components.empty] app/services/create_user.rb:1:1
  services must stay empty: behavior belongs on models
  evidence: app/services/create_user.rb belongs to services

The because: reason is optional but recommended. It appears in the failure message and tells the reader where the code should go instead.

When To Use It

Use it when a convention bans a whole category of objects. Vanilla Rails in the 37signals style has no service objects, form objects, or policy objects; preset :vanilla_rails uses this rule for those directories. See the Vanilla Rails guide.

It also helps during migrations: once a legacy directory has been emptied, must_be_empty keeps it empty.