Implementing Dependency Injection

Inject dependencies through the constructor using Hanami's auto_inject — never call the container directly.

Quick Reference

• Pattern: include Deps["provider_key"] in actions, operations, or repositories.
• Testing: Pass test doubles through the constructor in specs.
• Providers: Dependencies must be registered by a provider first.
• Rule: Never call Hanami.app["key"] outside of providers.

HARD-GATE

DO NOT call the container directly (`Hanami.app["key"]`) outside of providers.
DO NOT use global state or class-level constants for dependencies.
DO inject dependencies through the constructor — use `include Deps[...]`.

Core Process

1. Verify the provider — ensure the dependency is registered in a provider under a descriptive key.
2. Add injection in the consuming class:

   module Api
     module Actions
       module Users
         class Create < Api::Action
           include Deps["operations.users.create_user"]
   
           def handle(req, res)
             result = create_user.call(req.params.to_h)
             # ...
           end
         end
       end
     end
   end

3. For operations, inject repositories and other operations:

   module Users
     class CreateUser < Dry::Operation
       include Deps["repositories.user_repo", "operations.notifications.send_welcome"]
   
       def call(input)
         # ...
       end
     end
   end

4. For repositories, inject ROM if needed:

   module Api
     module Repositories
       class UserRepo < ROM::Repository[:users]
         # Auto-injected if using auto_registration
       end
     end
   end

5. Validate resolution — after adding the injection, confirm the key resolves correctly:

   bundle exec hanami console
   # Then in the console:
   # Hanami.app["operations.users.create_user"]  # should return the registered object

   If the key is unregistered, dry-system raises Dry::Container::Error: Nothing registered with the key. Check that the provider file exists, the key matches exactly (dot-namespaced, snake_case), and auto_registration covers the file path.
6. Test with stubs — inject test doubles through the constructor:

   RSpec.describe Api::Actions::Users::Create do
     let(:create_user) { instance_double(Users::CreateUser, call: Dry::Monads::Success(user)) }
     # Pass each dep as a keyword arg; use instance_double (not double) so RSpec
     # validates the method exists on the real class.
     # For failure paths, return Dry::Monads::Failure(...) to test error branches.
     # For multiple deps: described_class.new(repo:, notifier:)
     let(:action) { described_class.new(create_user:) }
   
     it "calls the operation with request params" do
       result = action.call({ "user" => { "email" => "test@example.com" } })
       expect(create_user).to have_received(:call).with(hash_including("user"))
     end
   end

For testing DI patterns, see TESTING_DI.md.

Integration