build-json-api

Use this skill when building JSON API endpoints in Hanami 2.x Actions.

End-to-End Workflow: Creating a JSON Endpoint

1. Create the action and set response.format = :json
2. Add a dedicated serializer for the entity being returned
3. Write a round-trip test asserting serialize → parse produces correct fields
4. Verify: run the test; fix serializer if fields are missing or misformatted

Core Rules

1. Set the response format to JSON:

   def handle(request, response)
     response.format = :json
     # ...
   end

2. Use dedicated serializers to encode response bodies. For implementation details and conventions, see SERIALIZERS.md.

   def handle(request, response)
     response.format = :json
     user = user_repo.by_id(request.params[:id]).one
     
     response.body = MyApp::Serializers::UserSerializer.new(user).to_json
   end

3. Verify round-trip serialization: Always assert that a serialized object can be parsed back into equivalent data. Format all date/time fields with .iso8601. If the assertion fails, check that the serializer maps all fields and that timestamps use .iso8601.

   # In tests:
   serialized = UserSerializer.new(user).to_json
   parsed = JSON.parse(serialized, symbolize_names: true)
   
   assert_equal user.email, parsed[:email]
   assert_equal user.created_at.iso8601, parsed[:created_at]
   # If assertion fails: verify serializer field mapping and timestamp formatting

4. Handle request body parsing via request.params:

   def handle(request, response)
     response.format = :json
     attrs = request.params[:user]
     result = user_repo.create(attrs)
   
     response.status = 201
     response.body = UserSerializer.new(result).to_json
   end

5. Return consistent error shapes: Ensure all error responses conform to a unified structure:

   halt 422, { error: { message: "Validation failed", details: [...] } }.to_json

6. Include pagination metadata for collections: Expose pagination fields inside a meta block sibling to data:

   {
     data: users.map { |u| UserSerializer.new(u).to_h },
     meta: {
       page: request.params[:page] || 1,
       per_page: request.params[:per_page] || 20,
       total: user_repo.count
     }
   }.to_json

7. Rescue parse errors and return bad request (400) status:

   rescue JSON::ParserError
     halt 400, { error: "Invalid JSON body" }.to_json
   end