Rails API controllers inherit from ActionController::API, which is a lightweight version of ActionController::Base.
Unlike the full controller, ActionController::API does not include MIME response functionality by default.
The respond_to method is part of the ActionController::MimeResponds module, which provides content negotiation
capabilities. When you use respond_to in an API controller without including this module, Rails cannot find the method and raises a
NoMethodError at runtime.
This error typically occurs when developers migrate from full Rails controllers to API-only controllers, or when they follow examples that assume
the full controller functionality is available. The error message can be confusing because Ruby suggests respond_to? (the Object method
for checking if an object responds to a method) instead of the missing respond_to method.
What is the potential impact?
This issue causes a runtime NoMethodError that will crash the application when the affected controller action is called. This leads
to:
- Application downtime and poor user experience
- Failed API requests that clients cannot handle gracefully
- Potential data loss if the error occurs during critical operations
- Debugging time spent tracking down what appears to be a missing method
