Fix Middleware Error in Rails

Rails Middleware Errors

Rails' middleware stack is built on Rack. Errors in custom middleware can be hard to debug because they occur before your application code runs.

Viewing the Middleware Stack

bin/rails middleware

This shows the exact order. Middleware at the top runs first.

Common Middleware Bug: Not Calling the App

# BAD — app never gets called on certain paths
class ApiKeyMiddleware
  def initialize(app)
    @app = app
  end

  def call(env)
    if env['PATH_INFO'].start_with?('/api')
      unless env['HTTP_X_API_KEY'] == ENV['API_KEY']
        return [401, {}, ['Unauthorized']]
      end
    end
    # Bug: @app.call(env) is missing for non-/api paths!
  end
end

# FIXED
def call(env)
  if env['PATH_INFO'].start_with?('/api')
    unless env['HTTP_X_API_KEY'] == ENV['API_KEY']
      return [401, { 'Content-Type' => 'text/plain' }, ['Unauthorized']]
    end
  end
  @app.call(env)
end

Middleware Ordering

# config/application.rb
config.middleware.insert_before ActionDispatch::Static, MyCustomMiddleware
config.middleware.insert_after Rack::Sendfile, AnotherMiddleware

Exception Handling in Middleware

def call(env)
  @app.call(env)
rescue StandardError => e
  Rails.logger.error "Middleware error: #{e.message}"
  [500, { 'Content-Type' => 'text/plain' }, ['Internal Server Error']]
end

Bugsly integrates as Rack middleware itself, so it captures exceptions from all middleware below it in the stack — insert it early for maximum coverage.