Return an error message in a GraphQL field with graphql-ruby

graphql_state_machine.rb

# This object returns the _new_ state and any error message during the transition. 
# So in the response, you could check for the presence of an error message. 
# (In the case of an error, presumably the state would not change.)
StateMachineTransitionType = GraphQL::ObjectType.define do 
  name "StateMachineTransition" 
  field :error_message, types.String 
  field :state, types.String # Could also define StateMachineStateEnum 
end

MutationType = GraphQL::ObjectType.define do 
  name "Mutation"
  # Attempt to move states. 
  # Rescue from a failed transition and grab the error. 
  # Return _both_ the new state (which may be unchanged) and the error message (which may be nil)
  field :transition_state_machine, StateMachineTransitionType do 
    argument :to_state, !types.String # Could also define StateMachineStateEnum 
    resolve -> (obj, args, ctx) {
      obj # => StateMachine 
      error_message = nil 
      begin
        obj.transition_to(args[:to_state])
      rescue StateMachine::TransitionFailedError => err 
        error_message = err.message 
      end 
      # IRL define a class like StateMachineTransitionResponse instead of OpenStruct
      OpenStruct.new(
        state: obj.state, 
        error_message: error_message, 
      )
    }
  end
end