module StateMachine
  module Integrations #:nodoc:
    # Adds support for integrating state machines with Sequel models.
    # 
    # == Examples
    # 
    # Below is an example of a simple state machine defined within a
    # Sequel model:
    # 
    #   class Vehicle < Sequel::Model
    #     state_machine :initial => :parked do
    #       event :ignite do
    #         transition :parked => :idling
    #       end
    #     end
    #   end
    # 
    # The examples in the sections below will use the above class as a
    # reference.
    # 
    # == Actions
    # 
    # By default, the action that will be invoked when a state is transitioned
    # is the +save+ action.  This will cause the resource to save the changes
    # made to the state machine's attribute.  *Note* that if any other changes
    # were made to the resource prior to transition, then those changes will
    # be made as well.
    # 
    # For example,
    # 
    #   vehicle = Vehicle.create          # => #<Vehicle @values={:state=>"parked", :name=>nil, :id=>1}>
    #   vehicle.name = 'Ford Explorer'
    #   vehicle.ignite                    # => true
    #   vehicle.refresh                   # => #<Vehicle @values={:state=>"idling", :name=>"Ford Explorer", :id=>1}>
    # 
    # == Events
    # 
    # As described in StateMachine::InstanceMethods#state_machine, event
    # attributes are created for every machine that allow transitions to be
    # performed automatically when the object's action (in this case, :save)
    # is called.
    # 
    # In Sequel, these automated events are run in the following order:
    # * before validation - Run before callbacks and persist new states, then validate
    # * before save - If validation was skipped, run before callbacks and persist new states, then save
    # * after save - Run after callbacks
    # 
    # For example,
    # 
    #   vehicle = Vehicle.create          # => #<Vehicle @values={:state=>"parked", :name=>nil, :id=>1}>
    #   vehicle.state_event               # => nil
    #   vehicle.state_event = 'invalid'
    #   vehicle.valid?                    # => false
    #   vehicle.errors.full_messages      # => ["state_event is invalid"]
    #   
    #   vehicle.state_event = 'ignite'
    #   vehicle.valid?                    # => true
    #   vehicle.save                      # => #<Vehicle @values={:state=>"idling", :name=>nil, :id=>1}>
    #   vehicle.state                     # => "idling"
    #   vehicle.state_event               # => nil
    # 
    # Note that this can also be done on a mass-assignment basis:
    # 
    #   vehicle = Vehicle.create(:state_event => 'ignite')  # => #<Vehicle @values={:state=>"idling", :name=>nil, :id=>1}>
    #   vehicle.state                                       # => "idling"
    # 
    # === Security implications
    # 
    # Beware that public event attributes mean that events can be fired
    # whenever mass-assignment is being used.  If you want to prevent malicious
    # users from tampering with events through URLs / forms, the attribute
    # should be protected like so:
    # 
    #   class Vehicle < Sequel::Model
    #     set_restricted_columns :state_event
    #     # set_allowed_columns ... # Alternative technique
    #     
    #     state_machine do
    #       ...
    #     end
    #   end
    # 
    # If you want to only have *some* events be able to fire via mass-assignment,
    # you can build two state machines (one public and one protected) like so:
    # 
    #   class Vehicle < Sequel::Model
    #     set_restricted_columns :state_event # Prevent access to events in the first machine
    #     
    #     state_machine do
    #       # Define private events here
    #     end
    #     
    #     # Allow both machines to share the same state
    #     state_machine :public_state, :attribute => :state do
    #       # Define public events here
    #     end
    #   end
    # 
    # == Transactions
    # 
    # In order to ensure that any changes made during transition callbacks
    # are rolled back during a failed attempt, every transition is wrapped
    # within a transaction.
    # 
    # For example,
    # 
    #   class Message < Sequel::Model
    #   end
    #   
    #   Vehicle.state_machine do
    #     before_transition do |transition|
    #       Message.create(:content => transition.inspect)
    #       false
    #     end
    #   end
    #   
    #   vehicle = Vehicle.create      # => #<Vehicle @values={:state=>"parked", :name=>nil, :id=>1}>
    #   vehicle.ignite                # => false
    #   Message.count                 # => 0
    # 
    # *Note* that only before callbacks that halt the callback chain and
    # failed attempts to save the record will result in the transaction being
    # rolled back.  If an after callback halts the chain, the previous result
    # still applies and the transaction is *not* rolled back.
    # 
    # To turn off transactions:
    # 
    #   class Vehicle < Sequel::Model
    #     state_machine :initial => :parked, :use_transactions => false do
    #       ...
    #     end
    #   end
    # 
    # == Validation errors
    # 
    # If an event fails to successfully fire because there are no matching
    # transitions for the current record, a validation error is added to the
    # record's state attribute to help in determining why it failed and for
    # reporting via the UI.
    # 
    # For example,
    # 
    #   vehicle = Vehicle.create(:state => 'idling')  # => #<Vehicle @values={:state=>"parked", :name=>nil, :id=>1}>
    #   vehicle.ignite                                # => false
    #   vehicle.errors.full_messages                  # => ["state cannot transition via \"ignite\""]
    # 
    # If an event fails to fire because of a validation error on the record and
    # *not* because a matching transition was not available, no error messages
    # will be added to the state attribute.
    # 
    # == Scopes
    # 
    # To assist in filtering models with specific states, a series of class
    # methods are defined on the model for finding records with or without a
    # particular set of states.
    # 
    # These named scopes are the functional equivalent of the following
    # definitions:
    # 
    #   class Vehicle < Sequel::Model
    #     class << self
    #       def with_states(*states)
    #         filter(:state => states)
    #       end
    #       alias_method :with_state, :with_states
    #       
    #       def without_states(*states)
    #         filter(~{:state => states})
    #       end
    #       alias_method :without_state, :without_states
    #     end
    #   end
    # 
    # *Note*, however, that the states are converted to their stored values
    # before being passed into the query.
    # 
    # Because of the way scopes work in Sequel, they can be chained like so:
    # 
    #   Vehicle.with_state(:parked).order(:id.desc)
    # 
    # == Callbacks
    # 
    # All before/after transition callbacks defined for Sequel resources
    # behave in the same way that other Sequel hooks behave.  Rather than
    # passing in the record as an argument to the callback, the callback is
    # instead bound to the object and evaluated within its context.
    # 
    # For example,
    # 
    #   class Vehicle < Sequel::Model
    #     state_machine :initial => :parked do
    #       before_transition any => :idling do
    #         put_on_seatbelt
    #       end
    #       
    #       before_transition do |transition|
    #         # log message
    #       end
    #       
    #       event :ignite do
    #         transition :parked => :idling
    #       end
    #     end
    #     
    #     def put_on_seatbelt
    #       ...
    #     end
    #   end
    # 
    # Note, also, that the transition can be accessed by simply defining
    # additional arguments in the callback block.
    module Sequel
      # The default options to use for state machines using this integration
      class << self; attr_reader :defaults; end
      @defaults = {:action => :save}
      
      # Should this integration be used for state machines in the given class?
      # Classes that include Sequel::Model will automatically use the Sequel
      # integration.
      def self.matches?(klass)
        defined?(::Sequel::Model) && klass <= ::Sequel::Model
      end
      
      # Loads additional files specific to Sequel
      def self.extended(base) #:nodoc:
        require 'sequel/extensions/inflector' if ::Sequel.const_defined?('VERSION') && ::Sequel::VERSION >= '2.12.0'
      end
      
      # Adds a validation error to the given object
      def invalidate(object, attribute, message, values = [])
        object.errors.add(self.attribute(attribute), generate_message(message, values))
      end
      
      # Resets any errors previously added when invalidating the given object
      def reset(object)
        object.errors.clear
      end
      
      protected
        # Skips defining reader/writer methods since this is done automatically
        def define_state_accessor
          name = self.name
          owner_class.validates_each(attribute) do |record, attr, value|
            machine = record.class.state_machine(name)
            machine.invalidate(record, attr, :invalid) unless machine.states.match(record)
          end
        end
        
        # Adds hooks into validation for automatically firing events
        def define_action_helpers
          if super && action == :save
            @instance_helper_module.class_eval do
              define_method(:valid?) do |*args|
                self.class.state_machines.fire_event_attributes(self, :save, false) { super(*args) }
              end
            end
          end
        end
        
        # Creates a scope for finding records *with* a particular state or
        # states for the attribute
        def create_with_scope(name)
          attribute = self.attribute
          lambda {|model, values| model.filter(attribute.to_sym => values)}
        end
        
        # Creates a scope for finding records *without* a particular state or
        # states for the attribute
        def create_without_scope(name)
          attribute = self.attribute
          lambda {|model, values| model.filter(~{attribute.to_sym => values})}
        end
        
        # Runs a new database transaction, rolling back any changes if the
        # yielded block fails (i.e. returns false).
        def transaction(object)
          object.db.transaction {raise ::Sequel::Error::Rollback unless yield}
        end
        
        # Creates a new callback in the callback chain, always ensuring that
        # it's configured to bind to the object as this is the convention for
        # Sequel callbacks
        def add_callback(type, options, &block)
          options[:bind_to_object] = true
          options[:terminator] = @terminator ||= lambda {|result| result == false}
          super
        end
    end
  end
end