A statesmanlike state machine library for Ruby 1.9.3 and 2.0.
Statesman is a little different from other state machine libraries which tack state behaviour directly onto a model. A statesman state machine is defined as a separate class which is instantiated with the model to which it should apply. State transitions are also modelled as a class which can optionally be persisted to the database for a full audit history, including JSON metadata which can be set during a transition.
This data model allows for interesting things like using a different state machine depending on the value of a model attribute.
TL;DR Usage
class OrderStateMachine
include Statesman::Machine
state :pending, initial: true
state :checking_out
state :purchased
state :shipped
state :cancelled
state :failed
state :refunded
transition from: :pending, to: [:checking_out, :cancelled]
transition from: :checking_out, to: [:purchased, :cancelled]
transition from: :purchased, to: [:shipped, :failed]
transition from: :shipped, to: :refunded
guard_transition(to: :checking_out) do |order|
order.products_in_stock?
end
before_transition(from: :checking_out, to: :cancelled) do |order, transition|
order.reallocate_stock
end
before_transition(to: :purchased) do |order, transition|
PaymentService.new(order).submit
end
after_transition(to: :purchased) do |order, transition|
MailerService.order_confirmation(order).deliver
end
end
class Order < ActiveRecord::Base
include Statesman::Adapters::ActiveRecordModel
has_many :order_transitions
def state_machine
OrderStateMachine.new(self, transition_class: OrderTransition)
end
private
def self.transition_class
OrderTransition
end
end
class OrderTransition < ActiveRecord::Base
include Statesman::Adapters::ActiveRecordTransition
belongs_to :order, inverse_of: :order_transitions
end
Order.first.state_machine.current_state
# => "pending"
Order.first.state_machine.allowed_transitions
# => ["checking_out", "cancelled"]
Order.first.state_machine.can_transition_to?(:cancelled)
# => true/false
Order.first.state_machine.transition_to(:cancelled, optional: :metadata)
# => true/false
Order.in_state(:cancelled)
# => [#<Order id: "123">]
Order.not_in_state(:checking_out)
# => [#<Order id: "123">]
Order.first.state_machine.transition_to!(:cancelled)
# => true/exception
Events
class TaskStateMachine
include Statesman::Machine
state :unstarted, initial: true
state :started
state :finished
state :delivered
state :accepted
state :rejected
event :start do
transition from: :unstarted, to: :started
end
event :finish do
transition from: :started, to: :finished
end
event :deliver do
transition from: :finished, to: :delivered
transition from: :started, to: :delivered
end
event :accept do
transition from: :delivered, to: :accepted
end
event :rejected do
transition from: :delivered, to: :rejected
end
event :restart do
transition from: :rejected, to: :started
end
end
class Task < ActiveRecord::Base
delegate :current_state, :trigger!, :available_events, to: :state_machine
def state_machine
@state_machine ||= TaskStateMachine.new(self)
end
end
task = Task.new
task.current_state
# => "unstarted"
task.trigger!(:start)
# => true/exception
task.current_state
# => "started"
task.available_events
# => [:finish, :deliver]
Persistence
By default Statesman stores transition history in memory only. It can be persisted by configuring Statesman to use a different adapter. For example, ActiveRecord within Rails:
config/initializers/statesman.rb
:
Statesman.configure do
storage_adapter(Statesman::Adapters::ActiveRecord)
end
Generate the transition model:
$ rails g statesman:active_record_transition Order OrderTransition
And add an association from the parent model:
app/models/order.rb
:
class Order < ActiveRecord::Base
has_many :order_transitions
# Initialize the state machine
def state_machine
@state_machine ||= OrderStateMachine.new(self, transition_class: OrderTransition)
end
# Optionally delegate some methods
delegate :can_transition_to?, :transition_to!, :transition_to, :current_state,
to: :state_machine
end
Using PostgreSQL JSON column
By default, Statesman uses serialize
to store the metadata in JSON format.
It is also possible to use the PostgreSQL JSON column if you are using Rails 4. To do that
-
Change
metadata
column type in the transition model migration tojson
# Before t.text :metadata, default: "{}" # After t.json :metadata, default: "{}"
-
Remove
include Statesman::Adapters::ActiveRecordTransition
statement from your transition model
Configuration
storage_adapter
Statesman.configure do
storage_adapter(Statesman::Adapters::ActiveRecord)
# ...or
storage_adapter(Statesman::Adapters::Mongoid)
end
Statesman defaults to storing transitions in memory. If you're using rails, you can instead configure it to persist transitions to the database by using the ActiveRecord or Mongoid adapter.
Statesman will fallback to memory unless you specify a transition_class when instantiating your state machine. This allows you to only persist transitions on certain state machines in your app.
Class methods
Machine.state
Machine.state(:some_state, initial: true)
Machine.state(:another_state)
Define a new state and optionally mark as the initial state.
Machine.transition
Machine.transition(from: :some_state, to: :another_state)
Define a transition rule. Both method parameters are required, to
can also be
an array of states (.transition(from: :some_state, to: [:another_state, :some_other_state])
).
Machine.guard_transition
Machine.guard_transition(from: :some_state, to: another_state) do |object|
object.some_boolean?
end
Define a guard. to
and from
parameters are optional, a nil parameter means
guard all transitions. The passed block should evaluate to a boolean and must
be idempotent as it could be called many times.
Machine.before_transition
Machine.before_transition(from: :some_state, to: another_state) do |object|
object.side_effect
end
Define a callback to run before a transition. to
and from
parameters are
optional, a nil parameter means run before all transitions. This callback can
have side-effects as it will only be run once immediately before the transition.
Machine.after_transition
Machine.after_transition(from: :some_state, to: another_state) do |object, transition|
object.side_effect
end
Define a callback to run after a successful transition. to
and from
parameters are optional, a nil parameter means run after all transitions. The
model object and transition object are passed as arguments to the callback.
This callback can have side-effects as it will only be run once immediately
after the transition.
Machine.new
my_machine = Machine.new(my_model, transition_class: MyTransitionModel)
Initialize a new state machine instance. my_model
is required. If using the
ActiveRecord adapter my_model
should have a has_many
association with
MyTransitionModel
.
Machine.retry_conflicts
Machine.retry_conflicts { instance.transition_to(:new_state) }
Automatically retry the given block if a TransitionConflictError
is raised.
If you know you want to retry a transition if it fails due to a race condition
call it from within this block. Takes an (optional) argument for the maximum
number of retry attempts (defaults to 1).
Instance methods
Machine#current_state
Returns the current state based on existing transition objects.
Machine#history
Returns a sorted array of all transition objects.
Machine#last_transition
Returns the most recent transition object.
Machine#allowed_transitions
Returns an array of states you can transition_to
from current state.
Machine#can_transition_to?(:state)
Returns true if the current state can transition to the passed state and all applicable guards pass.
Machine#transition_to!(:state)
Transition to the passed state, returning true
on success. Raises
Statesman::GuardFailedError
or Statesman::TransitionFailedError
on failure.
Machine#transition_to(:state)
Transition to the passed state, returning true
on success. Swallows all
Statesman exceptions and returns false on failure. (NB. if your guard or
callback code throws an exception, it will not be caught.)
Model scopes
A mixin is provided for the ActiveRecord adapter which adds scopes to easily
find all models currently in (or not in) a given state. Include it into your
model and define a transition_class
method.
class Order < ActiveRecord::Base
include Statesman::Adapters::ActiveRecordModel
private
def self.transition_class
OrderTransition
end
end
Model.in_state(:state_1, :state_2, etc)
Returns all models currently in any of the supplied states.
Model.not_in_state(:state_1, :state_2, etc)
Returns all models not currently in any of the supplied states.
Frequently Asked Questions
Model.in_state
does not work before first transition
This is expected. State is modelled as a series of tranisitions so initially where there has been no transition, there is no explict state. At GoCardless we get around this by transitioning immediately after creation:
after_create do
state_machine.transition_to! "pending"
end
Storing the state on the model object
If you wish to store the model state on the model directly, you can keep it up to date using an after_transition
hook:
after_transition do |model, transition|
model.state = transition.to_state
model.save!
end
Accessing metadata from the last transition
Given a field foo
that was stored in the metadata, you can access it like so:
model_instance.last_transition.metadata["foo"]
Testing Statesman Implementations
This answer was abstracted from this issue.
At GoCardless we focus on testing that:
- guards correctly prevent / allow transitions
- callbacks execute when expected and perform the expected actions
Testing Guards
Guards can be tested by asserting that transition_to!
does or does not raise a Statesman::GuardFailedError
:
describe "guards" do
it "cannot transition from state foo to state bar" do
expect { some_model.transition_to!(:bar) }.to raise_error(Statesman::GuardFailedError)
end
it "can transition from state foo to state baz" do
expect { some_model.transition_to!(:baz).to_not raise_error
end
end
Testing Callbacks
Callbacks are tested by asserting that the action they perform occurs:
describe "some callback" do
it "adds one to the count property on the model" do
expect { some_model.transition_to!(:some_state) }.
to change {
some_model.reload.count
}.by(1)
end
end
Creating models in certain states
Sometimes you'll want to test a guard/transition from one state to another, where the state you want to go from is not the initial state of the model. In this instance you'll need to construct a model instance in the state required. However, if you have strict guards, this can be a pain. One way to get around this in tests is to directly create the transitions in the database, hence avoiding the guards.
We use FactoryGirl for creating our test objects. Given an Order
model that is backed by Statesman, we can easily set it up to be in a particular state:
factory :order do
property "value"
...
trait :shipped do
after(:create) do |order|
FactoryGirl.create(:order_transition, :shipped, order: order)
end
end
end
factory :order_transition do
order
...
trait :shipped do
to_state "shipped"
end
end
This means you can easily create an Order
in the shipped
state:
let(:shipped_order) { FactoryGirl.create(:order, :shipped) }
GoCardless