A statesmanlike state machine library.
For our policy on compatibility with Ruby and Rails versions, see COMPATIBILITY.md.
Statesman is an opinionated state machine library designed to provide a robust audit trail and data integrity. It decouples the state machine logic from the underlying model and allows for easy composition with one or more model classes.
As such, the design of statesman is a little different from other state machine libraries:
- State behaviour is defined in a separate, "state machine" class, rather than added directly onto a model. State machines are then instantiated with the model to which they should apply.
- State transitions are also modelled as a class, which can optionally be persisted to the database for a full audit history. This audit history can include JSON metadata set during a transition.
- Database indices are used to offer database-level transaction duplication protection.
To get started, just add Statesman to your Gemfile
, and then run bundle
:
gem 'statesman', '~> 10.0.0'
First, create a state machine based on Statesman::Machine
:
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
Then, link it to your model:
class Order < ActiveRecord::Base
has_many :order_transitions, autosave: false
include Statesman::Adapters::ActiveRecordQueries[
transition_class: OrderTransition,
initial_state: :pending
]
def state_machine
@state_machine ||= OrderStateMachine.new(self, transition_class: OrderTransition)
end
end
Next, you'll need to create a further model to represent state transitions:
class OrderTransition < ActiveRecord::Base
include Statesman::Adapters::ActiveRecordTransition
validates :to_state, inclusion: { in: OrderStateMachine.states }
belongs_to :order, inverse_of: :order_transitions
end
Now, you can start working with your state machine:
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.first.state_machine.transition_to!(:cancelled) # => true/exception
Order.first.state_machine.last_transition # => transition model or nil
Order.first.state_machine.last_transition_to(:pending) # => transition model or nil
Order.in_state(:cancelled) # => [#<Order id: "123">]
Order.not_in_state(:checking_out) # => [#<Order id: "123">]
If you'd like, you can also define a template for a generic state machine, then alter classes which extend it as required:
module Template
def define_states
state :a, initial: true
state :b
state :c
end
def define_transitions
transition from: :a, to: :b
transition from: :b, to: :c
transition from: :c, to: :a
end
end
class Circular
include Statesman::Machine
extend Template
define_states
define_transitions
end
class Linear
include Statesman::Machine
extend Template
define_states
define_transitions
remove_transitions from: :c, to: :a
end
class Shorter
include Statesman::Machine
extend Template
define_states
define_transitions
remove_state :c
end
By default Statesman stores transition history in memory only. It can be persisted by configuring Statesman to use a different adapter. For example, for 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
Your transition class should
include Statesman::Adapters::ActiveRecordTransition
if you're using the
ActiveRecord adapter.
If you're using the ActiveRecord adapter and decide not to include the default
updated_at
column in your transition table, you'll need to configure the
updated_timestamp_column
option on the transition class, setting it to another column
name (e.g. :updated_on
) or nil
.
And add an association from the parent model:
app/models/order.rb
:
class Order < ActiveRecord::Base
has_many :transitions, class_name: "OrderTransition", autosave: false
# Initialize the state machine
def state_machine
@state_machine ||= OrderStateMachine.new(self, transition_class: OrderTransition,
association_name: :transitions)
end
# Optionally delegate some methods
delegate :can_transition_to?,
:current_state, :history, :last_transition, :last_transition_to,
:transition_to!, :transition_to, :in_state?, to: :state_machine
end
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
or 5. To do that
-
Change
metadata
column type in the transition model migration tojson
orjsonb
# Before t.text :metadata, default: "{}" # After (Rails 4) t.json :metadata, default: "{}" # After (Rails 5) t.json :metadata, default: {}
-
Remove the
include Statesman::Adapters::ActiveRecordTransition
statement from your transition model. (If you want to customise your transition class's "updated timestamp column", as described above, you should define a.updated_timestamp_column
method on your class and return the name of the column as a symbol, ornil
if you don't want to record an updated timestamp on transitions.)
Statesman.configure do
storage_adapter(Statesman::Adapters::ActiveRecord)
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 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.
Machine.state(:some_state, initial: true)
Machine.state(:another_state)
Define a new state and optionally mark as the initial state.
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(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. The guard will pass when it
evaluates to a truthy value and fail when it evaluates to a falsey value (nil
or false
).
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(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.
If you specify after_commit: true
, the callback will be executed once the
transition has been committed to the database.
Machine.after_transition_failure(from: :some_state, to: :another_state) do |object, exception|
Logger.info("transition to #{exception.to} failed for #{object.id}")
end
Define a callback to run if Statesman::TransitionFailedError
is raised
during the execution of transition callbacks. to
and from
parameters are optional, a nil parameter means run after all transitions.
The model object, and exception are passed as arguments to the callback.
This is executed outside of the transaction wrapping other callbacks.
If using transition!
the exception is re-raised after these callbacks are
executed.
Machine.after_guard_failure(from: :some_state, to: :another_state) do |object, exception|
Logger.info("guard failed during transition to #{exception.to} for #{object.id}")
end
Define a callback to run if Statesman::GuardFailedError
is raised
during the execution of guard callbacks. to
and from
parameters are optional, a nil parameter means run after all transitions.
The model object, and exception are passed as arguments to the callback.
This is executed outside of the transaction wrapping other callbacks.
If using transition!
the exception is re-raised after these callbacks are
executed.
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 { 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).
Returns an array of all possible state names as strings.
Returns a hash of states and the states it is valid for them to transition to.
Machine.successors
{
"pending" => ["checking_out", "cancelled"],
"checking_out" => ["purchased", "cancelled"],
"purchased" => ["shipped", "failed"],
"shipped" => ["refunded"]
}
Returns the current state based on existing transition objects.
Takes an optional keyword argument to force a reload of data from the
database.
e.g current_state(force_reload: true)
Returns true if the machine is in any of the given states.
Returns a sorted array of all transition objects.
Returns the most recent transition object.
Returns the most recent transition object to a given state.
Returns an array of states you can transition_to
from current state.
Returns true if the current state can transition to the passed state and all applicable guards pass.
Transition to the passed state, returning true
on success. Raises
Statesman::GuardFailedError
or Statesman::TransitionFailedError
on failure.
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.)
These errors are raised when the Machine and/or Model is initialized. A simple spec like
expect { OrderStateMachine.new(Order.new, transition_class: OrderTransition) }.to_not raise_error
will expose these errors as part of your test suite
Raised if:
- Attempting to define a transition without a
to
state. - Attempting to define a transition with a non-existent state.
- Attempting to define multiple states as
initial
.
Raised if:
- Attempting to define a callback
from
a state that has no valid transitions (A terminal state). - Attempting to define a callback
to
theinitial
state if that state has no transitions to it. - Attempting to define a callback with
from
andto
where any of the pairs have no transition between them.
Raised if:
- Attempting to define a callback without a block.
Raised if:
- ActiveRecord is configured to not serialize the
metadata
attribute into to Database column backing it. See theUsing PostgreSQL JSON column
section.
Raised if:
- There is a mismatch between the column type of the
metadata
in the Database and the model. See theUsing PostgreSQL JSON column
section.
Raised if:
- The model that
Statesman::Adapters::ActiveRecordQueries
is included in does not have ahas_many
association to thetransition_class
.
These errors are raised by transition_to!
. Using transition_to
will
supress GuardFailedError
and TransitionFailedError
and return false
instead.
Raised if:
- A guard callback between
from
andto
state returned a falsey value.
Raised if:
- A transition is attempted but
current_state -> new_state
is not a valid pair.
Raised if:
- A database conflict affecting the
sort_key
ormost_recent
columns occurs when attempting a transition. Retried automatically if it occurs wrapped inretry_conflicts
.
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 passing in transition_class
and initial_state
as options.
In 4.1.2 and below, these two options had to be defined as methods on the model, but 5.0.0 and above allow this style of configuration as well. The old method pollutes the model with extra class methods, and is deprecated, to be removed in 6.0.0.
class Order < ActiveRecord::Base
has_many :order_transitions, autosave: false
include Statesman::Adapters::ActiveRecordQueries[
transition_class: OrderTransition,
initial_state: OrderStateMachine.initial_state
]
end
If the transition class-name differs from the association name, you will also
need to pass transition_name
as an option:
class Order < ActiveRecord::Base
has_many :transitions, class_name: "OrderTransition", autosave: false
include Statesman::Adapters::ActiveRecordQueries[
transition_class: OrderTransition,
initial_state: OrderStateMachine.initial_state,
transition_name: :transitions
]
end
Returns all models currently in any of the supplied states.
Returns all models not currently in any of the supplied states.
This joins the model to its most recent transition whatever that may be.
We expose this method to ease use of ActiveRecord's or
e.g
Model.in_state(:state_1).or(
Model.most_recent_transition_join.where(model_field: 123)
)
If you wish to store the model state on the model directly, you can keep it up
to date using an after_transition
hook.
Combine it with the after_commit
option to ensure the model state will only be
saved once the transition has made it irreversibly to the database:
after_transition(after_commit: true) do |model, transition|
model.state = transition.to_state
model.save!
end
You could also use a calculated column or view in your database.
Given a field foo
that was stored in the metadata, you can access it like so:
model_instance.state_machine.last_transition.metadata["foo"]
Used to using a state machine with "events"? Support for events is provided by the statesman-events gem. Once that's included in your Gemfile you can include event functionality in your state machine as follows:
class OrderStateMachine
include Statesman::Machine
include Statesman::Events
...
end
If you need to delete the Parent model regularly you will need to change
either the association deletion behaviour or add a DELETE CASCADE
condition
to foreign key in your database.
E.g
has_many :order_transitions, autosave: false, dependent: :destroy
or when migrating the transition model
add_foreign_key :order_transitions, :orders, on_delete: :cascade
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
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
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
Including ActiveRecordQueries to your model can cause issues with type checkers such as Sorbet, this is because this technically is using a dynamic include, which is not supported by Sorbet.
To avoid these issues you can instead include the TypeSafeActiveRecordQueries module and pass in configuration.
class Order < ActiveRecord::Base
has_many :order_transitions, autosave: false
include Statesman::Adapters::TypeSafeActiveRecordQueries
configure_state_machine transition_class: OrderTransition,
initial_state: :pending
def state_machine
@state_machine ||= OrderStateMachine.new(self, transition_class: OrderTransition)
end
end
statesman-sequel - An adapter to make Statesman work with Sequel
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