This guide is based on guides.rubyonrails.org/activerecord_validations_callbacks.html
Model hooks are used to specify actions that occur at a given point in a model instance's lifecycle, such as before or after the model object is saved, created, updated, destroyed, or validated. There are also around hooks for all types, which wrap the before hooks, the behavior, and the after hooks.
Sequel::Model
uses instance methods for hooks. To define a
hook on a model, you just add an instance method to the model class:
class Album < Sequel::Model def before_create self.created_at ||= Time.now super end end
The one important thing to note here is the call to super
inside the hook. Whenever you override one of Sequel::Model's methods,
you should be calling super
to get the default behavior. Many
of the plugins that ship with Sequel work by
overriding the hook methods and calling super
. If you use
these plugins and override the hook methods but do not call
super
, it's likely the plugins will not work correctly.
Sequel calls hooks in the following order when saving/creating a new object (one that does not already exist in the database):
around_validation
before_validation
validate
method called
after_validation
around_save
before_save
around_create
before_create
INSERT QUERY
after_create
after_save
Sequel calls hooks in the following order when saving an existing object:
around_validation
before_validation
validate
method called
after_validation
around_save
before_save
around_update
before_update
UPDATE QUERY
after_update
after_save
Note that all of the hook calls are the same, except that
around_create
, before_create
and
after_create
are used for a new object, and
around_update
, before_update
and
after_update
are used for an existing object. Note that
around_save
, before_save
, and
after_save
are called in both cases.
Note that the validation hooks are still called if validate:
false
option is passed to save. If you call
Model#valid?
manually, then only the validation hooks are
called:
around_validation
before_validation
validate
method called
after_validation
Sequel calls hooks in the following order when destroying an existing object:
around_destroy
before_destroy
DELETE QUERY
after_destroy
Note that these hooks are only called when using
Model#destroy
, they are not called if you use
Model#delete
.
Sequel::Model no longer offers
transaction hooks for model instances. However, you can use the database
transaction hooks inside model before_save
and
after_save
hooks:
class Album < Sequel::Model def before_save db.after_rollback{rollback_action} super end def after_save super db.after_commit{commit_action} end end
Sequel does not provide a simple way to turn off the running of save/create/update hooks. If you attempt to save a model object, the save hooks are always called. All model instance methods that modify the database call save in some manner, so you can be sure that if you define the hooks, they will be called when you save the object.
However, you should note that there are plenty of ways to modify the database without saving a model object. One example is by using plain datasets, or one of the model's dataset methods:
Album.where(name: 'RF').update(copies_sold: Sequel.+(:copies_sold, 1)) # UPDATE albums SET copies_sold = copies_sold + 1 WHERE name = 'RF'
In this case, the update
method is called on the dataset
returned by Album.where
. Even if there is only a single
object with the name RF, this will not call any hooks. If you want model
hooks to be called, you need to make sure to operate on a model object:
album = Album.first(name: 'RF') album.update(copies_sold: album.copies_sold + 1) # UPDATE albums SET copies_sold = 2 WHERE id = 1
For the destroy hooks, you need to make sure you call destroy
on the object:
album.destroy # runs destroy hooks
Sequel makes it easy to skip destroy hooks by
calling delete
instead of destroy
:
album.delete # does not run destroy hooks
However, skipping hooks is a bad idea in general and should be avoided. As
mentioned above, Sequel doesn't allow you
to turn off the running of save hooks. If you know what you are doing and
really want to skip them, you need to drop down to the dataset level to do
so. This can be done for a specific model object by using the
this
method for a dataset that represents a single object:
album.this # dataset
The this
dataset works just like any other dataset, so you can
call update
on it to modify it:
album.this.update(copies_sold: album.copies_sold + 1)
If you want to insert a row into the model's table without running the
creation hooks, you can use Model.insert
instead of
Model.create
:
Album.insert(name: 'RF') # does not run hooks
Sometimes want to cancel an action in a before hook, so the action is not
performed. For example, you may want to not allow destroying or saving a
record in certain cases. In those cases, you can call
cancel_action
inside the before_*
hook, which
will stop processing the hook and will either raise a
Sequel::HookFailed
exception (the default), or return
nil
if raise_on_save_failure
is
false
). You can use this to implement validation-like
behavior, that will run even if validations are skipped:
class Album < Sequel::Model def before_save cancel_action if name == '' super end end
For around hooks, neglecting to call super
halts hook
processing in the same way as calling cancel_action
in a
before hook. It's probably a bad idea to use
cancel_action
hook processing in after hooks, or after
yielding in around hooks, since by then the main processing has already
taken place.
By default, Sequel runs hooks other than
validation hooks inside a transaction, so if you cancel the action by
calling cancel_action
in any hook, Sequel will rollback the transaction. However,
note that the implicit use of transactions when saving and destroying model
objects is conditional (it depends on the model instance's
use_transactions
setting and the :transaction
option passed to save).
Sometimes you only take to take a certain action in a hook if the object meets a certain condition. For example, let's say you only want to make sure a timestamp is set when updating if the object is at a certain status level:
class Album < Sequel::Model def before_update self.timestamp ||= Time.now if status_id > 3 super end end
Note how this hook action is made conditional just be using the standard
ruby if
conditional. Sequel
makes it easy to handle conditional hook actions by using standard ruby
conditionals inside the instance methods.
If you want all your model classes to use the same hook, you can just define that hook in Sequel::Model:
class Sequel::Model def before_create self.created_at ||= Time.now super end end
Just remember to call super
whenever you override the method
in a subclass. Note that super
is also used when overriding
the hook in Sequel::Model
itself. This is important as if you
add any plugins to Sequel::Model itself,
if you override a hook in Sequel::Model
and do not call
super
, the plugin may not work correctly.
If you don't want all classes to use the same hook, but want to reuse hooks in multiple classes, you should use a plugin or a simple module:
module SetCreatedAt module InstanceMethods def before_create self.created_at ||= Time.now super end end end Album.plugin(SetCreatedAt) Artist.plugin(SetCreatedAt)
module SetCreatedAt def before_create self.created_at ||= Time.now super end end Album.send(:include, SetCreatedAt) Artist.send(:include, SetCreatedAt)
super
Ordering¶ ↑While it's not enforced anywhere, it's a good idea to make
super
the last expression when you override a before hook, and
the first expression when you override an after hook:
class Album < Sequel::Model def before_save self.updated_at ||= Time.now super end def after_save super AuditLog.create(:log=>"Album #{name} created") end end
This allows the following general principles to be true:
before hooks are run in reverse order of inclusion
after hooks are run in order of inclusion
So if you define the same before hook in both a model and a plugin that the model uses, the hooks will be called in this order:
model before hook
plugin before hook
plugin after hook
model after hook
Again, Sequel does not enforce that, and you
are free to call super
in an order other than the recommended
one (just make sure that you call it).
Around hooks should only be used if you cannot accomplish the same results
with before and after hooks. For example, if you want to catch database
errors caused by the INSERT
or UPDATE
query when
saving a model object and raise them as validation errors, you cannot use a
before or after hook. You have use an around_save
hook:
class Album < Sequel::Model def around_save super rescue Sequel::DatabaseError => e # parse database error, set error on self, and reraise a Sequel::ValidationFailed end end
Likewise, let's say that upon retrieval, you associate an object with a
file descriptor, and you want to ensure that the file descriptor is closed
after the object is saved to the database. Let's assume you are always
saving the object and you are not using validations. You could not use an
after_save
hook safely, since if the database raises an error,
the after_save
method will not be called. In this case, an
around_save
hook is also the correct choice:
class Album < Sequel::Model def around_save super ensure @file_descriptor.close end end
instance_hooks
¶ ↑Sequel also ships with an
instance_hooks
plugin that allows you to define before and
after hooks on a per instance basis. It's very useful as it allows you
to delay action on an instance until before or after saving. This can be
important if you want to modify a group of related objects together (which
is how the nested_attributes
plugin uses
instance_hooks
).
hook_class_methods
¶ ↑While it's recommended to write your hooks as instance methods, Sequel ships with a
hook_class_methods
plugin that allows you to define hooks via
class methods. It exists mostly for legacy compatibility, but is still
supported. However, it does not implement around hooks.
after_initialize
¶ ↑The after_initialize plugin adds an after_initialize hook, that is called for all model instances on creation (both new instances and instances retrieved from the database). It exists mostly for legacy compatibility, but it is still supported.