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Model Hooks

This guide is based on guides.rubyonrails.org/activerecord_validations_callbacks.html

Overview

Model hooks, also known as model callbacks, 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.

Basic Usage

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 Sequel's built in plugins 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.

Available Hooks

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

      • INSERT 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.

Also note that the validation hooks are not called if the :validate => false option is passed to save. However, the validation hooks are called if you call Model#valid? manually:

  • 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 does support one additional hook, after_intialize, which is called after the model object has been initalized. It can be used to set default attribute values for new objects, since by default new Sequel::Model objects have no attributes, and the attributes are not filled in until the model object is saved. You should be careful when you are using after_initialize, since it is called for every created record. So if you run a query that returns 1000 model objects, it will be called 1000 times. If you only want to change the behavior for new records, you can override the initialize_set private method, which is called with the hash passed to initialize.

Running Hooks

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.filter(:name=>'RF').update(:copies_sold=>: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.filter. 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

Skipping 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=>: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

Halting Hook Processing

Sequel uses a convention that if any before_* hook method returns false (but not nil), that the action will be canceled and a Sequel::HookFailed raised (or nil to be returned by save if raise_on_save_failure is false). You can use this to implement validation-like behavior, that will run even if validations are skipped. For example:

class Album < Sequel::Model
  def before_save
    return false if name == ''
    super
  end
end

While returning false is not really recommended, you should be aware of this behavior so that you do not inadvertently return false. For around hooks, neglecting to call super halts hook processing in the same way as returning false in a before hook. You can't halt hook processing in after 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 abort the hook by returning false in a before hook or by raising an exception 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).

Conditional Hooks

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.

Using Hooks in Multiple Classes

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:

Plugin

module SetCreatedAt
  module InstanceMethods
    def before_create
      self.created_at ||= Time.now
      super
    end
  end
end
Album.plugin(SetCreatedAt)
Artist.plugin(SetCreatedAt)

Simple Module

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

  • returning false in any before hook will pass the false value down the hook method chain, halting the hook processing.

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

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

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.

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).

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