model_hooks.rdoc

doc/model_hooks.rdoc
Last Update: 2015-01-29 16:13:26 -0800

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 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.

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

      • 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.

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.

Special Hook-Related Instance Variables

For after_save hooks, a @was_new instance variable is present that indicates whether the record was a new record that was just inserted, or an existing record that was updated. Sequel marks a record as existing as soon as it inserts the record, so in an after_save or after_create hook, the instance is no longer considered new. You have to check @was_new to see if the record was inserted. This exists so that you don't have to have separate after_create and after_update hooks that are mostly the same and only differ slightly depending on whether the record was a new record.

For after_update hooks, a @columns_updated instance variable is present that is a hash of the values used to update the row (keys are column symbols, values are column values). This should be used by any code that wants to check what columns and values were used during the update. You can't just check the current values of the instance, since Sequel offers ways to manually specify which columns to use during the save.

Transaction-related Hooks

There are four other model hooks that Sequel::Model supports, all related to transactions. These are after_commit, after_rollback, after_destroy_commit, and after_destroy_rollback. after_commit is called after the transaction in which you saved the object commits, only if it commits. after_rollback is called after the transaction in which you saved the object rolls back, if it rolls back. after_destroy_commit is called after the transaction in which you destroyed the object commits, if it commits. after_destroy_rollback is called after the transaction in which you destroyed the object rolls back, if it rolls back.

If you aren't using transactions when saving or destroying model objects, and there isn't a currently open transaction, after_commit and after_destroy_commit will be called after after_save and after_destroy, respectively, and after_rollback and after_destroy_rollback won't be called at all.

The purpose of these hooks is dealing with external systems that are interacting with the same database. For example, let's say you have a model that stores a picture, and you have a background job library that makes thumbnails of all of the pictures. So when a model object is created, you want to add a background job that will create the thumbnail for the picture. If you used after_save for this and transactions are being used, you are subject to a race condition where the background job library will check the database table for the record before the transaction that saved the record commits, and it won't be able to see the record's data. Using after_commit, you are guaranteed that the background job library will not get notified of the record until after the transaction commits and the data is viewable.

Note that when using the after_commit or after_rollback hooks, you don't know whether the saved object was newly created or updated. If you only want to run an action after commit of a newly created record, you need to use the Database's after_commit inside the model's after_create hook:

class Album < Sequel::Model
  def after_create
    super
    db.after_commit{update_external_cache}
  end
end

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.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

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=>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

Canceling Actions in 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 backwards compatibility, Sequel also treats the before_* hook methods returning false as an indication that the action should be canceled. This usage is discouraged in new code, but 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 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).

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 related plugins

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.