= Model Hooks

This guide is based on http://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

<tt>Sequel::Model</tt> 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 <tt>:validate => false</tt> option is passed to save.  However, the validation hooks are called if you call <tt>Model#valid?</tt> 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 <tt>Model#destroy</tt>, they are not called if you use <tt>Model#delete</tt>.

<tt>Sequel::Model</tt> does support a few additional hooks. One is +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 <tt>Sequel::Model</tt> 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+.

== 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 <tt>Sequel::Model</tt> 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.

== 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 <tt>Album.filter</tt>.  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 <tt>Model.insert</tt> instead of <tt>Model.create</tt>:

  Album.insert(:name=>'RF') # does not run hooks

== Halting Hook Processing

Sequel uses a convention that if any <tt>before_*</tt> hook method returns false (but not nil), that the action will be canceled and a <tt>Sequel::HookFailed</tt> 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 <tt>:transaction</tt> 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 <tt>Sequel::Model</tt> itself.  This is important as if you add any plugins to Sequel::Model itself, if you override a hook in <tt>Sequel::Model</tt> 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+).