Active Record Callbacks

所有的callback：

3.1 Creating an Object

3.2 Updating an Object

3.3 Destroying an Object

callback用于注册：

class User &lt; ApplicationRecord
&nbsp; validates :login, :email, presence: true
&nbsp; before_validation :ensure_login_has_a_value
&nbsp; private
&nbsp;&nbsp;&nbsp; def ensure_login_has_a_value
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if login.nil?
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; self.login = email unless email.blank?
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; end
&nbsp;&nbsp;&nbsp; end
end

如果块内的代码太短以至于它适合一行，请考虑使用这种样式

class User &lt; ApplicationRecord
&nbsp; validates :login, :email, presence: true
&nbsp; before_create do
&nbsp;&nbsp;&nbsp; self.name = login.capitalize if name.blank?
&nbsp; end
end

注意：callback必须放在private方法中：

class User &lt; ApplicationRecord<br />
&nbsp; validates :login, :email, presence: true

&nbsp; before_validation :ensure_login_has_a_value

&nbsp; private
&nbsp;&nbsp;&nbsp; def ensure_login_has_a_value
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if login.nil?
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; self.login = email unless email.blank?
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; end
&nbsp;&nbsp;&nbsp; end
end

The following methods trigger callbacks:

```
create
```
```
create!
```
```
destroy
```
```
destroy!
```
```
destroy_all
```
```
destroy_by
```
```
save
```
```
save!
```
```
save(validate: false)
```
```
toggle!
```
```
touch
```
```
update_attribute
```
```
update
```
```
update!
```
```
valid?
```

Additionally, the

after_find

callback is triggered by the following finder methods:

```
all
```
```
first
```
```
find
```
```
find_by
```
```
find_by_*
```
```
find_by_*!
```
```
find_by_sql
```
```
last
```

跳过callback：

```
decrement!
```
```
decrement_counter
```
```
delete
```
```
delete_all
```
```
delete_by
```
```
increment!
```
```
increment_counter
```
```
insert
```
```
insert!
```
```
insert_all
```
```
insert_all!
```
```
touch_all
```
```
update_column
```
```
update_columns
```
```
update_all
```
```
update_counters
```
```
upsert
```
```
upsert_all
```

Relational Callbacks

删除后进行调用：

class User &lt; ApplicationRecord
&nbsp; has_many :articles, dependent: :destroy
end
class Article &lt; ApplicationRecord
&nbsp; after_destroy :log_destroy_action
&nbsp; def log_destroy_action
&nbsp;&nbsp;&nbsp; puts &#39;Article destroyed&#39;
&nbsp; end
end

验证：

user = User.first
user.articles.create!
user.destroy

8个条件回调(进官网看吧）