migration ":dependent" option

in migration file, what is ":dependent" option used for, what could be the possible values for this option? And what does each value mean? Can anybody explain to me?

e.g. here it uses :dependent => :delete

def self.up
    create_table :car, :id => false do |t|
      t.integer :brand_id, :null =>false
      t.timestamp :buy_at, :null开发者_如何学Go => false

    end

    add_index :car, :brand_id
    add_foreign_key :car, :brands, :dependent => :delete
  end

 def self.down
     drop_table :balance_updated
 end

In "normal" Rails, the :dependent option is used in the model like so:

has_many :models, :dependent => :destroy

However, I think you are using the foreign_key_migrations plugin? If so, take a look at this source code line to know what the :dependent option is doing.

A FOREIGN KEY is a key used to link two tables together.

A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table. Reference

So the car's column brand_id is depending on the (primary key) of the brand's table typically going to be called brand_id (default naming in sqlite3, if your using the default rails database).

So this ensures that if the brand is deleted i.e Mercedes from the Brand table then ALL cars that are branded as Mercedes in the Car table are also deleted.