Rails 迁移: self. up 和 self. down 与 change

看起来新的 Railsversion 具有“ change”与 self. up 和 self. down 方法的区别。

那么，当必须回滚迁移时会发生什么情况呢? 它如何知道要执行什么操作呢。我有以下的方法，我需要实现的基础上的在线教程:

class AddImageToUsers < ActiveRecord::Migration
def self.up
add_column :users, :image_file_name, :string
add_column :users, :image_content_type, :string
add_column :users, :image_file_size, :integer
add_column :users, :image_updated_at, :datetime
end


def self.down
remove_column :users, :image_file_name, :string
remove_column :users, :image_content_type, :string
remove_column :users, :image_file_size, :integer
remove_column :users, :image_updated_at, :datetime
end
end

如何使用新的更改方法执行相同操作？

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小开 
class AddImageToUsers < ActiveRecord::Migration
def change
add_column :users, :image_file_name, :string
add_column :users, :image_content_type, :string
add_column :users, :image_file_size, :integer
add_column :users, :image_updated_at, :datetime
end
end
小开
最佳答案

For many operations rails can guess what is the inverse operation (without problems). For example, in your case what is the reverse operation of add_column to call when you rollback? Of course it's remove_column. What is the inverse of create_table? It's drop_table. So in these cases rails know how to rollback and define a down method is superfluous (you can see in the documentation the methods currently supported from the change method).

But pay attention because for some kind of operation you still need to define the down method, for example if you change the precision of a decimal column how to guess the original precision on rollback? It's not possible, so you need to define the down method.

As said, I suggest you to read the Rails Migrations Guide.

小开

Better to use Up, Down, Change:

On Rails 3 (Reversible): which should add new column on up and fill all records in table only on up, and only delete this column on down

def up
add_column :users, :location, :string
User.update_all(location: 'Minsk')
end


def down
remove_column :users, :location
end

But:

You had to avoid using change method which allows to save some time. For example, if you didn’t need to update column value immediately after it’s adding you would cut this code down to like this:

def change
add_column :users, :location, :string
end

On up it will add column to table and remove it on down. Much less code and it’s a profit.

On Rails 4: one more useful way to write what we need in one place:

def change
add_column :users, :location, :string
reversible do |direction|
direction.up { User.update_all(location: 'Minsk') }
end
end

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