Ruby-on-rails 检查 Rails 中是否存在表

Check if a table exists in Rails

提问by thenengah

I have a rake task that won't work unless a table exists. I'm working with more than 20 engineers on a website so I want to make sure they have migrated the table before they can do a rake task which will populate that respective table.

我有一个 rake 任务，除非存在表，否则它将无法工作。我在一个网站上与 20 多名工程师合作，所以我想确保他们在执行将填充相应表的 rake 任务之前已经迁移了表。

Does AR have a method such as Table.exists? How can I make sure they have migrated the table successfully?

AR 有没有类似的方法Table.exists？我如何确保他们已成功迁移表？

回答by captainpete

In Rails 5 the API became explicit regarding tables/views, collectively data sources.

在 Rails 5 中，API 变得明确了表/视图，统称为数据源。

# Tables and views
ActiveRecord::Base.connection.data_sources
ActiveRecord::Base.connection.data_source_exists? 'kittens'

# Tables
ActiveRecord::Base.connection.tables
ActiveRecord::Base.connection.table_exists? 'kittens'

# Views
ActiveRecord::Base.connection.views
ActiveRecord::Base.connection.view_exists? 'kittens'

In Rails 2, 3 & 4 the API is about tables.

在 Rails 2、3 和 4 中，API 是关于表的。

# Listing of all tables and views
ActiveRecord::Base.connection.tables

# Checks for existence of kittens table/view (Kitten model)
ActiveRecord::Base.connection.table_exists? 'kittens'

Getting the status of migrations:

获取迁移状态：

# Tells you all migrations run
ActiveRecord::Migrator.get_all_versions

# Tells you the current schema version
ActiveRecord::Migrator.current_version

If you need more APIs for migrations or metadata see:

如果您需要更多用于迁移或元数据的 API，请参阅：

• ActiveRecord::SchemaMigration
  this is the ActiveRecord::Baseclass for the schema_migrationstable
• ActiveRecord::Migrator
  where all the action happens when migrations are run

• ActiveRecord::SchemaMigration
  这是表的ActiveRecord::Base类schema_migrations
• ActiveRecord::Migrator
  运行迁移时发生的所有操作

回答by alexey_the_cat

even if table is not exists:

即使表不存在：

model Kitten, expected table kittensrails 3:

型号Kitten，预期桌kittens轨 3：

Kitten.table_exists? #=> false

Kitten.table_exists？#=> 假

回答by kangkyu

I found this out while I was trying to remove a table via a migration:

我在尝试通过迁移删除表时发现了这一点：

drop_table :kittens if (table_exists? :kittens)
ActiveRecord::Migration.drop_table :kittens if (ActiveRecord::Base.connection.table_exists? :kittens)

works for Rails 3.2

适用于 Rails 3.2

This simpler form will become available in Rails 5:

这种更简单的形式将在 Rails 5 中可用：

drop_table :kittens, if_exists: true

Reference: https://github.com/rails/rails/pull/16366

参考：https: //github.com/rails/rails/pull/16366

And here's the Rails 5 ActiveRecord's CHANGELOG:

这是 Rails 5 ActiveRecord 的CHANGELOG：

Introduce the :if_exists option for drop_table.

Example:

drop_table(:posts, if_exists: true)

That would execute:

DROP TABLE IF EXISTS posts

If the table doesn't exist, if_exists: false (the default) raises an exception whereas if_exists: true does nothing.

为 drop_table 引入 :if_exists 选项。

例子：

drop_table(:posts, if_exists: true)

那将执行：

DROP TABLE IF EXISTS posts

如果表不存在， if_exists: false （默认值）会引发异常，而 if_exists: true 什么也不做。

回答by Vitor Oliveira

Rails 5.1

导轨 5.1

if ActiveRecord::Base.connection.data_source_exists? 'table_name'
   drop_table :table_name
end

or

或者

drop_table :table_name, if_exists: true

回答by Juan Furattini

The proper way to do this is Model.table_exists?

这样做的正确方法是 Model.table_exists？

class Dog < ApplicationRecord
  # something
end

do_something if Dog.table_exists?