检查Rails中是否存在表

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

# 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.

# 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:

# 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:

• ActiveRecord::SchemaMigration
  this is the ActiveRecord::Base class for the schema_migrations table
• ActiveRecord::Migrator
  where all the action happens when migrations are run

even if table is not exists:

model Kitten, expected table kittens rails 3:

Kitten.table_exists? #=> false

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

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

works for Rails 3.2

This simpler form will become available in Rails 5:

drop_table :kittens, if_exists: true

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

And here's the Rails 5 ActiveRecord's 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.

Rails 5.1

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

or

drop_table :table_name, if_exists: true

The proper way to do this is Model.table_exists?

class Dog < ApplicationRecord
# something
end


do_something if Dog.table_exists?