Check if a table exists in Rails

问题

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.

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

回答1:

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

回答2:

even if table is not exists:

model Kitten, expected table kittens rails 3:

Kitten.table_exists? #=> false

回答3:

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.

回答4:

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

回答5:

The proper way to do this is Model.table_exists?

class Dog < ApplicationRecord
  # something
end

do_something if Dog.table_exists?

来源：https://stackoverflow.com/questions/6590107/check-if-a-table-exists-in-rails