Check if a table exists in Rails
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 T开发者_C百科able.exists
? How can I make sure they have migrated the table successfully?
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 theActiveRecord::Base
class for theschema_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?
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