Check if a table exists in Rails

Question

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?

The joke goes.. how many engineers does it take to migrate a table :) — Zabba, Jul 05 '11 at 23:47
On production 1. On Staging dozens and multiple times each . — thenengah, Jul 05 '11 at 23:48
Wouldn't it be easier just run the migrations on the start of your rake task? So you don't have to worry about missing tables. — raskhadafi, Jul 24 '12 at 20:48
@raskhadafi : Note that missing tables are going to give you a problem if your config/initializers use them. (i.e. even `rake db:migrate` will fail.) — ocodo, Sep 23 '16 at 05:45

captainpete · Accepted Answer · 2017-03-29T02:24:59.603

343

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

edited Mar 29 '17 at 02:24

answered Jul 05 '11 at 23:49

captainpete

6,162
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4

`ActiveRecord::Base.connection.table_exist 'users'` would check for a users table. – thenengah Jul 05 '11 at 23:56
4

`ActiveRecord::Base.connection.table_exists? 'kittens` would check for a Kitten table. That is unless I destroyed all kittens! `drop_table :kittens` – thenengah Jul 05 '11 at 23:59
1

Thanks guys! I just used `.index_exists?('kittens', 'paws')` – Trip Oct 26 '12 at 15:34
14

This works for ActiveRecord 3.2.11 `drop_table(:hosts_users) if table_exists? :hosts_users` – Greg Feb 13 '13 at 13:46
`current_version` was just what I needed to disable a model method long enough to allow its migrations to run when I finally deployed it to production. Thanks! – Becca Royal-Gordon Feb 26 '15 at 20:57
1

`ActiveRecord::Base.connection.data_source_exists? 'table_name'` is the correct one now – Dorian Dec 30 '16 at 01:35

score 62 · Answer 2 · answered Jan 31 '12 at 14:48

62

even if table is not exists:

model Kitten, expected table kittens rails 3:

Kitten.table_exists? #=> false

answered Jan 31 '12 at 14:48

alexey_the_cat

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19
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+ 1 More elegant solution. Also works if the model overrides the table name. – Daniel Rikowski Sep 21 '13 at 14:03
2

Confirming this works for Rails 2.3.18-lts (tested with one table present, one missing before running script/console) – iheggie Oct 18 '14 at 04:36

score 39 · Answer 3 · edited Mar 09 '16 at 10:39

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.

This will fail if the table is in fact a view, as the table will appear to exist, but DROP TABLE can not drop it. — mcr, Mar 29 '16 at 14:34

score 14 · Answer 4 · answered Dec 16 '16 at 18:14

14

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

answered Dec 16 '16 at 18:14

Vitor Oliveira

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table_exists still works in rails-5, but its behavior will be changing to only check tables. As of 5.0.1 it checks views and tables. data_source_exists keeps that behavior and table_exists will be changing to only check tables. – John Naegle Jan 14 '17 at 20:19
He is not asking to check the table on a migration, he needs to be sure that the table exists on a rake task – Juan Furattini Jan 27 '19 at 06:25

score 3 · Answer 5 · answered Oct 10 '18 at 20:39

3

The proper way to do this is Model.table_exists?

class Dog < ApplicationRecord
  # something
end

do_something if Dog.table_exists?

answered Oct 10 '18 at 20:39

Juan Furattini

776
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9

Check if a table exists in Rails

5 Answers5

Introduce the :if_exists option for drop_table.

Linked