In a Rails 5 app with Postgres, I have this migration file:
class MigrateStuff < ActiveRecord::Migration[5.0]
def up
enable_extension 'uuid-ossp'
# migrate stuff
end
def down
# rollback stuff
end
end
My question is:
Is it necessary/cleaner to revert the enable_extension call?
And if so, how?
I've tried drop_extension but to no avail.
In your down block you have to use
disable_extension
and revert the corresponding migration which will call the drop extension statement.
More info can be found at the Rails source
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?
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?
I have inherited a Ruby on Rails project where the programmer didn't use rake to create the db schema, so it seems very out of synch, is there a way to rectify this?
First create a schema.rb file
rake db:schema:dump
Then make a migration ot of it.
class CreateMigration < ActiveRecord::Migration
def self.up
# insert schema.rb here
end
def self.down
end
end
You might also need to create the schema_migrations table, and manually add the timestamp for this migration to it.
I added a table that I thought I was going to need, but now no longer plan on using it. How should I remove that table?
I've already run migrations, so the table is in my database. I figure rails generate migration should be able to handle this, but I haven't figured out how yet.
I've tried:
rails generate migration drop_tablename
but that just generated an empty migration.
What is the "official" way to drop a table in Rails?
You won't always be able to simply generate the migration to already have the code you want. You can create an empty migration and then populate it with the code you need.
You can find information about how to accomplish different tasks in a migration here:
http://api.rubyonrails.org/classes/ActiveRecord/Migration.html
More specifically, you can see how to drop a table using the following approach:
drop_table :table_name
Write your migration manually. E.g. run rails g migration DropUsers.
As for the code of the migration I'm just gonna quote Maxwell Holder's post Rails Migration Checklist
BAD - running rake db:migrate and then rake db:rollback will fail
class DropUsers < ActiveRecord::Migration
def change
drop_table :users
end
end
GOOD - reveals intent that migration should not be reversible
class DropUsers < ActiveRecord::Migration
def up
drop_table :users
end
def down
fail ActiveRecord::IrreversibleMigration
end
end
BETTER - is actually reversible
class DropUsers < ActiveRecord::Migration
def change
drop_table :users do |t|
t.string :email, null: false
t.timestamps null: false
end
end
end
First generate an empty migration with any name you'd like. It's important to do it this way since it creates the appropriate date.
rails generate migration DropProductsTable
This will generate a .rb file in /db/migrate/ like 20111015185025_drop_products_table.rb
Now edit that file to look like this:
class DropProductsTable < ActiveRecord::Migration
def up
drop_table :products
end
def down
raise ActiveRecord::IrreversibleMigration
end
end
The only thing I added was drop_table :products and raise ActiveRecord::IrreversibleMigration.
Then run rake db:migrate and it'll drop the table for you.
Warning: Do this at your own risk, as #z-atef and #nzifnab correctly point out, Rails will not be aware of these changes, your migration sequence fill fail and your schema will be different from your coworkers'. This is meant as a resource for locally tinkering with development only.
While the answers provided here work properly, I wanted something a bit more 'straightforward', I found it here: link
First enter rails console:
$rails console
Then just type:
ActiveRecord::Migration.drop_table(:table_name)
And done, worked for me!
You need to to create a new migration file using following command
rails generate migration drop_table_xyz
and write drop_table code in newly generated migration file (db/migration/xxxxxxx_drop_table_xyz) like
drop_table :tablename
Or if you wanted to drop table without migration, simply open rails console by
$ rails c
and execute following command
ActiveRecord::Base.connection.execute("drop table table_name")
or you can use more simplified command
ActiveRecord::Migration.drop_table(:table_name)
rails g migration drop_users
edit the migration
class DropUsers < ActiveRecord::Migration
def change
drop_table :users do |t|
t.string :name
t.timestamps
end
end
end
rake db:migrate
The simple and official way would be this:
rails g migration drop_tablename
Now go to your db/migrate and look for your file which contains the drop_tablename as the filename and edit it to this.
def change
drop_table :table_name
end
Then you need to run
rake db:migrate
on your console.
I wasn't able to make it work with migration script so I went ahead with this solution. Enter rails console using the terminal:
rails c
Type
ActiveRecord::Migration.drop_table(:tablename)
It works well for me. This will remove the previous table. Don't forget to run
rails db:migrate
I think, to be completely "official", you would need to create a new migration, and put drop_table in self.up. The self.down method should then contain all the code to recreate the table in full. Presumably that code could just be taken from schema.rb at the time you create the migration.
It seems a little odd, to put in code to create a table you know you aren't going to need anymore, but that would keep all the migration code complete and "official", right?
I just did this for a table I needed to drop, but honestly didn't test the "down" and not sure why I would.
you can simply drop a table from rails console.
first open the console
$ rails c
then paste this command in console
ActiveRecord::Migration.drop_table(:table_name)
replace table_name with the table you want to delete.
you can also drop table directly from the terminal. just enter in the root directory of your application and run this command
$ rails runner "Util::Table.clobber 'table_name'"
You can roll back a migration the way it is in the guide:
http://guides.rubyonrails.org/active_record_migrations.html#reverting-previous-migrations
Generate a migration:
rails generate migration revert_create_tablename
Write the migration:
require_relative '20121212123456_create_tablename'
class RevertCreateTablename < ActiveRecord::Migration[5.0]
def change
revert CreateTablename
end
end
This way you can also rollback and can use to revert any migration
Alternative to raising exception or attempting to recreate a now empty table - while still enabling migration rollback, redo etc -
def change
drop_table(:users, force: true) if ActiveRecord::Base.connection.tables.include?('users')
end
You can't simply run drop_table :table_name, instead you can create an empty migration by running:
rails g migration DropInstalls
You can then add this into that empty migration:
class DropInstalls < ActiveRecord::Migration
def change
drop_table :installs
end
end
Then run rails db:migrate in the command line which should remove the Installs table
The solution was found here
Open you rails console
ActiveRecord::Base.connection.execute("drop table table_name")
ActiveRecord::Base.connection.drop_table :table_name
if anybody is looking for how to do it in SQL.
type rails dbconsole from terminal
enter password
In console do
USE db_name;
DROP TABLE table_name;
exit
Please dont forget to remove the migration file and table structure from schema
I needed to delete our migration scripts along with the tables themselves ...
class Util::Table < ActiveRecord::Migration
def self.clobber(table_name)
# drop the table
if ActiveRecord::Base.connection.table_exists? table_name
puts "\n== " + table_name.upcase.cyan + " ! "
<< Time.now.strftime("%H:%M:%S").yellow
drop_table table_name
end
# locate any existing migrations for a table and delete them
base_folder = File.join(Rails.root.to_s, 'db', 'migrate')
Dir[File.join(base_folder, '**', '*.rb')].each do |file|
if file =~ /create_#{table_name}.rb/
puts "== deleting migration: " + file.cyan + " ! "
<< Time.now.strftime("%H:%M:%S").yellow
FileUtils.rm_rf(file)
break
end
end
end
def self.clobber_all
# delete every table in the db, along with every corresponding migration
ActiveRecord::Base.connection.tables.each {|t| clobber t}
end
end
from terminal window run:
$ rails runner "Util::Table.clobber 'your_table_name'"
or
$ rails runner "Util::Table.clobber_all"
Helpful documentation
In migration you can drop table by:
drop_table(table_name, **options)
options:
:force
Set to :cascade to drop dependent objects as well. Defaults to false
:if_exists
Set to true to only drop the table if it exists. Defaults to false
Example:
Create migration for drop table, for example we are want to drop User table
rails g migration DropUsers
Running via Spring preloader in process 13189
invoke active_record
create db/migrate/20211110174028_drop_users.rb
Edit migration file, in our case it is db/migrate/20211110174028_drop_users.rb
class DropUsers < ActiveRecord::Migration[6.1]
def change
drop_table :users, if_exist: true
end
end
Run migration for dropping User table
rails db:migrate
== 20211110174028 DropUsers: migrating ===============================
-- drop_table(:users, {:if_exist=>true})
-> 0.4607s
the best way you can do is
rails g migration Drop_table_Users
then do the following
rake db:migrate
Run
rake db:migrate:down VERSION=<version>
Where <version> is the version number of your migration file you want to revert.
Example:-
rake db:migrate:down VERSION=3846656238
Drop Table/Migration
run:-
$ rails generate migration DropTablename
exp:- $ rails generate migration DropProducts
if you want to drop a specific table you can do
$ rails db:migrate:up VERSION=[Here you can insert timestamp of table]
otherwise if you want to drop all your database you can do
$rails db:drop
Run this command:-
rails g migration drop_table_name
then:
rake db:migrate
or if you are using MySql database then:
login with database
show databases;
show tables;
drop table_name;
If you want to delete the table from the schema perform below operation --
rails db:rollback
In Rails 3,
I created a table with a migration, then added a column with a migration which creates a has_many, belongs_to relationship....
I then ran rake db:migrate
I'd like to now add an Index because I forgot to add it before I can migrate. can I add that to one of the existing migration files (the create table one) or do I need to create a new migration for adding an index to an existing table?
Thanks
I usually create a new migration if I forget something like that (especially if I have already migrated in production). But if you are still in development, then you can alter your last migration file and use the redo rake command:
rake db:migrate:redo
There is also:
rake db:rollback # Rolls the schema back to the previous version (specify steps w/ STEP=n).
Run this command to view all the different rake tasks:
rake -T db
Here is the section in the Rails Guide that talks about it:
http://guides.rubyonrails.org/migrations.html#rolling-back
If you would like to add your index without losing the data, you must create a new migration to add an index. Assuming your model is called Widget and the foreign model is called Zidget;
rails generate migration AddIndexToWidgets
and inside your new migration file at db/migrate/xxxxxxxxxx_add_index_to_widgets
class AddIndexToWidgets < ActiveRecord::Migration
def self.up
change_table :widgets do |t|
t.index :zidget_id # add ':unique => true' option if necessary
end
end
def self.down
change_table :widgets do |t|
t.remove_index :zidget_id
end
end
end
and then rake db:migrate as usual and voilà, you have your indexed column.
Update: If adding an index is all you're doing, there is a more concise way to write the same thing. There is no difference regarding the results. It's just that the former syntax is meant to DRY your code if you have more than one change for your table.
class AddIndexToWidgets < ActiveRecord::Migration
def self.up
add_index :widgets, :zidget_id # add ':unique => true' option if necessary
end
def self.down
remove_index :widgets, :column => :zidget_id
end
end