After installing devise MODEL User i got this.

class DeviseCreateUsers < ActiveRecord::Migration
  def self.up
    create_table(:users) do |t|
      t.database_authenticatable :null => false
      t.recoverable
      t.rememberable
      t.trackable

      # t.encryptable
      # t.confirmable
      # t.lockable :lock_strategy => :failed_attempts, :unlock_strategy 开发者_如何学Go=> :both
      # t.token_authenticatable


      t.timestamps
    end

    add_index :users, :email,                :unique => true
    add_index :users, :reset_password_token, :unique => true
    # add_index :users, :confirmation_token,   :unique => true
    # add_index :users, :unlock_token,         :unique => true
    # add_index :users, :authentication_token, :unique => true
  end

  def self.down
    drop_table :users
  end
end

Now if i do rake db:migrate the users table will be created.

How can i revert this migration, i.e. how can I delete the users table using rake again ?

---

Run the following command

rake db:migrate:down VERSION=<version>

where <version> is the version number of your migration file you want to revert.

eg. if you want to revert a migration with file name 3846656238_create_users.rb

rake db:migrate:down VERSION=3846656238

---

Just run this command:

rake db:rollback

---

I believe there are three options available for reverting migrations (they also overlap):

1. Roll down the most recent migration:

   rake db:migrate:down # Rails 2 only.

2. Roll down a number(n) of recent migrations:

   rake db:rollback STEP=n

3. Roll down to a previous, specific version:

   $ rake db:migrate:down VERSION=nnn # Rails 3 (provide version number also).

Version Number means the SHA(Secure Hash Algorithm) for the commit which is a long hexadecimal number which looks something like 886af3194768917c78e... You can see it by doing git log

You can see these commands (and others) with their descriptions by using rake -T db: which for rails 3.2 includes:

rake db:migrate         # Migrate the database (options: VERSION=x, VERBOSE=false)
rake db:migrate:status  # Display status of migrations
rake db:rollback        # Rolls the schema back to the previous version (specify steps w/ STEP=n)

---

You can do rollback and specify how many last migrations will be rollbacked, e.g.

rake db:rollback STEP=3

for 3 last migrations.

---

As an new programmer (or to other new programmers)

rake db:rollback works about half the time. I start there.

If not, rake db:migrate:down VERSION=3846656238

plug in VERSION for the version number of your migration file you want to revert.

---

rake db:migrate:redo

It will undo and reapply the last migration.

---

For rails 5 we can use rails command instead of rake

rails db:migrate:down VERSION=<version>

example

rails db:migrate:down VERSION=20170330090327

---

Run this command in your terminal:

rake db:migrate:status

or

bundle exec rake db:migrate:status

It shows the status, migration ID's, migration name for all migration we ran previously. select your migration id (i.e your version number) and put that id in the following command after version= ,,, and press enter

bundle exec rake db:migrate:down VERSION=

---

How to Roll back a migration

Another way. I prefer this because you need to be explicit, rather than rolling back - just in case you might make a mistake.

(1) First Identify The Migration ID

rake db:migrate:status

• Copy the ID number.

(2) Then Roll back the migration

rake db:migrate:down VERSION=20190802023239

• Paste the relevant ID number above. Of course, in your case, the migration ID will be different!

.......and now you're off to the races!