Adding a column to an existing table in a Rails migration

If you have already run your original migration (before editing it), then you need to generate a new migration (rails generate migration add_email_to_users email:string will do the trick). It will create a migration file containing line: add_column :users, email, string Then do a rake db:migrate and it'll run the new migration, creating the new column.

If you have not yet run the original migration you can just edit it, like you're trying to do. Your migration code is almost perfect: you just need to remove the add_column line completely (that code is trying to add a column to a table, before the table has been created, and your table creation code has already been updated to include a t.string :email anyway).

Sometimes rails generate migration add_email_to_users email:string produces a migration like this

class AddEmailToUsers < ActiveRecord::Migration[5.0]
  def change
  end
end

In that case you have to manually an add_column to change:

class AddEmailToUsers < ActiveRecord::Migration[5.0]
  def change
    add_column :users, :email, :string
  end
end

And then run rake db:migrate

You can also do

rake db:rollback

if you have not added any data to the tables.Then edit the migration file by adding the email column to it and then call

rake db:migrate

This will work if you have rails 3.1 onwards installed in your system.

Much simpler way of doing it is change let the change in migration file be as it is. use

$rake db:migrate:redo

This will roll back the last migration and migrate it again.

Use this command on the terminal:

rails generate migration add_fieldname_to_tablename fieldname:string

and

rake db:migrate

to run this migration