If you need to create a user_id
then it would be a reasonable assumption that you are referencing a user table. In which case the migration shall be:
rails generate migration AddUserRefToProducts user:references
This command will generate the following migration:
class AddUserRefToProducts < ActiveRecord::Migration
def change
add_reference :user, :product, index: true
end
end
After running rake db:migrate
both a user_id
column and an index will be added to the products
table.
In case you just need to add an index to an existing column, e.g. name
of a user
table, the following technique may be helpful:
rails generate migration AddIndexToUsers name:string:index
will generate the following migration:
class AddIndexToUsers < ActiveRecord::Migration
def change
add_column :users, :name, :string
add_index :users, :name
end
end
Delete add_column
line and run the migration.
In the case described you could have issued rails generate migration AddIndexIdToTable index_id:integer:index
command and then delete add_column
line from the generated migration. But I'd rather recommended to undo the initial migration and add reference instead:
rails generate migration RemoveUserIdFromProducts user_id:integer
rails generate migration AddUserRefToProducts user:references