Suppose I created a table table in a Rails app. Some time later, I add a column running:
rails generate migration AddUser_idColumnToTable user_id:string. Then I realize I need to add user_id as an index. I know about the add_index method, but where should this method be called? Am I supposed to run a migration (if yes, which one ?), then adding by hand this method?
6 Answers
You can run another migration, just for the index:
class AddIndexToTable < ActiveRecord::Migration def change add_index :table, :user_id end end 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 Add in the generated migration after creating the column the following (example)
add_index :photographers, :email, :unique => true For references you can call
rails generate migration AddUserIdColumnToTable user:references If in the future you need to add a general index you can launch this
rails g migration AddOrdinationNumberToTable ordination_number:integer:index Generated code:
class AddOrdinationNumberToTable < ActiveRecord::Migration def change add_column :tables, :ordination_number, :integer add_index :tables, :ordination_number, unique: true end end For those who are using postgresql db and facing error
StandardError: An error has occurred, this and all later migrations canceled: === Dangerous operation detected #strong_migrations === Adding an index non-concurrently blocks writes please refer this article
example:
class AddAncestryToWasteCodes < ActiveRecord::Migration[6.0] disable_ddl_transaction! def change add_column :waste_codes, :ancestry, :string add_index :waste_codes, :ancestry, algorithm: :concurrently end end You can use this, just think Job is the name of the model to which you are adding index cader_id:
class AddCaderIdToJob < ActiveRecord::Migration[5.2] def change change_table :jobs do |t| t.integer :cader_id t.index :cader_id end end end 
