Rails: Adding an index after adding column - ruby-on-rails

Question

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?

Answer 1

You can run another migration, just for the index:
class AddIndexToTable < ActiveRecord::Migration
def change
add_index :table, :user_id
end
end

Answer 2

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

Answer 3

Add in the generated migration after creating the column the following (example)
add_index :photographers, :email, :unique => true

Answer 4

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

Answer 5

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

Answer 6

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