readonly!

Prevents records from being written to the database:

customer = Customer.new
customer.readonly!
customer.save # raises ActiveRecord::ReadOnlyRecord

customer = Customer.first
customer.readonly!
customer.update(name: 'New Name') # raises ActiveRecord::ReadOnlyRecord

Read-only records cannot be deleted from the database either:

customer = Customer.first
customer.readonly!
customer.destroy # raises ActiveRecord::ReadOnlyRecord

Please, note that the objects themselves are still mutable in memory:

customer = Customer.new
customer.readonly!
customer.name = 'New Name' # OK

but you won’t be able to persist the changes.