Adding validations to existing model in Rails

Question

I would like to add a :presence and :uniqueness validation to a model in Rails. I'm using MongoDB and the Mongoid gem.

I have a model that is already in use and have existing records in the DB. I'd like to add a new :field and then add validations for :presence and :uniqueness for the field.

I know that by default, any existing records will simply add the :field and the result will be null unless I specify a :default.

My question is, since I want this new :field to be unique, will this cause an error for the existing records in the DB that will be null? Will I lose these records or does Rails just apply the validations to the new records?

Answer 1

Rails/AciveRecord will not ignore or delete your old records...

Validations work as follows: they are only used/checked when you try to write stuff to the database, e.g. when you create a new record or when you update a record. So in your case you will get validation errors when you try to update an old record without adding the new required fields.

I suggest that you try to clean up your database when adding the new fields (meaning adding sensible defaults to old records for the new required fields).