Is it possible to put a minimum length on a string in the database column or can it only be done through validations on the model?

Question

Rails migrations allow you to specify a max length for a string column with the limit attribute. Is there a corresponding attribute for minimum length or can it only be done through validations on the model?

Answer 1

No, there is no corresponding min_length declaration. The best solution is validations at the model level. You can see the available options here:

https://github.com/rails/rails/blob/acbcec8ea869849f98213fea5e554bb3a82fea61/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb#L55

Answer 2

The max length is a database feature which controls how much memory the DB allocates for the column. This is very different compared to a validation as the DB just truncates anything beyond that length.

There is no corresponding minimum on any db that I know of.

Some databases (such as Postgres) allow you to place constraints on the columns to enforce the buisness logic on the database layer but thats a very different story. Constraints are mostly useful if you need to prevent invalid data from being inserted if validations are bypassed or for avoiding potential race conditions.

In most cases an application (Rails) level validation will be sufficient. And constraints must be complemented with application layer validations anyways.

See:

• MySQL CREATE TABLE Syntax
• Postgres Docs - CREATE TABLE

Answer 3

...can it only be done through validations on the model?

Probably, not exactly what you are looking for but if you use Postgresql, you can use CONSTRAINT:

ALTER TABLE table_name
  ADD CONSTRAINT check_minimum_length CHECK (LENGTH(column_name) >= 5);

Still, I would highly recommend using ActiveModel::Validations as validations should be done at application layer.

Answer 4

Yes, it's posible. Like Rails Guides show, you should use in the model:

validates :column_name, length: { minimum: 2 }

Visit the link for more validation types.