ColumnPresenceChecker

Imagine your model has a validates :email, presence: true validation on some field or a required belongs_to :user association but doesn't have a not-null constraint in the database. In that case, your model's definition assumes (in most cases) you won't have null values in the database but it's possible to skip validations or directly write improper data in the table.

Keep in mind that belongs_to is required by default starting from Rails 5 given config.load_defaults is in place and unless config.active_record.belongs_to_required_by_default is explicitly set to false.

To avoid the inconsistency and be always sure your value won't be null you should add not-null constraint.