Comparison Validator Also Validates Presence

In ActiveModel, a comparison validation on an attribute will first perform a presence validation, and return a :blank error if the attribute is missing. So you don't have to explicitly add a presence validation if you're also doing a comparison validation (unless you really really want to (I probably still will)).

class SomeClass
  include ActiveModel::Model
  include ActiveModel::Attributes

  attribute :some_number

  validates(
    :some_number,
    comparison: {greater_than: 8}
  )
end

If you don't provide some_number you'll get a :blank error, and the comparison validation will not run:

pry(main)> foo = SomeClass.new
pry(main)> foo.valid?; foo.errors.full_messages
=> ["Some number can't be blank"]

But if you do provide some_number, then the comparison validation will run as expected:

pry(main)> bar = SomeClass.new(some_number: 3)
pry(main)> bar.valid?; bar.errors.full_messages
=> ["Some number must be greater than 8"]

Source