Since MONGOID-5842 (https://github.com/mongodb/mongoid/pull/6108), Mongoid::Validatable::NumericalityValidator reads every attribute through read_attribute_before_type_cast. An attribute that is not a field (such as a method that derives a value from a field) has no stored value, so the validator always sees nil:

• without allow_nil/allow_blank, every value fails with "is not a number"
• with allow_nil/allow_blank, the validation is skipped, so out-of-range values pass

This worked in 9.0.x, and ActiveModel's numericality validator reads such attributes through the attribute reader.

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'mongoid', ENV.fetch('MONGOID_VERSION', '9.1.1')
end

Mongoid.configure do |config|
  config.clients.default = \{ hosts: ['localhost:27017'], database: 'mongoid_test' }
end

class Stake
  include Mongoid::Document

  field :ratio, type: Float

  # Not a field: a virtual attribute derived from one
  def percentage
    ratio * 100 if ratio
  end

  validates :percentage, numericality: \{ less_than_or_equal_to: 100 }
end

pp Mongoid::VERSION
stake = Stake.new(ratio: 0.25)
pp stake.percentage # => 25.0
pp stake.valid?
pp stake.errors.full_messages

With 9.0.12:

"9.0.12"
25.0
true
[]

With 9.1.1:

"9.1.1"
25.0
false
["Percentage is not a number"]

The earlier approach in https://github.com/mongodb/mongoid/pull/6074 read the raw value only for numeric fields, so it did not have this problem. A fix that reads attributes that are not fields through read_attribute_for_validation is coming in a PR.