app/validators/object_count_validator.rb
class ObjectCountValidator < ActiveModel::EachValidator
def validate_each(record, attribute, value)
min, max = options[:min].call, options[:max].call
values = value.reject(&:marked_for_destruction?)
return if values.size.between?(min, max)
association = options[:association] || attribute
record.errors.add(association, :out_of_range, **{ min: min, max: max })
end
end