example.rb

class MailboxQuery
  include Virtus.model
  include ActiveModel::Validations

  attribute :page, Integer, default: 1
  attribute :per_page, Integer, default: 25

  validates :page, presence: true, numericality: {only_integer: true, greater_than: 0}
  validates :per_page, presence: true, numericality: {only_integer: true, greater_than: 0, less_than_or_equal_to: 100}

  def offset
    per_page * (page - 1)
  end
end

# In my controller / endpoint

query = MailboxQuery.new(params)
unless query.valid?
  # render bad request with errors
else
  # do the proper thing
  mails = mailbox_store.load(limit: query.per_page, offset: query.offset)

  # render json or whatever
end