Validate presence of one field or another (XOR)

Your code will work if you add conditionals to the numericality validations like so:

class Transaction < ActiveRecord::Base
    validates_presence_of :date
    validates_presence_of :name

    validates_numericality_of :charge, allow_nil: true
    validates_numericality_of :payment, allow_nil: true


    validate :charge_xor_payment

  private

    def charge_xor_payment
      unless charge.blank? ^ payment.blank?
        errors.add(:base, "Specify a charge or a payment, not both")
      end
    end

end

I think this is more idiomatic in Rails 3+:

e.g: For validating that one of user_name or email is present:

validates :user_name, presence: true, unless: ->(user){user.email.present?}
validates :email, presence: true, unless: ->(user){user.user_name.present?}