A handy Rails model for storing emails with a little logic to let a User have multiple email addresses but a single primary address.

email.rb

class Email < ActiveRecord::Base

  # Nope, it's not RFC compliant. F*** that regex.
  # http://www.ex-parrot.com/~pdw/Mail-RFC822-Address.html
  EmailRegex = /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i.freeze

  before_validation :strip_spaces

  # Public: The email address.
  # column :address
  # Returns a String.
  validates :address,
    :presence   => true,
    :length     => { :in => 3..254 },
    :format     => { :with => EmailRegex, :message => "does not look like an email address" },
    :uniqueness => { :case_sensitive => false, :message => "is taken" }

  # Public: User email address belongs to.
  # column :user_id
  # Returns a User.
  validates :user_id, :presence => true
  belongs_to :user

  # Public: Email address validation as a service.
  #
  # Returns a boolean
  def self.valid_email_address?(email)
    EmailRegex.match(email)
  end

  # Public: Is this a primary email address?
  # column :primary
  # Returns a TrueClass or FalseClass.

  # TODO: Validate that only one Email#address per User is the primary.

  # Public: Set this email as the primary email. Sets primary to false for any
  # other emails for this User.
  def set_primary
    self.class.transaction do
      self.class.where(:user_id => user_id, :primary => true).update_all(:primary => false)
      self.primary = true
      save
    end
  end

  # Internal: Strip all space from the email.
  def strip_spaces
    self.address = address.strip
  end
end

Dont know if this will help, but this is how I validated 1 primary email per user:

def _only_1_primary_email_address
return unless user_id && primary
primary_email = UserEmail.where(user_id: user_id, primary: true)
return unless emails.any?
if primary_email.first.primary && primary_email.first.email_address != email_address
errors.add :primary, 'cannot be a second primary'
end
end