How do I display Ruby on Rails form validation error messages one at a time?

ActiveRecord stores validation errors in an array called errors. If you have a User model then you would access the validation errors in a given instance like so:

@user = User.create[params[:user]] # create will automatically call validators

if @user.errors.any? # If there are errors, do something

  # You can iterate through all messages by attribute type and validation message
  # This will be something like:
  # attribute="name"
  # message="cannot be left blank"
  @user.errors.each do |attribute, message|
    # do stuff for each error
  end

  # Or if you prefer, you can get the full message in single string, like so:
  # message="Name cannot be left blank"
  @users.errors.full_messages.each do |message|
    # do stuff for each error
  end

  # To get all errors associated with a single attribute, do the following:
  if @user.errors.include?(:name)
    name_errors = @user.errors[:name]

    if name_errors.kind_of?(Array)
      name_errors.each do |error|
        # do stuff for each error on the name attribute
      end
    else
      error = name_errors
      # do stuff for the one error on the name attribute.
    end
  end
end

Of course you can also do any of this in the views instead of the controller, should you want to just display the first error to the user or something.