validates associated with model's error message

You can write your own custom validator, based on the code for the built-in validator.

Looking up the source code for validates_associated, we see that it uses the "AssociatedValidator". The source code for that is:

module ActiveRecord
  module Validations
    class AssociatedValidator < ActiveModel::EachValidator
      def validate_each(record, attribute, value)
        if Array.wrap(value).reject {|r| r.marked_for_destruction? || r.valid?}.any?
          record.errors.add(attribute, :invalid, options.merge(:value => value))
        end
      end
    end

    module ClassMethods
      
      def validates_associated(*attr_names)
        validates_with AssociatedValidator, _merge_attributes(attr_names)
      end
    end
  end
end

So you can use this as an example to create a custom validator that bubbles error messages like this (for instance, add this code to an initializer in config/initializers/associated_bubbling_validator.rb):

module ActiveRecord
  module Validations
    class AssociatedBubblingValidator < ActiveModel::EachValidator
      def validate_each(record, attribute, value)
        ((value.kind_of?(Enumerable) || value.kind_of?(ActiveRecord::Relation)) ? value : [value]).each do |v|
          unless v.valid?
            v.errors.full_messages.each do |msg|
              record.errors.add(attribute, msg, options.merge(:value => value))
            end
          end
        end
      end
    end

    module ClassMethods
      def validates_associated_bubbling(*attr_names)
        validates_with AssociatedBubblingValidator, _merge_attributes(attr_names)
      end
    end
  end
end

So you can now validate like so:

class User < ActiveRecord::Base
 validates_associated_bubbling :account
end

Also, be sure to add a validate: false in your has_many association, otherwise, Rails will validate the association by default and you'll end up with two error messages, one given by your new AssociatedBubblingValidator and one generic given by Rails.

May be you can try something as given below

validates_associated :account , :message=> lambda{|class_obj, obj| obj[:value].errors.full_messages.join(",") }

Through obj[:value] you're accessing the validated Account object. So will obj[:value].errors give you errors.

A vanilla solution would be to have a second error's rendering for the associated object:

<%= render :partial => 'shared/errors', :locals => {:instance => @account.owner} %>
<%= render :partial => 'shared/errors', :locals => {:instance => @account} %>