Customizing Model Validation error messages alerts - ruby-on-rails-3

Question

I'm trying to customize the error message alert that users see at the top of a form when they input data incorrectly. The error message alert I'm trying to customize are for Model Attributes which are in a nested form.
I've tried the solution here which says to edit the config/locales/en.yml file but this only changes the message not the name of the model & attribute which are displayed before the error message.
I've also tried what Billy's suggested in his answer bellow which has the same result. i.e.
1 error prohibited this hikingtrail from being saved:
- Directions directions from 'My Custom Blank Error Message'
Is there a way for me to display a more user friendly Model & attribute name in my error message or remove them entirely from the error message?
Here is what I have:
config/locales/en.yml
# Sample localization file for English. Add more files in this directory for other locales.
# See https://github.com/svenfuchs/rails-i18n/tree/master/rails%2Flocale for starting points.
en:
activerecord:
models:
direction: "In the Getting There section"
attributes:
direction:
directions_from: "From field"
errors:
full_messages:
format: "%{message}"
models:
direction:
attributes:
directions_from:
blank: "My Custom Blank Error Message"
Model
class Direction < ActiveRecord::Base
belongs_to :hikingtrail
attr_accessible :directions_by, :directions_desc, :directions_from
validates :directions_from, :presence => {message: "'My Custom Error Message'", :if => Proc.new { |a| a.directions_by? || a.directions_desc? } }
validates :directions_by, :presence => {message: "'My Custom Error Message'", :if => Proc.new { |a| a.directions_from? || a.directions_desc? } }
validates :directions_desc, :presence => {message: "'My Custom Error Message'", :if => Proc.new { |a| a.directions_from? || a.directions_by? } }
end

Answer 1

You can use :message option to assign custom error message.
Example:
validates :directions_from, presence: true,
message: "'Direction from' really really really can't be blank!"
Then this custom error message will appear as <%= msg %> in the form view.
Ref: http://edgeguides.rubyonrails.org/active_record_validations.html#message
Add
To answer OP's question on the comment, i.e. the message shown in web page is not very friendly, showing result as "Directions directions from 'Direction from' really really really can't be blank"
The reason is the view template use errors.full_messages to show the error messages. You can easily customize it with two options:
Option 1: Write the custom message without subject. i.e. really can't be blank
Option 2: Write the message as before in full sentence, but refer to message only in view, instead of full_message
Example:
<% #hikingtrail.errors.messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
Ref: http://rubydoc.info/docs/rails/3.2.8/ActiveModel/Errors (full_message is nothing more but a mix of attribute and message)

Answer 2

With Rails 6, it's now possible to customize the format used by full_messages helper at the model or at the attribute level.
en:
activerecord:
errors:
models:
direction:
attributes:
directions_from:
format: "%{message}"
blank: "'Direction from' really really really can't be blank!"
This way the generated message for this error would be your message without the model prefix, leaving intact the default full_messages for other models and attributes.
More on that in this article :
https://blog.bigbinary.com/2019/04/22/rails-6-allows-to-override-the-activemodel-errors-full_message-format-at-the-model-level-and-at-the-attribute-level.html