I found myself needing to override the validation of "has_many" active record objects after using build to create them. The reason being that I was doing custom validation on each of the built objects anyway and wanted to display each of the child object's validation error message, not the generic "<object> is not valid".

For example:

class List < ActiveRecord::Base
  
  has_many :phone_numbers, :dependent => :destroy

  validate :validate_numbers

  def build_items_from_file_stream( stream )
    stream.split( "\n" ).each_with_index do |line, index|
      next if line.blank?
      phone = self.phone_numbers.build( :phone_number => line )
      phone.file_line_number = index + 1
      phone.unformatted_number = line
    end
  end

  # this method overrides the automatically generated method
  def validate_associated_records_for_phone_numbers
    true
  end

  def validate_numbers
    self.phone_numbers.each do |phone|
      if !phone.valid?
        error_prefix = "File line #{phone.file_line_number}, '#{phone.unformatted_number}': "
        errors.add error_prefix, target.errors.full_messages.join( ", " )
      end
    end
  end

end


Basically ActiveRecord will automatically create a validate method of the name:
validate_associated_records_for_
That method steps across all of the associated objects, and if !valid? does:
errors.add , "is not valid"

Without overriding that method, if you were to create a list (in my example above) that had an invalid phone number (assuming the phone_numbers class has validation too), you would get an error string something like:
- Phone number is not valid
- File line 1 '805-555-123': national number must contain 10 digits
- Phone number is not valid
- File line 2 '+11 805-555-1234': country code not found

Clearly having "phone number is invalid" multiple times is not adding any value, and in fact making the list validation message more confusing.