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

# this method overrides the automatically generated method
def validate_associated_records_for_phone_numbers

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( ", " )


Basically ActiveRecord will automatically create a validate method of the name:
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.