Quick&drty monkey patch to fix the https://github.com/rails/rails/issues/4321 issue in the 3.2 branch.

monkey_patch_pull_4370.rb

# https://github.com/rails/rails/issues/4321
#
# https://github.com/rails/rails/pull/4370/files
#

raise "delete me" if ActiveModel::VERSION::MAJOR >= 4

module ActiveRecord
  module Validations
    class UniquenessValidator < ActiveModel::EachValidator
      def validate_each(record, attribute, value)
        finder_class = find_finder_class_for(record)
        table = finder_class.arel_table

        coder = record.class.serialized_attributes[attribute.to_s]

        if value && coder
          value = coder.dump value
        end

        relation = build_relation(finder_class, table, attribute, value)
        relation = relation.and(table[finder_class.primary_key.to_sym].not_eq(record.send(:id))) if record.persisted?

        Array.wrap(options[:scope]).each do |scope_item|
          scope_value = record.read_attribute(scope_item)
          reflection = record.class.reflect_on_association(scope_item)
          if reflection
            scope_value = record.send(reflection.foreign_key)
            scope_item = reflection.foreign_key
          end
          relation = relation.and(table[scope_item].eq(scope_value))
        end

        if finder_class.unscoped.where(relation).exists?
          record.errors.add(attribute, :taken, options.except(:case_sensitive, :scope).merge(:value => value))
        end
      end

      protected

      def build_relation(klass, table, attribute, value) #:nodoc:
        reflection = klass.reflect_on_association(attribute)
        column = nil
        if(reflection)
          column = klass.columns_hash[reflection.foreign_key]
          attribute = reflection.foreign_key
          value = value.attributes[reflection.primary_key_column.name]
        else
          column = klass.columns_hash[attribute.to_s]
        end
        value = column.limit ? value.to_s.mb_chars[0, column.limit] : value.to_s if value && column.text?

        if !options[:case_sensitive] && value && column.text?
          # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
          relation = klass.connection.case_insensitive_comparison(table, attribute, column, value)
        else
          value    = klass.connection.case_sensitive_modifier(value) if value
          relation = table[attribute].eq(value)
        end

        relation
      end
    end
  end
end