Reimplementation of validates_uniqueness_of that works optimally with MySQL.

validates_uniqueness_of.rb

module ActiveRecord::Validations::ClassMethods
  def validates_uniqueness_of(*attr_names)
    configuration = { :case_sensitive => true }
    configuration.update(attr_names.extract_options!)
  
    validates_each(attr_names,configuration) do |record, attr_name, value|
      # The check for an existing value should be run from a class that
      # isn't abstract. This means working down from the current class
      # (self), to the first non-abstract class. Since classes don't know
      # their subclasses, we have to build the hierarchy between self and
      # the record's class.
      class_hierarchy = [record.class]
      while class_hierarchy.first != self
        class_hierarchy.insert(0, class_hierarchy.first.superclass)
      end
    
      # Now we can work our way down the tree to the first non-abstract
      # class (which has a database table to query from).
      finder_class = class_hierarchy.detect { |klass| !klass.abstract_class? }
    
      column = finder_class.columns_hash[attr_name.to_s]
    
      if value.nil?
        comparison_operator = "IS ?"
      elsif column.text?
        if configuration[:case_sensitive] then
          comparison_operator = "#{connection.case_sensitive_equality_operator} BINARY ?"
        else
          comparison_operator = "#{connection.case_sensitive_equality_operator} ?"
        end
        value = column.limit ? value.to_s[0, column.limit] : value.to_s
      else
        comparison_operator = "= ?"
      end
    
      sql_attribute = "#{record.class.quoted_table_name}.#{connection.quote_column_name(attr_name)}"
    
      condition_sql = "#{sql_attribute} #{comparison_operator}"
      condition_params = [value]
    
      if scope = configuration[:scope]
        Array(scope).map do |scope_item|
          scope_value = record.send(scope_item)
          condition_sql << " AND " << attribute_condition("#{record.class.quoted_table_name}.#{scope_item}", scope_value)
          condition_params << scope_value
        end
      end
    
      unless record.new_record?
        condition_sql << " AND #{record.class.quoted_table_name}.#{record.class.primary_key} <> ?"
        condition_params << record.send(:id)
      end
    
      finder_class.with_exclusive_scope do
        if finder_class.exists?([condition_sql, *condition_params])
          record.errors.add(attr_name, :taken, :default => configuration[:message], :value => value)
        end
      end
    end
  end
end