Validates whether the value of the specified attributes are unique across the system. Useful for making sure that only one user can be named "davidhh".

class Person < ActiveRecord::Base
  validates_uniqueness_of :user_name, :scope => :account_id
end

It can also validate whether the value of the specified attributes are unique based on multiple scope parameters. For example, making sure that a teacher can only be on the schedule once per semester for a particular class.

class TeacherSchedule < ActiveRecord::Base
  validates_uniqueness_of :teacher_id, :scope => [:semester_id, :class_id]
end

When the record is created, a check is performed to make sure that no record exists in the database with the given value for the specified attribute (that maps to a column). When the record is updated, the same check is made but disregarding the record itself.

Because this check is performed outside the database there is still a chance that duplicate values will be inserted in two parallel transactions. To guarantee against this you should create a unique index on the field. See create_index for more information.

Configuration options:

• message - Specifies a custom error message (default is: "has already been taken")
• scope - One or more columns by which to limit the scope of the uniquness constraint.
• case_sensitive - Looks for an exact match. Ignored by non-text columns (true by default).
• allow_nil - If set to true, skips this validation if the attribute is null (default is: false)
• allow_blank - If set to true, skips this validation if the attribute is blank (default is: false)
• if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
• unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.

Source Code

# File active_record/validations.rb, line 650
def validates_uniqueness_of(*attr_names)
  configuration = { :message => ActiveRecord::Errors.default_error_messages[:taken], :case_sensitive => true }
  configuration.update(attr_names.extract_options!)

  validates_each(attr_names,configuration) do |record, attr_name, value|
    if value.nil? || (configuration[:case_sensitive] || !columns_hash[attr_name.to_s].text?)
      condition_sql = "#{record.class.table_name}.#{attr_name} #{attribute_condition(value)}"
      condition_params = [value]
    else
      condition_sql = "LOWER(#{record.class.table_name}.#{attr_name}) #{attribute_condition(value)}"
      condition_params = [value.downcase]
    end

    if scope = configuration[:scope]
      Array(scope).map do |scope_item|
        scope_value = record.send(scope_item)
        condition_sql << " AND #{record.class.table_name}.#{scope_item} #{attribute_condition(scope_value)}"
        condition_params << scope_value
      end
    end

    unless record.new_record?
      condition_sql << " AND #{record.class.table_name}.#{record.class.primary_key} <> ?"
      condition_params << record.send(:id)
    end

    # 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? }

    if finder_class.find(:first, :conditions => [condition_sql, *condition_params])
      record.errors.add(attr_name, configuration[:message])
    end
  end
end