Module: AWS::Record::Validations

Defined in:
lib/simple_unique/validations.rb

Instance Method Summary collapse

Instance Method Details

#validates_uniqueness_of(*args) ⇒ Object

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

class Person < AWS::Record::Model
  validates_uniqueness_of :user_name
end

It can also validate whether the value of the specified attributes are unique based on a scope parameter:

class Person < AWS::Record::Model
  validates_uniqueness_of :user_name, :scope => :account_id
end

Or even 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 < AWS::Record::Model
  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.

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 uniqueness constraint.

  • :case_sensitive - Always true. SimpleDB does not support case insensitive search.

  • :allow_nil - Always true.

  • :allow_blank - Always true.

  • :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.

Concurrency and integrity

Using this validation method in conjunction with AWS::Record::Model#save does not guarantee the absence of duplicate record insertions, because uniqueness checks on the application level are inherently prone to race conditions. For example, suppose that two users try to post a Comment at the same time, and a Comment’s title must be unique. At the database-level, the actions performed by these users could be interleaved in the following manner:

             User 1                 |               User 2
------------------------------------+--------------------------------------
# User 1 checks whether there's     |
# already a comment with the title  |
# 'My Post'. This is not the case.  |
SELECT * FROM comments              |
WHERE title = 'My Post'             |
                                    |
                                    | # User 2 does the same thing and also
                                    | # infers that his title is unique.
                                    | SELECT * FROM comments
                                    | WHERE title = 'My Post'
                                    |
# User 1 inserts his comment.       |
INSERT INTO comments                |
(title, content) VALUES             |
('My Post', 'hi!')                  |
                                    |
                                    | # User 2 does the same thing.
                                    | INSERT INTO comments
                                    | (title, content) VALUES
                                    | ('My Post', 'hello!')
                                    |
                                    | # ^^^^^^
                                    | # Boom! We now have a duplicate
                                    | # title!

To guard against duplicates you should create a process to periodically scan your data and take appropriate action when duplicates are found. (Ick!) This means that this validation is useful only in systems where the data creation events that trigger it are few and far between (relatively speaking).



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# File 'lib/simple_unique/validations.rb', line 86

def validates_uniqueness_of *args
  validators << UniquenessValidator.new(self, *args)
end