validates_uniqueness_of(*attr_names) public

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
end

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

class Person < ActiveRecord::Base
  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 < 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.

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 - 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 nil (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.

Concurrency and integrity

Using this validation method in conjunction with ActiveRecord::Base#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!

This could even happen if you use transactions with the ‘serializable’ isolation level. The best way to work around this problem is to add a unique index to the database table using ActiveRecord::ConnectionAdapters::SchemaStatements#add_index. In the rare case that a race condition occurs, the database will guarantee the field’s uniqueness.

When the database catches such a duplicate insertion, ActiveRecord::Base#save will raise an ActiveRecord::StatementInvalid exception. You can either choose to let this error propagate (which will result in the default Rails exception page being shown), or you can catch it and restart the transaction (e.g. by telling the user that the title already exists, and asking him to re-enter the title). This technique is also known as optimistic concurrency control: http://en.wikipedia.org/wiki/Optimistic_concurrency_control

The bundled ActiveRecord::ConnectionAdapters distinguish unique index constraint errors from other types of database errors by throwing an ActiveRecord::RecordNotUnique exception. For other adapters you will have to parse the (database-specific) exception message to detect such a case. The following bundled adapters throw the ActiveRecord::RecordNotUnique exception:

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October 24, 2008
10 thanks

Back it up with a unique index

As mentioned briefly above, as well as using this validation in your model you should ensure the underlying database table also has a unique index to avoid a race condition.

For example:

class User < ActiveRecord::Base
  validates_uniqueness_of :login_name
end

The index can be specified in the migration for the User model using add_index like this:

add_index :users, :login_name, :unique => true

You do a similar thing when using the :scope option:

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

Should have a migration like this:

add_index :people, [ :account_id, :user_name ], :unique => true

Note that both the attribute being validated (:user_name) and the attribute(s) used in the :scope (:account_id) must be part of the index.

For a clear and concise explanation of the potential for a race condition see Hongli Lai’s blog.

April 20, 2010
2 thanks

Does not work with polymorphic relations

If you have polymorphic relations, e.g.:

class Bookmark < ActiveRecord::Base
  belongs_to :thing, :polymorphic => true
  belongs_to :owner, :polymorphic => true
end

and you want to ensure that a thing can bookmarked by an owner at most once, you can’t do this:

validates_uniqueness_of :thing, :scope => :owner

Instead, you must use the real column names, e.g.:

validates_uniqueness_of :thing_id, :scope => [:thing_type, :owner_id, :owner_type]
June 3, 2009
0 thanks

multi scope to sql

validates_uniqueness_of :name, :scope => [:big_category_id, :small_category_id]

SELECT * FROM schedules WHERE (products.name = 'xxxx' AND products.big_category_id= 1 AND products.small_category_id = 1) LIMIT 1
December 6, 2013
0 thanks

Migration for uniqueness with existent data in DB

I’m using sub-transaction to update existent records on DB. I use this approach to update the uniqueness field when it value dependent on another existent field without uniqueness restriction.

Migration for uniqueness with existent dependent data in DB

class AddUniquenessBarToFoo < ActiveRecord::Migration
  class Foo < ActiveRecord::Base
  end

  def change

    add_column :foos, :bar, :string
    execute "ALTER TABLE foos ADD CONSTRAINT uk_foods_bar UNIQUE (bar)"    

    Foo.reset_column_information
    Foo.all.each do |f|
      begin
        #try get unique value in a new sub-transaction
        Foo.transaction(requires_new: true) do
          f.update_attributes!(:bar => "some ops. with another non-unique existent field to set this")
        end
      rescue ActiveRecord::StatementInvalid
         #We can't reuse a crashed transaction. New one.
         Foo.transaction(requires_new: true) do
          #Alternative unique value, if another error exist it's another
          #migration problem and then raise new error.
          f.update_attributes!(:bar => "some operation to set this-#{f.id}")
        end
      end
    end   
    change_column :foos, :bar, :string, :null => false

  end
end

Be aware about performance that is transaction per record for big DB.