Module: ActiveRecord::Validations::ClassMethods

Validates that the specified attributes are not blank (as defined by Object#blank?), and, if the attribute is an association, that the associated object is not marked for destruction. Happens by default on save.

class Person < ActiveRecord::Base
  has_one :face
  validates_presence_of :face
end

The face attribute must be in the object and it cannot be blank or marked for destruction.

If you want to validate the presence of a boolean field (where the real values are true and false), you will want to use validates_inclusion_of :field_name, in: [true, false].

This is due to the way Object#blank? handles boolean values: false.blank? # => true.

This validator defers to the ::ActiveModel validation for presence, adding the check to see that an associated object is not marked for destruction. This prevents the parent object from validating successfully and saving, which then deletes the associated object, thus putting the parent object into an invalid state.

Configuration options:

• :message - A custom error message (default is: “can't be blank”).

• :on - Specifies the contexts where this validation is active. Runs in all validation contexts by default (nil). You can pass a symbol or an array of symbols. (e.g. on: :create or on: :custom_validation_context or on: [:create, :custom_validation_context])

• :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.

• :strict - Specifies whether validation should be strict. See ActiveModel::Validation#validates! for more information.

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

It is also possible to limit the uniqueness constraint to a set of records matching certain conditions. In this example archived articles are not being taken into consideration when validating uniqueness of the title attribute:

class Article < ActiveRecord::Base
  validates_uniqueness_of :title, conditions: -> { where.not(status: 'archived') }
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.

• :conditions - Specify the conditions to be included as a WHERE SQL fragment to limit the uniqueness constraint lookup (e.g. conditions: -> { where(status: 'active') }).

• :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 their title is unique.
                                    | SELECT * FROM comments
                                    | WHERE title = 'My Post'
                                    |
# User 1 inserts their 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 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 them to re-enter the title). This technique is also known as 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:

• ::ActiveRecord::ConnectionAdapters::MysqlAdapter.

• ::ActiveRecord::ConnectionAdapters::Mysql2Adapter.

• ::ActiveRecord::ConnectionAdapters::SQLite3Adapter.

• ::ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.