Module | ActiveRecord::Validations::ClassMethods |
In: |
lib/active_record/validations/associated.rb
lib/active_record/validations/uniqueness.rb lib/active_record/validations.rb |
Creates an object just like Base.create but calls save! instead of save so an exception is raised if the record is invalid.
Validates whether the associated object or objects are all valid themselves. Works with any kind of association.
class Book < ActiveRecord::Base has_many :pages belongs_to :library validates_associated :pages, :library end
WARNING: This validation must not be used on both ends of an association. Doing so will lead to a circular dependency and cause infinite recursion.
NOTE: This validation will not fail if the association hasn‘t been assigned. If you want to ensure that the association is both present and guaranteed to be valid, you also need to use validates_presence_of.
Configuration options:
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:
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: 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: