Rails%203%20In%20Action

78 CHAPTER 3 Developing a real Rails application
Listing 3.28 app/models/project.rb
class Project < ActiveRecord::Base
validates :name, :presence => true
end
This validates method tells the model that you want to validate the name field and
that you want to validate its presence. There are other kinds of validations as well,
such as the :uniqueness key, which, when passed true as the value, validates the
uniqueness of this field as well. In prior versions of Rails, to do this you would have to
use the validates_presence_of method instead:
class Project < ActiveRecord::Base
validates_presence_of :name
end
This syntax is still supported in Rails 3, but with the new syntax in Rails 3, you can
specify multiple validation types for multiple fields on the same line, thereby reducing
duplication in your model.
Beware race conditions with the uniqueness validator
The validates_uniqueness_of validator works by checking to see if a record
matching the validation criteria exists already. If this record doesn’t exist, the validation
will pass.
A problem arises if two connections to the database both make this check at almost
exactly the same time. Both connections will claim that a record doesn’t exist and
therefore will allow a record to be inserted for each connection, resulting in nonunique
records.
A way to prevent this is to use a database uniqueness index so the database, not
Rails, does the uniqueness validation. For information about how to do this, consult
your database’s manual.
Although this problem doesn’t happen all the time, it can happen, so it’s something
to watch out for.
With the presence validation in place, you can experiment with the validation by using
the Rails console, which allows you to have all the classes and the environment from
your application loaded in a sandbox environment. You can launch the console with
this command
rails console
or with its shorter alternative:
rails c
If you’re familiar with Ruby, you may realize that this is effectively IRB with some Rails
sugar on top. For those of you new to both, IRB stands for Interactive Ruby, and it pro-