Of course, user_with_same_email.valid? is true, because the uniqueness vali- dation is currently case-sensitive, but we want it to be false. Fortunately, :uniqueness accepts an option, :case_sensitive, for just this purpose (Listing 6.21).

Note that we have simply replaced true with case_sensitive: false; Rails infers in this case that :uniqueness should be true. At this point, our application—with an important caveat—enforces email uniqueness, and our test suite should pass.

The Uniqueness Caveat
There’s just one small problem, the caveat alluded to above:
Using validates :uniqueness does not guarantee uniqueness.
D’oh! But what can go wrong? Here’s what:

1.  Alice signs up for the sample app, with address alice@wonderland.com.
2.  Alice accidentally clicks on ‘‘Submit’’ twice, sending two requests in quick succession.
3 . The following sequence occurs: request 1 creates a user in memory that passes validation, request 2 does the same, request 1’s user gets saved, request 2’s user gets saved.
4.  Result: two user records with the exact same email address, despite the uniqueness validation.

If the above sequence seems implausible, believe me, it isn’t: It can happen on any Rails website with significant traffic. Luckily, the solution is straightforward to implement; we just need to enforce uniqueness at the database level as well. Our method is to create a database index on the email column, and then require that the index be unique.

              The email index represents an update to our data modeling requirements, which (as discussed in Section 6.1.1) is handled in Rails using migrations. Wesaw in Section 6.1.1 that generating the User model automatically created a new migration (Listing 6.2); in the present case, we are adding structure to an existing model, so we need to create a migration directly using the migration generator:

Unlike the migration for users, the email uniqueness migration is not pre-defined, so we need to fill in its contents with Listing 6.22.14

This uses a Rails method called add_index to add an index on the email column of the users table. The index by itself doesn’t enforce uniqueness, but the option unique: true does.  

           The final step is to migrate the database:

(If this fails, try exiting any running sandbox console sessions, which can lock the database and prevent migrations.) If you’re interested in seeing the practical effect