Case-sensitivity in RailsEdit

Doing Behaviour-Driven Development with Rails I recently discovered that MySQL does case-insensitive comparisons by default, and more importantly, that even if you tell Rails to force case-sensitive comparisons you won't get them.

This means that if you have validations like the following in your ActiveRecord subclasses:

validates_uniqueness_of :login_name, :case_sensitive => false

Then you'll get case-insensitive comparisons. But if you change this to:

validates_uniqueness_of :login_name, :case_sensitive => true

Then you will still get case-insensitive comparisons. Note that case_sensitive defaults to true.

I discovered this because the BDD development model gets you to work in a cycle:

1. Write a spec describing the behaviour you want: it should fail because you haven't written the code to make it pass yet
2. Write the code to make the spec pass
3. Repeat

The order of the steps is important: you want the spec to fail first before you make it pass because that confirms that your spec tests the thing you actually think it tests.

So this was an unexpected benefit of the BDD approach: it made me learn something about the way Rails and MySQL interact that I might not have found out about otherwise, and which could potentially one day help me to identify particularly tricky case-sensitivity bugs which might crop up.