Rails could be DRYer
I'm playing with Ruby on Rails these days and I've been thinking about how Rails isn't entirely DRY. (i.e., doesn't implement the Don't Repeat Yourself philosophy fully.) The area where it falls short of DRY is in the duplication between defining the database schema (either manually, or through the use of migrations) and defining the Model.
In Rails you can define constraints and validations in the Model. You repeat the Model definition when you create migrations to create your database structure (or if you create you create your schema manually, when you do that.) You repeat the constraints (e.g., foreign keys) in the database also. For Rails to be completely DRY, it should generate the schema and update the database automatically based on the current state of your Model classes. (Allowing overrides that go low-level, where necessary, of course for specific optimizations or advanced SQL.)
Googling around, I came across this discussion of a suggestion for ActiveSchema where people are discussing the same thing.