pre-commit

#!/usr/bin/env ruby
# vim: set syntax=ruby

# Ensures that changes to the Rails schema.rb file may only be committed if a
# migration file is also committed at the same time.
# put this in .git/hooks/pre-commit

def schema_modified?
  %x[ git diff --cached |grep schema.rb ] == ''
end

def no_migrations_staged?
  %x[ git diff --cached |grep 'db\/migrate\/20[0-9]*' ] == ''
end

# regex for typical migration
# db/migrate/20110901091759_remove_invalid_venue_phone_numbers.rb
# db\/migrate\/20[0-9]{12}_.*\.rb$
# or
# db\/migrate\/20[0-9]*
# Warning may only work for like 100 years or something.

if schema_modified? && no_migrations_staged?
  puts "Cannot commit modified schema.rb without any migrations. You can override this warning by committing with the --no-verify flag."
  exit 1
end

exit 0

The most frequent problems that I've seen with schema.rb would be covered by these conditions. We have a separate rspec test that checks for a migration without a schema.rb change.