I have a table called city, and a table called city_city. city_city correlates two city records, so it has a fromcity_id and a tocity_id. I can enforce uniqueness on fromcity_id and and tocity_id through a unique key, but how do I enforce uniqueness so that I cant insert a record if fromcity_id and tocity_id are reversed.

For example, the following records are conceptually the same:

id    fromcity_id  tocity_id
1     100          200
2     200          100