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I want to validate that the postal address filled in by a user is correct.
Assuming that we have the fields street, street_number, postal_code, city and country
How can show errors when the user, types in that his country is France and is city is New York?
or
How can I detect that a postal code doesn’t correspond to the city?
Is there a strategy to address this issue with Ruby On Rails or shall I just trust the user entries ?
There are several companies that publish US ZIP code directories, typically not terribly expensive, and some even come with lat/lon data. This is the most common way of validating city and state in the US or North America. Worldwide postal codes to country and city is likely to be a harder problem, or at least a more expensive one.
Another approach would be to use the Google Maps API, perhaps in conjunction with the data you can get from location-aware browsers (all modern ones). To be honest, it’s been a while since I have used the API, but seem to recall you can get pretty detailed information given a lat/lon. Google now charges for high use of the Maps API, but if your site has even moderate traffic, you’re probably still in the free zone.
At some level, you’ll have to trust users. I remember a while back we found anomalies in user reported zipcodes in our database — seemed like everyone lived in Beverly Hills, CA. This was around 2000, when the original version of the TV show “90210” was still around — that’s the zipcode :-).