should I be using strings instead of lookup tables in my ruby on rails web project (mysql and active record)?
I usually try to avoid lookup tables in my ruby on rails projects. Instead, I tend to store the user representations in the language files, e.g.:
(Or whatever kind of structure you prefer, I am not religious about this.)
Then, in the model file, I only introduce Ruby constants for those values that I actually reference in Ruby code:
STATUS_ACTIVE = 1
self.status == User::STATUS_ACTIVE
Here as well, I am not religious about this particular syntax. If you feel like it, you could hide some of these lines with meta-programming. The letters "a-c-t-i-v-e" show up awfully often, so some guys may prefer a more DRY approach. ;)
Is this something specific to ruby on rails, or to dynamically typed languages?
No, I have seen both variants (lookup tables vs. hardcoded enumerations) in all kinds of languages.
Or for some particular circumstances?
This is more like it. For example, if you have an application with a significant chance that these values would change over time; or if you have a multi-language application, it would be crazy to always have to go back to the code to change anything.
But there are definitely cases where it would not make any sense at all to change these lookup tables without also changing the code. In this example, the code presumably has to know when a user is active or not active; i.e., it would not make sense to have some translator nilly-willy add a third value
3: maybe-active without modifying the code to actually implement some behaviour. In this case, having a separate table USER_STATUS would be useless.
In short: if "some guy" should be able to modify these values at any point of time, this is much easier in the DB (maybe with an admin "self-service" functionality in your application). Then, these lookups are proper data of your application. Obviously, some values are obvious candidates for a DB (for example, a list of all ISO country codes in all their variants).
Or is it just bad database design?
Not per se, as shown above.