I'm new to Ruby, but coming up to speed quickly.  One question I still
have never seen a good explanation to is this: When is it preferred to a
key a hash with a symbol, and when is it keyed by string?  Is this just
personal preference, or is there a rule of thumb?

For example, in the Rails book, the session variable is always populated
with symbols, i.e.:

session[:user] = User.new

It's also obviously completely common throughout the Rails framework
(e.g., :controller =>, :action =>, etc.)

So, when should I use what...or what should I prefer?

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