Railtie initializer and generators

I am creating a gem for a Rails project and I got some troubles to
understand how generators and initializers work. I would like to
initialize my module loading some stuff from the database from models
related to tables my gem should create with a migration file.
Problem is : if I create a Railtie and put it my gem lib directory,
when I try to run my generator (to create the migration template file
for example), it's already trying to run the Railtie even though
tables required don't exist yet.

How to restrict the "scope" of the Railtie ? I would like it only to
run when booting Rails from a server (webrick, thin, ...) or from the
console, but not for any rake tasks (including generators). I think
that rake tasks (like generators) load the entire Rails environment
and my problem should come from that.
Is there a simpler way to do that ?

any help or advice appreciated.