I guess what you want to do is find a script that will look at your models and create factories for them. I tried this once (with something other than factory_girl), and found that it is full of bad data.
I would suggest that you move slowly to using factories. When you write new tests or update old ones, create the necessary factories. After the lapse of time, if you have time and energy, you can select an individual test case and replace all calls with verified data using the corresponding factory calls and delete the specific fastening requirement at the top of the test case.
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