When developing a Rails app with a legacy schema that is in-use by an existing application, if tables have NOT NULL constraints on foreign id columns throughout the schema, in order to create/save models for tests, models need to exist for those associations, and their associations, etc. So, it isn't an easy as just creating one model at a time as you need it and testing with it.
As far as testing goes, this seems to be a problem if you are using FactoryGirl and want to create and save model instances to return from controllers, etc. when all of the association dependencies are involved. Another option is to mock, but mocking can be a little more time consuming and it doesn't allow you to do integration testing as easily. Another is to use fixtures, but those are time consuming and brittle. Another is to pre-populate the test database with production data, but that doesn't solve the need for factories, etc./known data to expect in tests, and Rails usually expects to start with a clean DB for the test environment.
What strategies for developing models, tests, etc. do you use when you have an existing complex schema that you are bolting a Rails app onto- not just for reading data, but also writing to the existing schema that is in use by an existing production application? (i.e. the "rebuild the ship while at sea" problem)