This change uses Module.redefine_method as defined in ActiveSupport. Making Module.define_method public would be as clean in the code, and would also emit warnings when redefining an association. That is pretty messy given current tests, so I'm leaving it for someone else to decide what approach is better.
Instead of generating association methods directly in the model class, they are generated in an anonymous module which is then included in the model class. There is one such module for each association. The only subtlety is that the generated_attributes_methods module (from ActiveModel) must be forced to be included before association methods are created so that attribute methods will not shadow association methods.
After a long list of discussion about the performance problem from using varargs and the reason that we can't find a great pair for it, it would be best to remove support for it for now. It will come back if we can find a good pair for it. For now, Bon Voyage, `#among?`.
There're a lot of places in Rails source code which make a lot of sense to switching to Object#in? or Object#either? instead of using .include?.
…ethods, callbacks etc) rather than calling a whole bunch of methods with rather long names.