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?.

Use proper objects to do the work to build the associations (adding methods, callbacks etc) rather than calling a whole bunch of methods with rather long names.