Fix issue with polymorphic association
Fix issue with polymorphic association

The only fallout from this change is that reciprocal_type for
many_to_one associations now needs access to the associated class
if not cached, so when caching is disabled, if there isn't a
single associated class (such as when using manually creating a
custom polymorphic association), you should manually set the
:reciprocal_type option.

https://github.com/jeremyevans/sequel/commit/ae283c45550b999ad5cefa2dfff
4da5c8ef1bb5c

bitzesty/trade-tariff-backend,bitzesty/trade-tariff-backend,alphagov/trade-tariff-backend,alphagov/trade-tariff-backend,alphagov/trade-tariff-backend,bitzesty/trade-tariff-backend