Ah, yes, sorry - this is not documented.
When working with the InMemory persistor - which in fact pre-dates the EntityPersistor by several years - reverse associations are not made automatically. You have to set them up. So you'll need to explicitly say in the
But what about outside fixtures? Well, if you are likely to be switching between the InMemory and Entity persistors, then the safest bet is to always make the reverse associations explicilty. EF will just ignore it (they won't be doubled up).
And the safest way to do that is to always add things through an explicit AddTo method.
FYI, this was how we always built Naked Object domain models in the past - even to work with other persistors. EF provided a positive surprise for us in managing the reverse associations automatically, so we've tended to get out of that
habit - which does impact the ability to switch between the persistors.