Document behaviors around setting owned navigations to null

[LunicLynx]

Document behaviors around setting owned navigations to null

[ajcvickers]

Note for team: still repros on latest daily; not a regression.

[maumar]

Note: also repros for non-json owned entities

[LunicLynx]

The example only works if MoreData was something (not null) at the time it was attached.

[ajcvickers]

Note for triage: I investigated this more. If this was not an owned type, this would be the correct behavior. This is because, in effect, the MoreData navigation isn't loaded. What this should do for owned types depends on whether or not we allow optional owned types to be not loaded. From an aggregate perspective, not allowed them to be unloaded is fine. That is, we could assume that if the navigation is null, then the owned type does not exist in the database, as opposed to it just isn't loaded. From the perspective of how people actually use owned types, it may not be fine.

Given that we are unsure where owned types are going, I propose we do nothing here immediately.

It's also worth noting that this works:

myDbContext.Update(owner);
owner.Data.MoreData = null;
myDbContext.SaveChanges();

This is because in this case EF is tracking the entity when the nav is set to null, so EF correctly detects this as a delete.