Background:
Test relied on an `A->B->C` association:
* `A#id` being `B`
* `B#id` being `C`
* `C#id` being an auto-generated identifier (post-insert)
This cannot work, because it breaks the UnitOfWork's identity map.
Specifically, no entries for `A` and `B` can exist in the identity map until `C` entries
are persisted (post-insert).
That means that the identifier generator for `A` and `B` should not be an "assigned"
generator, but should instead be a post-insert generator waiting for other entities
to be persisted.
We cannot fix this in ORM 2.x, but we'll need to invent something for 3.x in order to
fix that (directed graph, or caching the order of operations in the metadata graph).
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