At least in PostgreSQL, at most one reference is allowed. My undirected
recursion code used a UNION of two recursive steps, one for each
direction. That is invalid, so instead I define a CTE that's a union of
the edges and their reverse, and do a single recursion step on that CTE
instead of on the edge table itself.
I thought SQL arrays were common and PersistList corresponded to SQL
array values. But that isn't the case. PersistList seems to be
serialized as a JSON list, and `filterClause` uses IN, not ANY. So I'm
doing the same thing here and using IN.
Note that I'm building the list myself using Text concatenation, not
using `filterClause`, because the latter takes a filter on an existing
`PersistEntity` while my filters often apply to temporary tables.
