As I might have expected, especially once I discovered that the issue was that the row was being inserted as expected but then called with an UPDATE function shortly after, the problem was some related code. I was returning the result as project_id (as you can see in the code above), and for an entirely unrelated reason (having to do with Blinker signals) the method was getting called again, with the returned value of project_id, which I had set thus:
project_id = result.inserted_primary_key if result.is_insert else id_The correct version of this line is only slightly different:
project_id = result.inserted_primary_key[0] if result.is_insert else id_From the SQLAlchemy docs (emphasis mine):
Return the primary key for the row just inserted.
The return value is a list of scalar values corresponding to the list of primary key columns in the target table.
The return value here has to be a list because primary keys can be a combination of more than one field in the database. (This should have been obvious to me; it's obvious I haven't done serious database work in over a year.) Since the primary key in this case is a single value, I just chose that value and returned it, and the problem is resolved.
Of course, now I have to go hunt down that Blinker signal issue—this method shouldn't be getting called twice—but c'est la vie...