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This is a really strange one...

When connecting through a mongos, I'm seeing an issue where find_one with PRIMARY_PREFERRED doesn't return any output.

c = ...some_collection ...

c.with_options(read_preference=ReadPreference.PRIMARY_PREFERRED)

c.find_one({'symbol': symbol}, sort=[('version', pymongo.DESCENDING)])

returns None. Whereas:

c.find_one({'symbol': symbol})

return an item.

len(list(c.find({'symbol': symbol}))

Out[76]: 25

In [79]: [x for x in list(c.find({'symbol': symbol})) if x is None]

Out[79]: []

Note a normal find finds nothing too.

In [84]: c = lib._versions.with_options(read_preference=ReadPreference.PRIMARY_PREFERRED)

In [85]: len(list(c.find({'symbol': symbol}, sort=[('version', pymongo.DESCENDING)])))

Out[85]: 0

If I change the read-preference, it works!

In [82]: c = lib._versions.with_options(read_preference=ReadPreference.PRIMARY)

In [83]: len(list(c.find({'symbol': symbol}, sort=[('version', pymongo.DESCENDING)])))

Out[83]: 25

is duplicated by

PYTHON-935 Setting $maxTimeMS raises OperationFailure when connecting through mongos (with ReadPreference.NEAREST)