django: crash when comparing TIMESTAMP and DATE

[timgraham]

======================================================================
ERROR: test_mixed_comparisons1 (expressions.tests.FTimeDeltaTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tim/.virtualenvs/django37/lib/python3.7/site-packages/google/api_core/grpc_helpers.py", line 79, in next
    return six.next(self._wrapped)
  File "/home/tim/.virtualenvs/django37/lib/python3.7/site-packages/grpc/_channel.py", line 392, in __next__
    return self._next()
  File "/home/tim/.virtualenvs/django37/lib/python3.7/site-packages/grpc/_channel.py", line 561, in _next
    raise self
grpc._channel._Rendezvous: <_Rendezvous of RPC that terminated with:
	status = StatusCode.INVALID_ARGUMENT
	details = "No matching signature for operator > for argument types: DATE, TIMESTAMP. Supported signature: ANY > ANY [at 1:263]\n...end` FROM expressions_ExPeRiMeNt WHERE expressions_ExPeRiMeNt.assigned > (...\n                                          ^"
	debug_error_string = "{"created":"@1576630086.782303263","description":"Error received from peer ipv4:172.217.6.234:443","file":"src/core/lib/surface/call.cc","file_line":1055,"grpc_message":"No matching signature for operator > for argument types: DATE, TIMESTAMP. Supported signature: ANY > ANY [at 1:263]\n...end` FROM expressions_ExPeRiMeNt WHERE expressions_ExPeRiMeNt.assigned > (...\n                                          ^","grpc_status":3}"
>

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/tim/code/django/django/test/testcases.py", line 1229, in skip_wrapper
    return test_func(*args, **kwargs)
  File "/home/tim/code/django/tests/expressions/tests.py", line 1202, in test_mixed_comparisons1
    test_set = [e.name for e in Experiment.objects.filter(assigned__gt=F('start') - delay)]
  File "/home/tim/code/django/django/db/models/query.py", line 274, in __iter__
    self._fetch_all()
  File "/home/tim/code/django/django/db/models/query.py", line 1242, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/home/tim/code/django/django/db/models/query.py", line 55, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
  File "/home/tim/code/django/django/db/models/sql/compiler.py", line 1166, in execute_sql
    return list(result)
  File "/home/tim/code/django/django/db/models/sql/compiler.py", line 1545, in cursor_iter
    for rows in iter((lambda: cursor.fetchmany(itersize)), sentinel):
  File "/home/tim/code/django/django/db/models/sql/compiler.py", line 1545, in <lambda>
    for rows in iter((lambda: cursor.fetchmany(itersize)), sentinel):
  File "/home/tim/code/django/django/db/utils.py", line 96, in inner
    return func(*args, **kwargs)
  File "/home/tim/code/spanner-orm/spanner/dbapi/cursor.py", line 250, in fetchmany
    items.append(tuple(self.__next__()))
  File "/home/tim/code/spanner-orm/spanner/dbapi/cursor.py", line 213, in __next__
    return next(self.__itr)
  File "/home/tim/.virtualenvs/django37/lib/python3.7/site-packages/google/cloud/spanner_v1/streamed.py", line 143, in __iter__
    self._consume_next()
  File "/home/tim/.virtualenvs/django37/lib/python3.7/site-packages/google/cloud/spanner_v1/streamed.py", line 116, in _consume_next
    response = six.next(self._response_iterator)
  File "/home/tim/.virtualenvs/django37/lib/python3.7/site-packages/google/cloud/spanner_v1/snapshot.py", line 45, in _restart_on_unavailable
    for item in iterator:
  File "/home/tim/.virtualenvs/django37/lib/python3.7/site-packages/google/api_core/grpc_helpers.py", line 81, in next
    six.raise_from(exceptions.from_grpc_error(exc), exc)
  File "<string>", line 3, in raise_from
google.api_core.exceptions.InvalidArgument: 400 No matching signature for operator > for argument types: DATE, TIMESTAMP. Supported signature: ANY > ANY [at 1:263]\n...end` FROM expressions_ExPeRiMeNt WHERE expressions_ExPeRiMeNt.assigned > (...\n                                          ^

[odeke-em]

Since there doesn't seem to be a Cloud Spanner function for casting DATE to TIMESTAMP and vice versa, I think that given that ISO 8601 allows for string comparison by magnitude, we'll need to format both to YYYYMMDD and then compare them in this normalized form.

[odeke-em]

Perhaps we can convert DATE to TIMESTAMP with

TIMESTAMP(date_field, "GMT")

[timgraham]

I believe this is another case of "we-cannot-typecheck-spanner-columns".

[asthamohta]

Adding support to this would have serious performance impacts of this. In addition, real Spanner supports non-typed parameters, and the missing support for non-typed parameters for some type (DATE and TIMESTAMP) in the emulator is considered a bug: GoogleCloudPlatform/cloud-spanner-emulator#31
Further details about the decisions can be checked here: googleapis/python-spanner#200