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$dateToString aggregation with timezone outputs invalid ISO8601 string #1768

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$dateToString aggregation with timezone outputs invalid ISO8601 string #1768

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281c7a3
make format optional and create testing case about $dateToString aggr…
NathanQingyangXu Jul 17, 2025
9b6b7a3
Update driver-core/src/test/functional/com/mongodb/client/model/mql/T…
NathanQingyangXu Jul 17, 2025
f4f59bd
code change as per review comments
NathanQingyangXu Jul 17, 2025
0954ac5
Update driver-core/src/main/com/mongodb/client/model/mql/MqlExpressio…
NathanQingyangXu Jul 18, 2025
3a4b76b
follow up of the last suggestion commit
NathanQingyangXu Jul 18, 2025
8f68b79
remove unnecessary new method to MqlDate
NathanQingyangXu Jul 28, 2025
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14 changes: 14 additions & 0 deletions driver-core/src/main/com/mongodb/client/model/mql/MqlDate.java
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Original file line number Diff line number Diff line change
Expand Up @@ -138,6 +138,20 @@ public interface MqlDate extends MqlValue {
*/
MqlString asString(MqlString timezone, MqlString format);

/**
* The string representation of {@code this} date as determined by the
* provided {@code timezone} with default format as below:
*
* <ul>
* <li>If {@code timezone} is UTC timezone, {@code "%Y-%m-%dT%H:%M:%S.%LZ"} will be the default
* <li>Otherwise the default format will be {@code "%Y-%m-%dT%H:%M:%S.%L"}
* </ul>
*
* @param timezone the UTC Offset or Olson Timezone Identifier.
* @return the resulting value.
*/
MqlString asString(MqlString timezone);
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Why add this to the external API? This would expose a method that would always fail.

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removed. I misunderstood something.


/**
* The result of passing {@code this} value to the provided function.
* Equivalent to {@code f.apply(this)}, and allows lambdas and static,
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9 changes: 9 additions & 0 deletions driver-core/src/main/com/mongodb/client/model/mql/MqlExpression.java
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Expand Up @@ -953,6 +953,15 @@ public MqlString asString(final MqlString timezone, final MqlString format) {
.append("timezone", toBsonValue(cr, timezone))));
}

@Override
public MqlString asString(final MqlString timezone) {
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I believe that we do not need this method, and that it might actually be introducing the bug described in the Jira. That is, the API as it is now intentionally prevents the user from specifying a timezone without also forcing that user to explicitly specify a format, thus preventing the situation from arising.

For reference:

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thanks for sharing. Yeah, also saw you raising the point in our team slack channel. I have no objection of closing this PR.

Assertions.notNull("timezone", timezone);
return newMqlExpression((cr) -> astDoc("$dateToString", new BsonDocument()
.append("date", this.toBsonValue(cr))
.append("timezone", toBsonValue(cr, timezone))));
}


@Override
public MqlDate parseDate(final MqlString timezone, final MqlString format) {
Assertions.notNull("timezone", timezone);
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30 changes: 30 additions & 0 deletions ...er-core/src/test/functional/com/mongodb/client/model/mql/TypeMqlValuesFunctionalTest.java
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Expand Up @@ -33,6 +33,7 @@
import static com.mongodb.client.model.mql.MqlValues.ofIntegerArray;
import static com.mongodb.client.model.mql.MqlValues.ofMap;
import static com.mongodb.client.model.mql.MqlValues.ofNull;
import static java.time.format.DateTimeFormatter.ISO_INSTANT;
import static java.time.format.DateTimeFormatter.ISO_LOCAL_DATE_TIME;
import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.junit.jupiter.api.Assumptions.assumeTrue;
Expand Down Expand Up @@ -225,6 +226,35 @@ public void dateAsStringTest() {
of(instant).asString(of("America/New_York"), of("%Y-%m-%dT%H:%M:%S.%L%z")));
}

/**
* Tests that with server default format and non-UTC timezone, {@code $dateToString} won't output invalid ISO 8601 string ending with 'Z'.
* Ticket: JAVA-5044
*/
@Test
void dateAsStringWithDefaultFormat() {
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This is testing a new API. But, since this is a bugfix, I would expect it to test an existing API, where the test would fail before the prod change, and pass afterward. Is that not the case?

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yeah, correct. The testing would only succeed for v7.1..0+; it would fail for v6, for instance.

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I am not referring to the difference in test outcome based on the server version. The server would write such a test.

I mean, the ticket suggests that we need to make changes in the driver. When we write a regression test (or, all tests actually), the idea is that the test should not work before the change, but should work after. If the test works both before and after the change, the test has no relation to the change and is not testing it. If it fails both times, the change did not fix the issue.

Before the change to the production API, it is not actually possible to write a test that exhibits the bug via our API. So, no error. After the change, we can now write a test that exhibits the bug. But, we block the test from running on the server version necessary for the bug to show up. So things are "green" at first for that version, but after the prod code change, they are "red". This means we introduced a bug.

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If I didn't misunderstand, you said the same testing case would fail or be red on v6, correct? That is true and I understand that is an issue.

// the bug was reported on v6.0.3, but was fixed in v7.1 without backporting
// due to the concern that it is a breaking change.
assumeTrue(serverVersionAtLeast(7, 1));

final Instant instant = Instant.parse("2025-07-17T18:00:36.052Z");
MqlDate date = of(instant);
ZonedDateTime utcDateTime = ZonedDateTime.ofInstant(instant, ZoneId.of(ZoneOffset.UTC.getId()));

// UTC timezone's default format should be "%Y-%m-%dT%H:%M:%S.%LZ"
assertExpression(
utcDateTime.format(ISO_INSTANT),
date.asString(of("UTC")),
"{'$dateToString': {'date': {'$date': '2025-07-17T18:00:36.052Z'}, "
+ "'timezone': 'UTC'}}");

// non-UTC timezone's default format should be "%Y-%m-%dT%H:%M:%S.%L"
assertExpression(
utcDateTime.withZoneSameInstant(ZoneId.of("America/New_York")).format(ISO_LOCAL_DATE_TIME),
date.asString(of("America/New_York")),
"{'$dateToString': {'date': {'$date': '2025-07-17T18:00:36.052Z'}, "
+ "'timezone': 'America/New_York'}}");
}

// parse string

@Test
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