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Type errors when applying aggregation array operators on result of $split #214
Description
Hi, I'm seeing an error when attempting to apply an array operator on the result of a string $split.
If I run an aggregation like this:
db.MY_DOC.aggregate([
{
$addFields: {
pathSegments: {
$split: ["$path", "/"]
},
firstSegment: {
$arrayElemAt: ["$pathSegments", 0]
}
}
}
])
I see an error like this:
MongoServerError: PlanExecutor error during aggregation :: caused by :: $arrayElemAt's first argument must be an array, but is [Ljava.lang.String;
When I run from a java environment, I get this error
org.springframework.data.mongodb.UncategorizedMongoDbException: Command failed with error 28689 (Location28689): 'PlanExecutor error during aggregation :: caused by :: $arrayElemAt's first argument must be an array, but is [Ljava.lang.String;' on server localhost:12345. The full response is {"$err": "PlanExecutor error during aggregation :: caused by :: $arrayElemAt's first argument must be an array, but is [Ljava.lang.String;", "errmsg": "PlanExecutor error during aggregation :: caused by :: $arrayElemAt's first argument must be an array, but is [Ljava.lang.String;", "code": 28689, "codeName": "Location28689", "ok": 0}; nested exception is com.mongodb.MongoCommandException: Command failed with error 28689 (Location28689): 'PlanExecutor error during aggregation :: caused by :: $arrayElemAt's first argument must be an array, but is [Ljava.lang.String;' on server localhost:12345. The full response is {"$err": "PlanExecutor error during aggregation :: caused by :: $arrayElemAt's first argument must be an array, but is [Ljava.lang.String;", "errmsg": "PlanExecutor error during aggregation :: caused by :: $arrayElemAt's first argument must be an array, but is [Ljava.lang.String;", "code": 28689, "codeName": "Location28689", "ok": 0}
at org.springframework.data.mongodb.core.MongoExceptionTranslator.translateExceptionIfPossible(MongoExceptionTranslator.java:133)
at org.springframework.data.mongodb.core.MongoTemplate.potentiallyConvertRuntimeException(MongoTemplate.java:2882)
at org.springframework.data.mongodb.core.MongoTemplate.execute(MongoTemplate.java:564)
at org.springframework.data.mongodb.core.MongoTemplate.doAggregate(MongoTemplate.java:2130)
at org.springframework.data.mongodb.core.MongoTemplate.aggregate(MongoTemplate.java:2099)
at org.springframework.data.mongodb.core.MongoTemplate.aggregate(MongoTemplate.java:1989)
Looking at the mongo-java-server code. It looks like $arrayElemAt is expecting a List type, but the $split return type is a String[]:
Activity
Fix return type of $split aggregation #214
bwaldvogel commentedon May 31, 2023
Thanks for the bug report. This should be fixed via 5e478bc.
Please note that your example aggregation doesn’t work as you probably expected with a real MongoDB either. At least in my tests with MongoDB
5.0.18and6.0.6. Can you confirm?TBH, this behavior surprised me. I would be interested if the behavior is documented?
bwaldvogel commentedon Jun 25, 2023
The fix is available via
v1.44.0Fix return type of $split aggregation bwaldvogel#214