Invalid reference when using group on lookup pipeline

[ghimisradu]

Running a lookup with pipeline and including an group stage results in invalid reference for the group id field:
org.springframework.dao.InvalidDataAccessApiUsageException: Invalid reference 'myField'

Aggregation.lookup()
    .from('my-collection')
    .let(ExpressionVariable.newVariable("variable").forField("$myObject.field"))
    .pipeline(
        Aggregation.match(Expr.valueOf(Eq.valueOf("$myField").equalTo("$$variable"))),
        Aggregation.group("$myField")
    )
    .as("myObjects")

Running the same stage in mongo shell returns the correct results.

{
  "$lookup": {
      "from": "my-collection",
      "let": {
          "variable": "$myObject.field"
      },
      "pipeline": [{
              "$match": {
                  "$expr": {
                      "$eq": ["$myField", "$$variable"]
                  }
              }
          }, {
              "$group": {
                  "_id": "$myField"
              }
          }
      ],
      "as": "myObjects"
  }
}

The objects from target collection my-collection look like this:

{
  _id: ObjectId,
  myField: String,
  _class: String
}

If I remove the group field reference the stage works, but of course wrong results will be returned

• Spring boot version: 3.1.3
• Spring data mongodb version: 4.1.3
• Reactive driver version: 4.9.1

[christophstrobl]

@ghimisradu Thank you for getting in touch. It would be great if you could take the time to provide a complete minimal sample (something that we can unzip or git clone, build, and deploy) that reproduces the problem so we can also see the domain types involved and how to aggregation is triggered.

[spring-projects-issues]

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