Why MongoDB aggregation is rewritten by Spring Data in a wrong way? - spring-data-mongodb

Question

There is a collection
{
"field": {
"nested": 0
}
}
It is necessary to get a sum of nested fields. As direct operation like below is not allowed
{
"$group": {
"_id": null,
"field.nested": {
"$sum": "$field.nested"
}
}
}
It is done by the following aggregation with pre and post projection.
[
{
"$project": {
"fieldNested": "$field.nested"
}
},
{
"$group": {
"_id": null,
"fieldNested": {
"$sum": "$fieldNested"
}
}
},
{
"$project": {
"total": {
"field" : {
"nested" : "$fieldNested"
}
}
}
}
]
And it works correctly in the Mongo DB shell. The problem comes with using this aggregation in Spring Data in a json format. For example,
#Repository
public interface Sample extends MongoRepository<InData, String> {
#Aggregation({
"" +
"{" +
" '$project': {" +
" 'fieldNested': '$field.nested'" +
" }" +
"}",
"" +
"{" +
" '$group': {" +
" '_id': null," +
" 'fieldNested': {" +
" '$sum': '$fieldNested'" +
" }" +
" }" +
"}",
"" +
"{" +
" '$project': {" +
" 'total': {" +
" 'field': {" +
" 'nested': '$fieldNested'" +
" }" +
" }" +
" }" +
"}"
})
List<OutData> aggregateBy();
}
Somehow this json is rewritten by Spring Data so that the pipelines sent to the Mongo looks like the following
[
{
"$project": {
"field.nested": "$field.nested"
}
},
{
"$group": {
"_id": null,
"field.nested": {
"$sum": "$fieldNested"
}
}
},
{
"$project": {
"total": {
"field" : {
"nested" : "$fieldNested"
}
}
}
}
]
fieldNested is replaced by field.nested. As a result the aggregation fails.
Why it happens? Is there any way to avoid this rewrite by Spring Data for MongoDB (Spring Boot 2.2.13.RELEASE, MongoDB 3.6.8)?

Answer 1

Spring Data MongoDB is converting fieldNested to field.nested because your InData class & its collection is having fields in that hierarchy. If you want to project that nested field using #Aggregation, then project it using any other name that is not part of your InData class & its collection's fields so Spring Data won't convert it.