MongoDB embedded Document Array: Get only one embedded document with a spezific attribute - spring

Question

I want to get one Embedded Document with a specific field (version) from an array with mongodb and spring boot.
This is the data structure:
{
"_id": 5f25882d28e40663719d0b52,
"versions": [
{
"versionNr": 1
"content": "This is the first Version of some Text"
},
{
"versionNr": 2
"content": "This is the second Version of some Text"
},
...
]
...
}
Here are my entities:
#Data
#Document(collection = "letters")
public class Letter {
#Id
#Field("_id")
private ObjectId _id;
#Field("versions")
private List<Version> versions;
}
//There is no id for embedded documents
#Data
#Document(collection = "Version")
public class Version{
#Field("content")
private String content;
#Field("version")
private Long version;
}
And this is the query that doesn't work. I think the "join" isn't correct. But can't figure out the right way.
public Optional<Version> findByIdAndVersion(ObjectId id, Long version) {
Query query = new Query(Criteria.where("_id").is(id).and("versions.version").is(version));
return Optional.ofNullable(mongoTemplate.findOne(query,Version.class,"letters"));
}
}
EDIT: This is a working Aggregation, I'm sure it isn't a pretty solution but it works
#Override
public Optional<Version> findByIdAndVersion(ObjectId id, Long version) {
MatchOperation match = new MatchOperation(Criteria.where("_id").is(id).and("versions.version").is(version));
Aggregation aggregate = Aggregation.newAggregation(
match,
Aggregation.unwind("versions"),
match,
Aggregation.project()
.andInclude("versions.content")
.andInclude("versions.version")
);
AggregationResults<Version> aggregateResult = mongoTemplate.aggregate(aggregate, "letters", Version.class);
Version version = aggregateResult.getUniqueMappedResult();
return Optional.ofNullable(mongoRawPage);
}

Answer 1

Query query = new Query(Criteria.where("_id").is(id).and("versions.version").is(version));
return Optional.ofNullable(mongoTemplate.findOne(query,Version.class,"letters"));
You are querying the Letter document but your entity class is specified as Version.class, since findOne from MongoDB doesn't return the subdocument by itself but rather the whole document, you need to have Letter.class as return type and filter (project) what fields to get back. So you can either project the single version subdocument that you want to receive, like so:
Query query = new Query()
.addCriteria(Criteria.where("_id").is(id).and("versions.version").is(version))
.fields().position("versions", 1);
Optional.ofNullable(mongoTemplate.findOne(query, Letter.class))
.map(Letter::getVersions)
.findFirst()
.orElse(null);
or use aggregation pipeline:
newAggregation(
Letter.class,
match(Criteria.where("_id").is(id)),
unwind("versions"),
replaceRoot("versions"),
match(Criteria.where("version").is(version))),
Version.class)
Note -- I typed this on a fly.