如何在Spring中编写此Mongo聚合查询？

Question

I've got an aggregation query in MongoDB that works when I run it directly in the shell. Here's the shell query:

db.MyCollection.aggregate([
    {$match: {_id: {$in: ['A', 'B', 'C']}}},
    {$project: {"versions": "$nested.field.version"}},
    {$unwind: "$versions"},
    {$group: {_id: "$_id", "maxVersion": {$max: "$versions"}}}
])

So as you can see, this does the following:

1. Matches only certain documents with specified IDs
2. Projects a nested field down to a base-level field (and effectively filters out all other fields from the pipeline, but still retaining the IDs)
3. Unwinds the array elements of the $versions field that we projected into individual documents in the pipeline
4. Finds the max value of those $versions per ID

Like I said, that query above already works. My question is how to translate that into Spring MongoDB syntax. Here's my first attempt, which does not work:

Aggregation aggregation = newAggregation(
    match(Criteria.where("_id").in(listOfIds))
    ,project().and("versions").nested(bind("versions", "nested.field.version"))
    ,unwind("versions")
    ,group("_id").max("versions").as("maxVersion")
);

When I try to run the code in debug mode, I can see that I actually get an IllegalArgumentException on newAggregation saying it cannot evaluate. If I comment out the line with the $group clause, then I can see this toString() representation of aggregation variable, which reveals a problem with the $project clause:

{
  "aggregate" : "__collection__" ,
  "pipeline" : [
    { "$match" : { "_id" : { "$in" : [ "A" , "B" , "C"]}}} ,
    { "$project" : { "versions" : { "versions" : "$nested.field.version"}}} ,
    { "$unwind" : "$versions"}
  ]
}

Clearly this doesn't match up with what I was intending so I'm not getting the syntax correct. But TBH I don't find the Spring MongoOps syntax very intuitive, and their documentation isn't great either.

I don't see any way to call that nested() method without first including the call to and() . I think that's the main problem since it's doubling up the nesting there. Are there any Spring MongoOps heroes here who can help me write the equivalent Java code properly?

EDIT: Here's a snapshot of the collection I'm using:

Answer 1

The $project pipeline is not necessary since you can still do an $unwind on the nested field, thus this aggregation pipeline can yield the same results as your current:

db.MyCollection.aggregate([
    {
        "$match": {
            "_id": { "$in": ['A', 'B', 'C'] }
        }
    },
    { "$unwind": "$nested.field" },
    {
        "$group": {
            "_id": "$_id", 
            "maxVersion": { "$max": "$nested.field.version" }
        }
    }
])

The Spring Data MongoDB aggregation equivalent:

Aggregation agg = newAggregation(
        match(Criteria.where("_id").in(ids)),
        unwind("nested.field"),        
        group("_id").max("nested.field.version").as("maxVersion")
    );

Back to your current aggregation, you need to $unwind on nested.field array, not nested.field.version field since that is a String, not array:

db.MyCollection.aggregate([
    {$match: {_id: {$in: ['A', 'B', 'C']}}},
    {$project: {"fields": "$nested.field"}},
    {$unwind: "$fields"},
    {$group: {_id: "$_id", "maxVersion": {$max: "$fields.version"}}}
])

the Sprind Data MongoDB equivalent would look like:

Aggregation agg = newAggregation(
        match(Criteria.where("_id").in(ids)),
        project().and("nested.field").as("fields")
        unwind("fields"),        
        group("_id").max("fields.version").as("maxVersion")
    );

Answer 2

use map reduce way before the underscore bug is fixed. like :

GroupBy groupBy = GroupBy.key("user_id")
        .initialDocument("{ total : 0, used : 0 }")
        .reduceFunction("function( curr, result ){ result.total++; if(curr.status == 1) {result.used++;} result.userID = curr.user_id;");
        GroupByResults<YourResult> yourResultInfo =
                mongoTemplate.group(Criteria.where("user_id").in(user_ids),
                                "your_collection_name", groupBy, YourResult.class);

class YourResult{
private String userID;
    private Long total = 0l;
    private Long used = 0l;
// getter and setter`enter code here
}

Answer 3

Spring uses _ as a wild cart for arrays, and split snake_case fields, when performing fields reference validation in aggregate operations.

To avoid validation, you can use the following MongoTemplate method, that performs aggregation, without fields transformation and validation.

public <O> AggregationResults<O> aggregate(Aggregation aggregation, String collectionName, Class<O> outputType)