Spring data mongodb Aggregation framework Invalid reference

I use: 1. Mongo 3.4v database. (I am new in mongo) 2. Spring boot 1.5.1.RELEASE (spring data mongodb 1.10.0.RELEASE)

I have database with two collections 'categories' and 'ads' (advertisements)

Categories

{
"_id" : ObjectId("58a4b39eca0cdc1ba88d9bf5"),
"_class" : "com.mongotest.model.Category",
"title" : {
    "en" : "My title"
},
"parentId" : ObjectId("58b1fca82dc4b81365110fe4")
}

Ads

{
"_id" : ObjectId("58a62e4a27dccb1bec742052"),
"_class" : "com.mongotest.model.Ads",
"categoryId" : ObjectId("58a4b39eca0cdc1ba88d9bf5"),
"description" : "Description",
"phone" : "29890000",
"email" : "[email protected]",
"price" : "65.0"
}

I have created mongo query which return for each category array of subcategories and count of ads for each subcategory.

This mongo query looks like this (working 100% i was double check it before question):

db.categories.aggregate([
{
    $match : {
       "parentId": null     
    }    
},
{
    $lookup:{
        from: "categories",
        localField: "_id",
        foreignField: "parentId",
        as: "subcategories"
    }   
},
{ 
    $unwind : { 
        path : "$subcategories",
        preserveNullAndEmptyArrays : true
    }
},
{
    $lookup:{
        from: "ads",
        localField: "subcategories._id",
        foreignField: "categoryId",
        as: "subcategories.adsList"
    }
},
{
    $project : {                
        "title" : "$title",
        "subcategories._id" : "$subcategories._id",
        "subcategories.title" : "$subcategories.title",
        "subcategories.adsCount" : {$size : "$subcategories.adsList"}
    }
},
{
    $group : { 
        "_id" : "$_id",
        "title" : { $first : "$title" },
        "subcategories" : { $push: "$subcategories" }
    }
}])

Here is my translation to Spring mongo data Aggregation framework:

    AggregationOperation match = Aggregation.match(Criteria.where("parentId").is(null));
    AggregationOperation lookup = Aggregation.lookup("categories", "_id", "parentId", "subcategories");
    AggregationOperation unwindSubcategories = Aggregation.unwind("subcategories", true);
    AggregationOperation lookupAds = Aggregation.lookup("ads", "subcategories._id", "categoryId", "subcategories.adsList");
    AggregationOperation project = Aggregation.project().and("subcategories._id").as("subcategories._id").and("title").as("title").and("subcategories.title").as("subcategories.title").and("subcategories.adsList").size().as("adsCount");
    AggregationOperation group = Aggregation.group("_id").first("title").as("title").push("subcategories").as("subcategories");
    Aggregation aggregation = Aggregation.newAggregation(match, lookup, unwindSubcategories, lookupAds, project, group); //Invalid reference 'subcategories'
    AggregationResults<CategoryTO> aggregate = mongoTemplate.aggregate(aggregation, Category.class, CategoryTO.class); 

Problem is in group operation

In this case i receive Invalid reverence 'subcategories'. I can't understand why 'subcategories' reference is unavailable in push() method if in project() operation it is available?!

How I can solve it?

Also i know about related issues Spring Data MongoDB - Aggregation Framework - invalid reference in group Operation

Spring Data MongoDB: aggregation framework - sort with nested property throws invalid reference

Many thanks for the feedback.