如何使用猫鼬只返回第三级的子文档？

Question

我具有以下文档结构：

{ 
    "_id" : ObjectId("5ccdccc61b704939740f6ad7"), 
    "Keywords" : [
        "Dine Out", 
        "Dine Out"
    ], 
    "NameAR" : "Dine Out", 
    "NameEN" : "Dine Out", 
    "DescriptionAR" : "Dine Out", 
    "DescriptionEN" : "Dine Out", 
    "Type" : "main", 
    "SubCategories" : [
        {
            "Keywords" : [
                "Fast food", 
                "Fast food"
            ], 
            "_id" : ObjectId("5ccdccc61b704939740f6adb"), 
            "NameAR" : "Fast food", 
            "NameEN" : "Fast food", 
            "DescriptionAR" : "Fast food", 
            "DescriptionEN" : "Fast food", 
            "Type" : "Outlet", 
            "SubCategories" : [
                {
                    "Keywords" : [

                    ], 
                    "_id" : ObjectId("5ccdccc61b704939740f6add"), 
                    "NameAR" : "Burger", 
                    "NameEN" : "Burget", 
                    "DescriptionAR" : "Burget", 
                    "DescriptionEN" : "Burget", 
                    "Type" : "Item", 
                    "SubCategories" : [

                    ]
                }, 
                {
                    "Keywords" : [

                    ], 
                    "_id" : ObjectId("5ccdccc61b704939740f6adc"), 
                    "NameAR" : "Fries", 
                    "NameEN" : "Fries", 
                    "DescriptionAR" : "Fries", 
                    "DescriptionEN" : "Fries", 
                    "Type" : "Item", 
                    "SubCategories" : [

                    ]
                }
            ]
        }, 
        {
            "Keywords" : [
                "Coffee Shops", 
                "Coffee Shops"
            ], 
            "_id" : ObjectId("5ccdccc61b704939740f6ad8"), 
            "NameAR" : "Coffee Shops", 
            "NameEN" : "Coffee Shops", 
            "DescriptionAR" : "Coffee Shops", 
            "DescriptionEN" : "Coffee Shops", 
            "Type" : "Outlet", 
            "SubCategories" : [
                {
                    "Keywords" : [
                        "black coffee", 
                        "green tea"
                    ], 
                    "_id" : ObjectId("5ccdccc61b704939740f6ada"), 
                    "NameAR" : "Coffee", 
                    "NameEN" : "Coffee", 
                    "DescriptionAR" : "Coffee", 
                    "DescriptionEN" : "Coffee", 
                    "Type" : "Item", 
                    "SubCategories" : [

                    ]
                }, 
                {
                    "Keywords" : [

                    ], 
                    "_id" : ObjectId("5ccdccc61b704939740f6ad9"), 
                    "NameAR" : "cheese cake", 
                    "NameEN" : "cheese cake", 
                    "DescriptionAR" : "cheese cake", 
                    "DescriptionEN" : "cheese cake", 
                    "Type" : "Item", 
                    "SubCategories" : [

                    ]
                }
            ]
        }
    ], 
    "__v" : NumberInt(0)
}

我已完成以下查询：

Category.findOne({ "SubCategories.SubCategories.Keywords": "black coffee" });

该查询工作正常，但返回了整个文档，我只想返回第3级的以下对象。

{
                    "Keywords" : [
                        "black coffee", 
                        "green tea"
                    ], 
                    "_id" : ObjectId("5ccdccc61b704939740f6ada"), 
                    "NameAR" : "Coffee", 
                    "NameEN" : "Coffee", 
                    "DescriptionAR" : "Coffee", 
                    "DescriptionEN" : "Coffee", 
                    "Type" : "Item", 
                    "SubCategories" : [

                    ]
                }

我尝试过

Category.findOne（{{“ SubCategories.SubCategories.Keywords”：“黑咖啡”}，{'SubCategories.SubCategories。$ ._ id'：1}）;

，但它仅返回第二级，而不是第三级。有什么线索吗？