Javascript Mongo Find/Aggregate将子数组返回为完整数组，将某些子数组返回为空数组

我很绝望：( 我有一个只包含一个文档的集合，其大小约为~200MB。 并且文档包含子数组作为属性。这意味着文档具有下一个形状：

    {
        "_id": ........
        "time": .......
        "categories": [
    {
        "name":"Cat1",
        "subCategories": 
        [
           {
               "name2": "SubCat11"
           },
           {
               "name2": "SubCat12"
           } 
        ]
    }, 
    {
        "name":"Cat2",
        "subCategories": 
        [
           {
               "name2": "SubCat21"
           },
           {
               "name2": "SubCat22"
           } 
        ]
    }, 
    {
        "name":"Cat3",
        "subCategories": 
        [
           {
               "name2": "SubCat31"
           },
           {
               "name2": "SubCat32"
           } 
        ]
    }, 
]

}

我尝试使用（aggregate/find）获取所有文档数据及其所有子数据，但是，返回的数据确实包含所有“subCategories”属性，并且只有前3个“subCategories”完整数组和后30个为空数组

我的问题是：

Model.find({}); 

返回值为：

{
        "_id": ........
        "time": .......
        "categories": [
    {
        "name":"Cat1",
        "subCategories": 
        [
           {
               "name2": "SubCat11"
           },
           {
               "name2": "SubCat12"
           } 
        ]
    }, 
    {
        "name":"Cat2",
        "subCategories": 
        [
           {
               "name2": "SubCat21"
           },
           {
               "name2": "SubCat22"
           } 
        ]
    }, 
    {
        "name":"Cat3",
        "subCategories": 
        [
        ] // >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> HERE IS THE BUG, THE EMPTY ARRAY !
    }, 
]

}

如果结果需要：

        {
        "_id": ........
        "time": .......
        "categories": [
    {
        "name":"Cat1",
        "subCategories": 
        [
           {
               "name2": "SubCat11"
           },
           {
               "name2": "SubCat12"
           } 
        ]
    }, 
    {
        "name":"Cat2",
        "subCategories": 
        [
           {
               "name2": "SubCat21"
           },
           {
               "name2": "SubCat22"
           } 
        ]
    }, 
    {
        "name":"Cat3",
        "subCategories": 
        [
           {
               "name2": "SubCat31"
           },
           {
               "name2": "SubCat32"
           } 
        ]
    }, 
]

}

我无法更改数据库的结构，我需要帮助：）