如何在聚合中按字符限制字符串字段？

Question

我的文档看起来像这样：

[
{
   "_id" : ObjectId("5e41877df4cebbeaebec5146"),
   "title": "this is a title",
   "Paragraph" : "My Name is John Smith.I am learning MongoDB database"
},
{
   "_id" : ObjectId("5e4187d7f4cebbeaebec5147"),
   "title": "this is a title",
   "Paragraph" : "David Miller is a good student and learning Spring and Hibernate Framework."
}
]

我想将段落字段文本限制为 6 个字符，如下所示：

[
{
   "_id" : ObjectId("5e41877df4cebbeaebec5146"),
   "title": "this is a title",
   "Paragraph" : "My Nam"
},
{
   "_id" : ObjectId("5e4187d7f4cebbeaebec5147"),
   "title": "this is a title",
   "Paragraph" : "David "
}
]

我试过这段代码，但它删除了所有其他字段：

{
        $project:         {
            Paragraph: { $substr: [ "$Paragraph", 0, 6] }
            }      
}

Answer 1

{
        $set:         {
            Paragraph: { $substr: [ "$Paragraph", 0, 6] }
            }      
}