在 MongoDB 中更新文档时如何插入字段？

Question

I have a collection that looks like this:

{ 
    "_id" : ObjectId("5e5777b74db43342073051e0"), 
    "a" : "b"
}
{ 
    "a" : "c", 
    "_id" : ObjectId("5e5777f24db43342073051e4")
}

And I want to copy the value of each document's a into a new array field so I end up with this:

{ 
    "_id" : ObjectId("5e5777b74db43342073051e0"), 
    "a" : "b",
    "d": ["b"]
}
{ 
    "_id" : ObjectId("5e5777f24db43342073051e4"),
    "a" : "c", 
    "d": ["c"]
}

I tried this command at the shell:

db.getCollection("C").updateMany({}, {
    $set: {
        "d.0": "$a"
    }
});

But that gave me:

{ 
    "_id" : ObjectId("5e5777b74db43342073051e0"), 
    "a" : "b", 
    "d" : {
        "0" : "$a"
    }
}
{ 
    "_id" : ObjectId("5e5777f24db43342073051e4"), 
    "a" : "c", 
    "d" : {
        "0" : "$a"
    }
}

$$a gave me the same result. How do I write this operation?

Answer 1

In Mongo 4.2 the <update> document can also be an aggregation pipeline. Try this one:

db.getCollection("C").updateMany(
   {},
   [{
      $set: {
         "d": ["$a"]
      }
   }]
)