Pymongo how to properly use $push to update an existing document

问题

I am doing something wrong or I don't understand $push (probably don't understand). Anyway I have the following document:

{ "_id" : ObjectId("501c83051d41c8753e000000"), 
   "node" : "denver", 
   "enc" : { "environment" : "production", "classes" : { "denver" : "" } }, 
   "inherit" : "default" }

And I am trying to make the document

{ "_id" : ObjectId("501c83051d41c8753e000000"), 
  "node" : "denver", 
  "enc" : { "environment" : "production", 
  "classes" : { "denver" : "", "boulder" : ""} }, 
  "inherit" : "default" }

This is what my update statement looks like:

col.update({ 'node' : 'denver'}, 
           { '$push': { 'enc.classes' : {'boulder': ''}}},
           True)

I don't get an error but the document never updates. If I change $push to $set then the denver is replaced with boulder.

Thanks for any assistance.

回答1:

This query works.

db.foo.update({"node": "denver"}, {"$set": {"enc.classes.boulder": ""}}

回答2:

$push does not work in this case because you are trying to use an array function on an object.

To use $push you would need to change your data structure to the following:

{
    "_id" : ObjectId("501c83051d41c8753e000000"), 
    "node" : "denver", 
    "enc" : {
        "environment" : "production", 
        "classes" : [
            "denver"
        ]
    }, 
    "inherit" : "default"
}

Then your query would be:

col.update(
    {
        'node' : 'denver'
    },
    {
        '$push': {
            'enc.classes' : 'boulder'
        }
    },
    True
)

来源：https://stackoverflow.com/questions/11809663/pymongo-how-to-properly-use-push-to-update-an-existing-document