how to update sequence number in mongodb safely

问题

In my mongodb database, there is a 'messages' collection, it has a field 'order' which values integer

msg1.order=1, msg2.order=2, msg3.order=3, msg4.order=4, msg5.order=5, ... 

for each message, represents an ordered sequence of a sub-collection of messages.

while, these messages can be re-sorted, via web page, using jquery.sortable. for example, If I move message on position No.3 to No.1, then I should change 'order' values to

msg1.order=2, msg2.order=3, msg3.order=1, msg4.order=4, msg5.order=5, ...

. are there any mongodb modifiers or other means of doing such update so that I can do such update in one step, or in a safe way?

3 sample documents:

{
"author_id": "a",
"class": "principle",
"content_id": null,
"host_id": null,
"modified_date": 1330935540,
"order": 1,
"pub_date": 1330935540,
"score": 0,
"text": "Hello World！",
"vote_down_count": 0,
"vote_up_count": 0
}

{
  "author_id": "a",
  "class": "principle",
  "content_id": null,
  "host_id": null,
  "modified_date": 1330935538,
  "order": 2,
  "pub_date": 1330935538,
  "score": 0,
  "text": "Nice to meet you.",
  "vote_down_count": 0,
  "vote_up_count": 0
}
{
  "author_id": "a",
  "class": "principle",
  "content_id": null,
  "host_id": null,
  "modified_date": 1330935548,
  "order": 3,
  "pub_date": 1330935548,
  "score": 0,
  "text": "Great!",
  "vote_down_count": 0,
  "vote_up_count": 0
}

回答1:

In order to do this atomically, all your three sample documents need to be part of the same document. MongoDB only does operations atomically on simple documents: http://www.mongodb.org/display/DOCS/Atomic+Operations

If they are part of a single document, the following would change the order of the 2nd and 3rd subdocument:

> db.so.find().pretty();
{
    "_id" : ObjectId("4f55e7ba362e2f2a734c92f8"),
    "subs" : [
        {
            "author_id" : "a",
            "class" : "principle",
            "content_id" : null,
            "host_id" : null,
            "modified_date" : 1330935540,
            "order" : 1,
            "pub_date" : 1330935540,
            "score" : 0,
            "text" : "Hello World！",
            "vote_down_count" : 0,
            "vote_up_count" : 0
        },
        {
            "author_id" : "a",
            "class" : "principle",
            "content_id" : null,
            "host_id" : null,
            "modified_date" : 1330935538,
            "order" : 2,
            "pub_date" : 1330935538,
            "score" : 0,
            "text" : "Nice to meet you.",
            "vote_down_count" : 0,
            "vote_up_count" : 0
        },
        {
            "author_id" : "a",
            "class" : "principle",
            "content_id" : null,
            "host_id" : null,
            "modified_date" : 1330935548,
            "order" : 3,
            "pub_date" : 1330935548,
            "score" : 0,
            "text" : "Great!",
            "vote_down_count" : 0,
            "vote_up_count" : 0
        }
    ]
}

Query:

db.so.update(
    { _id: new ObjectId("4f55e7ba362e2f2a734c92f8")},
    { $set : { 'subs.1.order' : 3, 'subs.2.order' : 2 } }
);

Result:

> db.so.find().pretty();
{
    "_id" : ObjectId("4f55e7ba362e2f2a734c92f8"),
    "subs" : [
        {
            "author_id" : "a",
            "class" : "principle",
            "content_id" : null,
            "host_id" : null,
            "modified_date" : 1330935540,
            "order" : 1,
            "pub_date" : 1330935540,
            "score" : 0,
            "text" : "Hello World！",
            "vote_down_count" : 0,
            "vote_up_count" : 0
        },
        {
            "author_id" : "a",
            "class" : "principle",
            "content_id" : null,
            "host_id" : null,
            "modified_date" : 1330935538,
            "order" : 3,
            "pub_date" : 1330935538,
            "score" : 0,
            "text" : "Nice to meet you.",
            "vote_down_count" : 0,
            "vote_up_count" : 0
        },
        {
            "author_id" : "a",
            "class" : "principle",
            "content_id" : null,
            "host_id" : null,
            "modified_date" : 1330935548,
            "order" : 2,
            "pub_date" : 1330935548,
            "score" : 0,
            "text" : "Great!",
            "vote_down_count" : 0,
            "vote_up_count" : 0
        }
    ]
}

来源：https://stackoverflow.com/questions/9566847/how-to-update-sequence-number-in-mongodb-safely