在MongoDB中，当文档具有嵌入式文档时，为什么更新/查找精确文档失败？

Question

I'm using pymongo for reference.

So if I have a document like this:

{
    'name': 'bill',
    'color': 'blue',
    'subdocument': {
        'title': 'Untitled',
        'content': 'Hello World',
    }
}

If I find this document doing db.users.find()[0] and then pass it as an argument to update, Mongo will not find the document to update unless I remove the subdocument. Passing the found document to find_one also fails unless the subdocument is removed.

I need to specify all the fields for the sake of optimized locking. How can I easily retrieve the document, update it, and pass the original fields as requirements for not having been changed?

Answer 1

Since I primarily was interested in this for the purpose of optimistic locking, I have found a better solution than my original attempt, thanks to Scott from the MongoDB Users Group

The solution is to use a "version" counter field, and only update if the field has not been changed. Upon each update of the document, the query doing the updating should increment the counter. This is faster , since all of the fields don't have to be scanned and compared for the change to be made.

Answer 2

I believe you are encountering a deep bug in MongoDB . You can see a similar issue in this bug .

Embedded documents are compared as binary rather than compared as documents. So subdocument: {a:1,b:2} is different from subdocument:{b:2,a:1} even though they should not be.

See this comment for why this may be a problem.

Of course, that first bug is both serious and well over a year old, so it's questionable that this will be fixed any time soon. In fact, it's currently unscheduled.

Answer 3

Did you put a '$set' term in?

Here's what I did:

d = {
    'name': 'bill',
    'color': 'blue',
    'subdocument': {
        'title': 'Untitled',
        'content': 'Hello World',
    }
}

db.name.insert(d)
f = db.name.find_one({'name':'bill'})
print f
{u'color': u'blue', u'_id': ObjectId('4f8dbc834647df1428000002'), u'name': u'bill', u'subdocument': {u'content': u'Hello World', u'title': u'Untitled'}}

db.name.update(f, {'$set':{'name':'fred'}})

f = db.name.find_one({'name':'fred'})
print f
{u'color': u'blue', u'_id': ObjectId('4f8dbc834647df1428000002'), u'name': u'fred', u'subdocument': {u'content': u'Hello World', u'title': u'Untitled'}}