MongoDB: Too many positional (i.e. '$') el

问题:

I just upgraded to Mongo 2.6.1 and one update statement that was working before is not returning an error. The update statement is:

db.post.update( { 'answers.comments.name': 'jeff' },
    { '$set': {
        'answers.$.comments.$.name': 'joe'
    }},
    { multi: true }
)

The error I get is:

WriteResult({
    "nMatched" : 0,
    "nUpserted" : 0,
    "nModified" : 0,
    "writeError" : {
        "code" : 2,
        "errmsg" : "Too many positional (i.e. '$') elements found in path 'answers.$.comments.$.createUsername'"
    }
})

When I update an element just one level deep instead of two (i.e. answers.$.name instead of answers.$.comments.$.name), it works fine. If I downgrade my mongo instance below 2.6, it also works fine.

回答1:

The positional operator can be used only once in a query. This is a limitation, there is an open ticket for improvement: https://jira.mongodb.org/browse/SERVER-831

回答2:

As mentioned; more than one positional elements not supported for now. You may update with mongodb cursor.forEach() method.

db.post
  .find({"answers.comments.name": "jeff"})
  .forEach(function(post) {
    if (post.answers) {
      post.answers.forEach(function(answer) {
        if (answer.comments) {
          answer.comments.forEach(function(comment) {
            if (comment.name === "jeff") {
              comment.name = "joe";
            }
          });
        }
      });

      db.post.save(post);
    }
  });

回答3:

I have faced the same issue for the as array inside Array update require much performance impact. So, mongo db doest not support it. Redesign your database as shown in the given link below.

https://pythonolyk.wordpress.com/2016/01/17/mongodb-update-nested-array-using-positional-operator/

回答4:

You CAN do this, you just need Mongo 3.6! Instead of redesigning your database, you could use the Array Filters feature in Mongo 3.6, which can be found here:

https://thecodebarbarian.com/a-nodejs-perspective-on-mongodb-36-array-filters

The beauty of this is that you can bind all matches in an array to a variable, and then reference that variable later. Here is the prime example from the link above:

回答5:

db.post.update( { 'answers.comments.name': 'jeff' },
    { '$set': {
        'answers.$.comments.$.name': 'joe'
    }},
    { multi: true }
)

Answer is

db.post.update( { 'answers.comments.name': 'jeff' },
    { '$set': {
        'answers.0.comments.1.name': 'joe'
    }},
    { multi: true }
)