Update mongo array elements by index with c-driver

Question

To update elements of a mongo array, I was using syntax like:

{"$set":{"a.0":1238},{"a.1":402}}

Or, more accurately, I was using the C-driver function calls that I think are equivalent to that. This seemed to work fine, but when I look at the object in MongoHub, I see:

a: {"0":1238,"1":402}

instead of:

a: [1238,402]

Does anyone know what is proper syntax to access array elements by index with C-driver? What I am doing now serves my immediate purpose, but I am not sure if there are significant performance differences. Also, I might later need to use operations that require true array.

Might be wrong but have you tried syntax like {"$set":{"a": {1238, 402}}} — Hank
– Hank, Commented Mar 3, 2012 at 3:14

Sergio Tulentsev · Accepted Answer · 2012-03-03 03:18:40Z

5

If a field didn't exist, then this dot-notation query will create it as a hash (object) and assign values to keys of that hash. If field exists and is an array, it will behave as you expect. See this session.

> db.arrays.insert({});
> db.arrays.find();
{ "_id" : ObjectId("4f518c8b58713e4dbadbfb9f") }
> db.arrays.update({ "_id" : ObjectId("4f518c8b58713e4dbadbfb9f") }, {$set: {"a.0": 123}});
> db.arrays.find();
{ "_id" : ObjectId("4f518c8b58713e4dbadbfb9f"), "a" : { "0" : 123 } }


> db.arrays.insert({a: []})
> db.arrays.find();
{ "_id" : ObjectId("4f518c8b58713e4dbadbfb9f"), "a" : { "0" : 123 } }
{ "_id" : ObjectId("4f518cca58713e4dbadbfba0"), "a" : [ ] }
> db.arrays.update({ "_id" : ObjectId("4f518cca58713e4dbadbfba0") }, {$set: {"a.0": 123}});
> db.arrays.find();
{ "_id" : ObjectId("4f518c8b58713e4dbadbfb9f"), "a" : { "0" : 123 } }
{ "_id" : ObjectId("4f518cca58713e4dbadbfba0"), "a" : [ 123 ] }

answered Mar 3, 2012 at 3:18

Sergio Tulentsev

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1 Comment

user1055568 Over a year ago

Thanks, this works. As long as I use explicit array syntax to initialize the field, then subsequent updates work.

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