mongodb positional operator error

You need to include the array field as part of the query document in order to use the positional operator.

For example, if you want to update the first array element i.e. with { "attributes.name": "x" } then you could follow the pattern:

db.spaces.update(
   { "attributes.name": "x" }, // <-- the array field must appear as part of the query document.
   { "$set": { "attributes.$.weight": 2 } },
   { "multi": true }
)

For the newer MongoDB versions 3.2.X, you could use the updateMany() method to update multiple documents within the collection based on the filter above.

The positional operator needs a match, from the match part of your update query.

for example:

db.spaces.update({ "attributes.name": "x" }, { $set: { "attributes.0.weight": 2 } }, {multi: true})

here the first parameter for update operation will match the array attributes where any element has a property name=="x", for any element that matches the condition the position operator can be used to update it.

So, because name='x', in this case the first matching element would be,

{
            "name" : "x",
            "color" : "0xd79c9c",
            "_id" : ObjectId("5742be02289512cf98bf63e8")
        }, 

and it will get updated.

Now from your question I understand you want to update the document in a way that in each document your first element, attribute gets a new value for weight=2.

you can do something like

db.spaces.update({ "attributes.name": { $regex: /^(?=[\S\s]{10,8000})[\S\s]*$/ } }, { $set: { "attributes.0.weight": 2 } }, {multi: true})

What we do here is match all element in array attribute. and we use the positional operator to update the first element of that array