An empty update path is not valid

[peacekeeper]

After upgrade of MongoDB from 2.4 to 2.6, empty "" indices seem to be no longer allowed in a collection, e.g.:

db.inventory.update(   { _id: ObjectId("556eb937d2c2ca09af5298e6") },   { $addToSet: { "": "accessories" } })

WriteResult({
    "nMatched" : 0,
    "nUpserted" : 0,
    "nModified" : 0,
    "writeError" : {
        "code" : 56,
        "errmsg" : "An empty update path is not valid."
    }
})

This is a problem, because the XDI address of a context node is used in the JSON objects stored in the MongoDB, and the XDI address of the common root node is the empty string.

Solution: Downgrade MongoDB to 2.4.x

Also see
https://github.com/projectdanube/xdi2-mongodb/wiki/Graph%20Factory%20Flags

[peacekeeper]

Also see
https://jira.mongodb.org/browse/SERVER-6852