ren@ren-desktop ~/mongo-copy $ ./mongo --writeMode legacy
MongoDB shell version: 2.6.0-rc1-pre-
connecting to: test
> db.user.drop()
true
> db.user.insert({ a: { "": 2 }})
> db.user.find()
{ "_id" : ObjectId("531f798aa676eb5ad3d44332"), "a" : { "" : 2 } }
> db.user.insert( { "": 2 })
> db.user.find()
{ "_id" : ObjectId("531f798aa676eb5ad3d44332"), "a" : { "" : 2 } }
{ "_id" : ObjectId("531f7a27a676eb5ad3d44333"), "" : 2 }

ren@ren-desktop ~/mongo-copy $ ./mongo
MongoDB shell version: 2.6.0-rc1-pre-
connecting to: test
> db.user.insert( { "": 3 })
WriteResult({ "nInserted" : 1 })
> db.user.find()
{ "_id" : ObjectId("531f798aa676eb5ad3d44332"), "a" : { "" : 2 } }
{ "_id" : ObjectId("531f7a27a676eb5ad3d44333"), "" : 2 }
{ "_id" : ObjectId("531f7a3248b214d47bdddd04"), "" : 3 }
> db.user.update({ }, { "": 4 })
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
> db.user.find()
{ "_id" : ObjectId("531f798aa676eb5ad3d44332"), "" : 4 }
{ "_id" : ObjectId("531f7a27a676eb5ad3d44333"), "" : 2 }
{ "_id" : ObjectId("531f7a3248b214d47bdddd04"), "" : 3 }
> db.user.ensureIndex({ "": 1 })
WriteResult({
	"nInserted" : 0,
	"writeError" : {
		"code" : 67,
		"errmsg" : "bad index key pattern { : 1.0 }: Index keys cannot be an empty field."
	}
})
> db.user.ensureIndex({ "a.": 1 })
WriteResult({
	"nInserted" : 0,
	"writeError" : {
		"code" : 67,
		"errmsg" : "bad index key pattern { a.: 1.0 }: Index keys cannot contain an empty field."
	}
})
> db.user.find({ "": 3 })
{ "_id" : ObjectId("531f7a3248b214d47bdddd04"), "" : 3 }