The upsert behaviour in update does not handle arrays the same way for insert and update.

Upsert is only useful in the case that you don't know if the object already exists.

I need a way to call upsert with arrays that creates the same document shape for both insert and update. This will allow efficient manipulation of array contents.

The example below is a simple demonstration of the problem, using the mongo shell.

Simplest version, mongo shell code. The first upsert adds to the 'arr' array, the second creates an 'arr' document, the third creates an 'arr' array.

 
db.onearray.insert({_id:"oa1",arr: [1]}) 
db.onearray.update({_id:"oa1"},{$set: {"arr.1":2}},true) 
db.onearray.update({_id:"oa2"},{$set: {"arr.1":3}},true) 
db.onearray.update({_id:"oa3"},{$set: {"arr":[4]}},true) 
db.onearray.find() 
{ "_id" : "oa1", "arr" : [ 1, 2 ] } 
{ "_id" : "oa2", "arr" : { "1" : 3 } } 
{ "_id" : "oa3", "arr" : [ 4 ] } 

Slightly more complex version.
The first upsert modifies the 'bottom' array. The second upsert creates a 'bottom' document with key "0". The third upsert creates a 'bottom' array.

 
db.twoarrays.insert({_id:"ta1",rnumber: 0,top:[{tnumber:1,bottom:[{a:1,b:2}]}]}) 
db.twoarrays.update({_id:"ta1"},{$set:{"top.0.bottom.0": {a:2,b:2}}},true) 
db.twoarrays.update({_id:"ta1"},{$set:{"top.1.bottom.0": {a:3,b:3}}},true) 
db.twoarrays.update({_id:"ta1"},{$set:{"top.2.bottom": [{a:4,b:4}]}},true) 
db.twoarrays.find()