Currently, mongoDB server creates object/hash by default during new record/element insertion when using dotted sub-paths inside $set:{} command:

collection.update(  
       { <search.filter> },
       { {$set:{"doc.1":{<doc-data>}} },
       { safe:true, upsert:true},
       function(err, ...

In this case, the following record is created:
{ "_id":<XXX>, ... , "doc" :

}

However, if the record is already created with doc=[] (array) element then the above update leads to an array update procedure.

MongoDB server should not desire by default that the doc is an object {} during record/element creation, because it can be an array too. Please add a new option like "CreateArrayOnInsert:true" so mongoDB server can create arrays[] if the element/record doesn't exist, like this:

collection.update(  
       { <search.filter> },
       { {$set:{"doc.1":{<doc-data>}} },
       { safe:true, upsert:true, CreateArrayOnInsert:true},
       function(err, ...

-> this should create a record with the following elements: