In 2.0.8, createNewFromQuery performs the following check to see if a
query field represents a value that should be incorporated in the
upsert document, or an operator:

if ( e.type() == Object && e.embeddedObject().firstElementFieldName()[0] == '$' ) {
    // this means this is a $gt type filter, so don't make part of the new object continue;
}

In 2.2.2, that check became the following:

if ( e.type() == Object && e.embeddedObject().firstElementFieldName()[0] == '$' ) {
    // we have something like { x : { $gt : 5 } }
    // this can be a query piece
    // or can be a dbref or something
 
    int op = e.embeddedObject().firstElement().getGtLtOp( -1 );
    if ( op >= 0 ) {
        // this means this is a $gt type filter, so don't make part of the new object continue;
    }
}

Now we only consider the object field to represent an operator if
getGtLtOp() can interpret the field name as an operator. However,
this is not a complete check because getGtLtOp() only understands
simple field-value operators. It does not parse $not (or $and, etc.).
This means that the {$not: {$elemMatch:

}} portion of the
query is interpreted by the upsert implementation a value, not an
operator. The upserted document starts with a value of the 'r' field
of {$not: {$elemMatch:

}}. $push cannot operate on this value
because it is not an array, which causes the error the user reported.

Aaron

------------------------------------

In 2.2.x $elemMatch breaks $push when used inside of $not and with an upsert. This is useful is you want to push only when an existing property doesn't exist inside of an array.

An update in 2.0 such as:
db.test.update({_id: 1, r: {$not: {$elemMatch: {"a": 1}}}}, {$push: {r:

}},

);
or even:
db.test.update({_id: 1, r: {$not: {$elemMatch: {$or: [

,

]}}}}, {$push: {r:

}},

);
Would successfully create a new document:
{ "_id" : 1, "r" : [

] }

If you called either of those updates again, it would expectantly throw a 11000 error:
E11000 duplicate key error index: test.test.$id dup key: { : "hello" }
Which meant that there already was a value inside "r" with either

or

.

Now in 2.2 either of those updates fails with error:
Cannot apply $push/$pushAll modifier to non-array

There was a proposed workaround (using $nin): http://stackoverflow.com/questions/12391334/mongodb-upsert-with-push-and-not
But that doesn't actually work. The $nin fails to actually look inside the array, which is expected as it wouldn't seem that it would anyways.