Comparing 2 fields within a Mongo Aggregation Query - php

Question

I have a collection in my Mongo Database called WorkOrder with 2 fields DateComplete and DateDue. Using those 2 fields I'd like to use the aggregation framework to count the number of 'Late' Work Orders by comparing the two fields. However the research I've found hasn't had any useful ways to format the query so that the 'Late' Work Orders will be filtered through. Does anyone know of a way to format a Mongo DB Aggregation Query (preferably in PHP) that can compare 2 fields in the collection?
EDIT:
An example entry in WorkOrder might look like
_id
some mongo id
DateDue
2014-10-10
DateCompleted
2014-10-12
This entry would want to be filtered through since DateCompleted is greater than DateDue. I didn't know about the $cond operator so I haven't tried anything for that yet.
EDIT:
After trying #BatScream's suggestion with the following query in my PHP script
array(
'$cond' => array(
'if' => array(
'dateDue' => array(
'$lt' => 'dateComplete
)
)
)
)
However the MongoCollection::Aggregate function told me that $cond wasn't a recognized operator.
EDIT: #BatScream's answer seems to work but I wasn't aware of the fact that the group operator doesn't work properly after a $project is applied. I was hoping to be able to group these document on another field cID, is that possible?

Answer 1

The below aggregation pipeline would give you the result, considering your fields are of ISODate type. If not i suggest you to store them as ISODate type and not Strings.
db.collection.aggregate([
{$project:{"isLateWorkOrder":{$cond:[{$lt:["$dateDue","$dateCompleted"]},
true,false]}}},
{$match:{"isLateWorkOrder":true}},
{$group:{"_id":null,"lateWorkOrders":{$sum:1}}},
{$project:{"_id":0,"lateWorkOrders":1}}
])
The PHP syntax should look similar to,
$projA = array("isLateWorkOrder" =>
array("$cond" =>
array(array("$lt" =>
array("$dateDue","$dateCompleted")),
true,false)))
$matchA = array("isLateWorkOrder" => true)
$grp = array("_id" => null,"lateWorkOrders" => array("$sum" => 1))
$projB = array("_id" => 0,"lateWorkOrders" => 1)
$pipeline = array($projA,$matchA,$grp,$projB);
$someCol -> aggregate($pipeline)
or, simply using the count function:
db.collection.count({$where:"this.dateDue < this.dateCompleted"})