inventory集合有下列文档：

{ "_id" : 1, "item" : "abc1", "description": "product 1", "qty": 300 }
{ "_id" : 2, "item" : "abc2", "description": "product 2", "qty": 200 }
{ "_id" : 3, "item" : "xyz1", "description": "product 3", "qty": 250 }
{ "_id" : 4, "item" : "VWZ1", "description": "product 4", "qty": 300 }
{ "_id" : 5, "item" : "VWZ2", "description": "product 5", "qty": 180 }

下面的聚合操作使用$gt运算符来判断qty是否大于250：

db.inventory.aggregate(
   [
     {
       $project:
          {
            item: 1,
            qty: 1,
            qtyEq250: { $gt: [ "$qty", 250 ] },
            _id: 0
          }
     }
   ]
)

操作返回下面的结果：

{ "item" : "abc1", "qty" : 300, "qtyGt250" : true }
{ "item" : "abc2", "qty" : 200, "qtyGt250" : false }
{ "item" : "xyz1", "qty" : 250, "qtyGt250" : false }
{ "item" : "VWZ1", "qty" : 300, "qtyGt250" : true }
{ "item" : "VWZ2", "qty" : 180, "qtyGt250" : false }