MongoDB查询空

我在地图上减少了我们网站流量来源的统计数据集。它托管在mongohq.com上，我在本地安装了mongo客户端，并且正在通过本地客户端连接到mongohq db。

如果我查询db.page_views_stats.find（{“ _ id.offer”：“ chihuahua-insurance”}）

我得到21行：

{ "_id" : { "month" : ISODate("2013-04-01T00:00:00Z"), "source" : "cpc", "offer" : "chihuahua-insurance" }, "value" : { "count" : 30 } }
{ "_id" : { "month" : ISODate("2013-04-01T00:00:00Z"), "source" : "none", "offer" : "chihuahua-insurance" }, "value" : { "count" : 2 } }
{ "_id" : { "month" : ISODate("2013-05-01T00:00:00Z"), "source" : null, "offer" : "chihuahua-insurance" }, "value" : { "count" : 5 } }
{ "_id" : { "month" : ISODate("2013-05-01T00:00:00Z"), "source" : "cpc", "offer" : "chihuahua-insurance" }, "value" : { "count" : 495 } }
{ "_id" : { "month" : ISODate("2013-05-01T00:00:00Z"), "source" : "email", "offer" : "chihuahua-insurance" }, "value" : { "count" : 8 } }
{ "_id" : { "month" : ISODate("2013-05-01T00:00:00Z"), "source" : "none", "offer" : "chihuahua-insurance" }, "value" : { "count" : 68 } }
{ "_id" : { "month" : ISODate("2013-06-01T00:00:00Z"), "source" : null, "offer" : "chihuahua-insurance" }, "value" : { "count" : 40 } }
{ "_id" : { "month" : ISODate("2013-06-01T00:00:00Z"), "source" : "cpc", "offer" : "chihuahua-insurance" }, "value" : { "count" : 63 } }
{ "_id" : { "month" : ISODate("2013-06-01T00:00:00Z"), "source" : "email", "offer" : "chihuahua-insurance" }, "value" : { "count" : 1 } }
{ "_id" : { "month" : ISODate("2013-07-01T00:00:00Z"), "source" : null, "offer" : "chihuahua-insurance" }, "value" : { "count" : 110 } }
{ "_id" : { "month" : ISODate("2013-07-01T00:00:00Z"), "source" : "cpc", "offer" : "chihuahua-insurance" }, "value" : { "count" : 48 } }
{ "_id" : { "month" : ISODate("2013-08-01T00:00:00Z"), "source" : null, "offer" : "chihuahua-insurance" }, "value" : { "count" : 219 } }
{ "_id" : { "month" : ISODate("2013-08-01T00:00:00Z"), "source" : "cpc", "offer" : "chihuahua-insurance" }, "value" : { "count" : 1 } }
{ "_id" : { "month" : ISODate("2013-08-01T00:00:00Z"), "source" : "tweet", "offer" : "chihuahua-insurance" }, "value" : { "count" : 1 } }
{ "_id" : { "month" : ISODate("2013-09-01T00:00:00Z"), "source" : null, "offer" : "chihuahua-insurance" }, "value" : { "count" : 65 } }
{ "_id" : { "month" : ISODate("2013-10-01T00:00:00Z"), "source" : null, "offer" : "chihuahua-insurance" }, "value" : { "count" : 48 } }
{ "_id" : { "month" : ISODate("2013-11-01T00:00:00Z"), "source" : null, "offer" : "chihuahua-insurance" }, "value" : { "count" : 67 } }
{ "_id" : { "month" : ISODate("2013-11-01T00:00:00Z"), "source" : "cpc", "offer" : "chihuahua-insurance" }, "value" : { "count" : 1 } }
{ "_id" : { "month" : ISODate("2013-12-01T00:00:00Z"), "source" : null, "offer" : "chihuahua-insurance" }, "value" : { "count" : 56 } }
{ "_id" : { "month" : ISODate("2013-12-01T00:00:00Z"), "source" : "cpc", "offer" : "chihuahua-insurance" }, "value" : { "count" : 6 } }
{ "_id" : { "month" : ISODate("2014-01-01T00:00:00Z"), "source" : null, "offer" : "chihuahua-insurance" }, "value" : { "count" : 7 } }

它们都有一个源字段，其中一些为空

如果我查询以下任何一项：

db.page_views_stats.find({$and:[{"_id.source": null},{"_id.offer":"chihuahua-insurance"}]})
db.page_views_stats.find({$and:[{"_id.source":{ $exists: false } },{"_id.offer":"chihuahua-insurance"}]})
db.page_views_stats.find({$and:[{"_id.source":{ $type: 10 } },{"_id.offer":"chihuahua-insurance"}]})
db.page_views_stats.find({"_id.source":{ $type: 10 },"_id.offer":"chihuahua-insurance"})
db.page_views_stats.find({"_id.source": null,"_id.offer":"chihuahua-insurance"})
db.page_views_stats.find({"_id.source":{ $exists: false },"_id.offer":"chihuahua-insurance"})

我得到0结果返回。

我如何找到source为null的结果？