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Operating System:
ALL
Steps To Reproduce:

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> db.test.insert({obj:{prop:null}})
> db.test.findOne()
{
"_id" : ObjectId("52298b4aaa95c6f468712074"),
"obj" :
{ "prop" : null }
}
> db.test.findOne({"obj.prop":{$exists: true}})
{
"_id" : ObjectId("52298b4aaa95c6f468712074"),
"obj" :
{ "prop" : null }
}

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> db.test.insert({obj:{prop:null}}) > db.test.findOne() { "_id" : ObjectId("52298b4aaa95c6f468712074"), "obj" : { "prop" : null } } > db.test.findOne({"obj.prop":{$exists: true}}) { "_id" : ObjectId("52298b4aaa95c6f468712074"), "obj" : { "prop" : null } }

According to the documentation[1] "Documents that contain the field but has the value null are not returned."

This doesn't appear to be true, if one is querying against subdocuments.