mongoDB distinct & where in same query?

Maybe you could try this

db.Article.aggregate([
{$unwind: "$Comment"},
{$unwind: "$Comment.Reply"},
{$match: {"Comment.Reply.email": "xxx"}},
{$group: {_id: "$Comment.Reply.ip"}}
])

The result of example should be

/* 1 */
{
    "_id" : "192.168.1.1"
}

/* 2 */
{
    "_id" : "128.168.1.1"
}

Distinct query in mongo with condition works like this

 db.Article.distinct("Comment.Reply.ip",{"Comment.Reply.email" : "xxx"})

not other way around

EDIT:

I understand the problem now, inorder to match/filter subdocuments we need to use $elemMatch operator, like this

  db.Article.distinct("Comment.Reply.ip",{Comment: {$elemMatch: {"Reply.email" : "xxx"}}})

but this will not work if the sub-document contains sub arrays (in your case, you have array of replies). There is an existing issue $elemMatch on subArray is opened. And its planned for mongo 2.1. You can check out the link for more info

db.collection.distinct("field_name", query)

will return all the distinct values from your collection in the form of an array

field_name should be the field for which you want to return distinct values.

query specifies the documents from where you want to retrieve the distinct values. ex :- {"Comment.Reply.email" : "xxx"} or {'country' : 'India','state' : 'MH','city' : 'mumbai'}