Querying against nested arrays - mongodb

Question

I have a users collection whose schema is as follow :
var userSchema = new mongoose.Schema({
id : Number,
tags :
[
{
tag_name : String,
tag_url : String,
posts :
[
{
post_id: String
}
]
}
]
});
What I would like to do is retrieving only tag_name whose post_id value is in the posts array.
So, I tried query as follow
db.users.find({'tags.posts.post_id':'000000'}, {'tags.tag_name':1})
Unfortunatelly, I got all tag_name although post_id is not in posts array.
Can you help me to write query for this?
Edit =================================================================
Let's say I have data as follow :
tags
[
{
tag_name: 'java',
posts : [{post_id:'000000'}, {post_id:'111111'}
},
{
tag_name: 'ruby',
posts : [{post_id:'000000'}, {post_id:'111111'}
},
{
tag_name: 'php',
posts : [{post_id:'111111'}
},
]
I want to have tag element by post_id, if I search by post_id is '000000' I want to get only tag elements whose tag_name is 'java' and 'ruby' not the last tag element, is it possible?

Answer 1

$ in tags.$.tag_name should help:
db.users.find({'tags.posts.post_id':'000000'}, {'tags.$.tag_name':1})
EDIT:
ok, I read your update. In this case I see a solution in aggregation framework. We can try build pipeline like this:
db.col.aggregate(
{$match: {'tags.posts.post_id':'000000'}},
{$unwind: '$tags'},
{$match: {'tags.posts.post_id':'000000'}},
{$group: {_id:'$_id', tags: {$push: '$tags'}}}
)
result will be:
{
"_id" : ObjectId("5209f5e4ef21a864f6f6ed54"),
"tags" : [
{
"tag_name" : "java",
"posts" : [
{ "post_id" : "000000" },
{ "post_id" : "111111" }
]
},
{
"tag_name" : "ruby",
"posts" : [
{ "post_id" : "000000" },
{ "post_id" : "111111" }
]
}
]
}
as you might see I did $match twice. It was for performance purposes. By first matching we reduce set of documents that contain post_id:000000 from a collection. Second match filters tags