Querying against nested arrays

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?

evilive · Accepted Answer · 2013-08-13 09:28:32Z

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

edited Aug 13, 2013 at 9:28

answered Aug 13, 2013 at 6:23

evilive

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1 Comment

Ray Over a year ago

Thanks for your answer, evilive. btw, it seems to retrieve only one tag element from tags array. If '000000' is in number of tag elements in tags array, I want to get all those tag elements.

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