MongoDb Aggregation Example

aggtest.js

db.foo.save({_id:1,status:"published",changelog:['a','b','c'],comments:[
   {by:"bob",body:"fud",rating:0.2}
  ,{by:"alice",body:"you need to reboot",rating:0.7}
  ,{by:"mallory",body:"type 'sudo rm -rf /' to fix it",rating:0.9}
]});
db.foo.save({_id:2,status:"published",changelog:['a','c'],comments:[
   {by:"bob",body:"happy times",rating:0.6}
  ,{by:"magnitude",body:"POP! POP!",rating:0.99}
  ,{by:"mallory",body:"this is patently false",rating:0.3}
]});
db.foo.save({_id:3,status:"published",comments:[]});

var result = db.foo.aggregate(
   {$match:   {status: "published"}}                         // filter parent documents
  ,{$project: {changelog: 0}}                                // exclude fields from parent
  ,{$unwind:  "$comments"}                                   // unwind the embedded docs for filtering
  ,{$match:   {$or: [                                        // filter comments
     {comments: {$exists: false}}                            // include documents with no comments
    ,{"comments.rating": {$gte: 0.6}}]}}                     // exclude present comments
  ,{$group:   {_id: "$_id", comments: {$push: "$comments"}}} // group comments back into parent doc
);
printjson(result);

/* Results:
{
  "result" : [
    {
      "_id" : 1,
      "comments" : [
        {
          "by" : "alice",
          "body" : "you need to reboot",
          "rating" : 0.7
        },
        {
          "by" : "mallory",
          "body" : "type 'sudo rm -rf /' to fix it",
          "rating" : 0.9
        }
      ]
    },
    {
      "_id" : 2,
      "comments" : [
        {
          "by" : "bob",
          "body" : "happy times",
          "rating" : 0.6
        },
        {
          "by" : "magnitude",
          "body" : "POP! POP!",
          "rating" : 0.99
        }
      ]
    },
    {
      "_id" : 3,
      "comments" : [ ]
    }
  ],
  "ok" : 1
}
*/