Getting a distinct aggregation of an array field across indexes

Question

I\'m trying to learn MongoDB and how it\'d be useful for analytics for me. I\'m simply playing around with the JavaScript console available on their website and have created

Answer 1

You can use the aggregation framework. Depending on how you'd like the results structured, you can use either

var pipeline = [ 
        {"$unwind": "$tags" } ,
        { "$group": { _id: "$tags" } }
    ];
R = db.tb.aggregate( pipeline );
printjson(R);

{
        "result" : [
                {
                        "_id" : "seventy"
                },
                {
                        "_id" : "ten"
                },
                {
                        "_id" : "sixty"
                },
                {
                        "_id" : "thirty"
                },
                {
                        "_id" : "twenty"
                }
        ],
        "ok" : 1
}

or

var pipeline = [ 
        {"$unwind": "$tags" } ,
        { "$group": 
            { _id: null, tags: {"$addToSet": "$tags" }  }
        }
    ];
R = db.tb.aggregate( pipeline );
printjson(R);

{
        "result" : [
                {
                        "_id" : null,
                        "tags" : [
                                "seventy",
                                "ten",
                                "sixty",
                                "thirty",
                                "twenty"
                        ]
                }
        ],
        "ok" : 1
}

Answer 2

You should be able to use this:

db.mycollection.distinct("tags").sort()

Answer 3

The code that fails on their website works on an actual MongoDB instance:

> db.posts.insert({title: "Hello", tags: ["one", "five"]});
> db.posts.insert({title: "World", tags: ["one", "three"]});
> db.posts.distinct("tags");
[ "one", "three", "five"]

Weird.

Answer 4

Another way of getting unique array elements using aggregation pipeline

db.blogs.aggregate(
  [
    {$group:{_id : null, uniqueTags : {$push : "$tags"}}},
    {$project:{
      _id : 0,
      uniqueTags : {
        $reduce : {
          input : "$uniqueTags", 
          initialValue :[], 
          in : {$let : {
            vars : {elem : { $concatArrays : ["$$this", "$$value"] }},
            in : {$setUnion : "$$elem"}
          }}
        }
      }
    }}
  ]
)

collection

> db.blogs.find()
{ "_id" : ObjectId("5a6d53faca11d88f428a2999"), "name" : "sdfdef", "tags" : [ "abc", "def", "efg", "abc" ] }
{ "_id" : ObjectId("5a6d5434ca11d88f428a299a"), "name" : "abcdef", "tags" : [ "abc", "ijk", "lmo", "zyx" ] }
> 

pipeline

>   db.blogs.aggregate(
...     [
...       {$group:{_id : null, uniqueTags : {$push : "$tags"}}},
...       {$project:{
...         _id : 0,
...         uniqueTags : {
...           $reduce : {
...             input : "$uniqueTags", 
...             initialValue :[], 
...             in : {$let : {
...               vars : {elem : { $concatArrays : ["$$this", "$$value"] }},
...               in : {$setUnion : "$$elem"}
...             }}
...           }
...         }
...       }}
...     ]
...   )

result

{ "uniqueTags" : [ "abc", "def", "efg", "ijk", "lmo", "zyx" ] }

Answer 5

There are couple of web mongo consoles available:

• http://try.mongodb.org/
• http://www.mongodb.org/#

But if you type help in them you will realise they only support a very small number of ops:

HELP
Note: Only a subset of MongoDB's features are provided here.
For everything else, download and install at mongodb.org.

db.foo.help()                 help on collection method
db.foo.find()                 list objects in collection foo
db.foo.save({a: 1})           save a document to collection foo
db.foo.update({a: 1}, {a: 2}) update document where a == 1
db.foo.find({a: 1})           list objects in foo where a == 1

it                            use to further iterate over a cursor

As such distinct does not work because it is not supported.