Getting a distinct aggregation of an array field a

问题:

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 the following items:

{"title": "Cool", "_id": {"$oid": "503e4dc0cc93742e0d0ccad3"}, "tags": ["twenty", "sixty"]}
{"title": "Other", "_id": {"$oid": "503e4e5bcc93742e0d0ccad4"}, "tags": ["ten", "thirty"]}
{"title": "Ouch", "_id": {"$oid": "503e4e72cc93742e0d0ccad5"}, "tags": ["twenty", "seventy"]}
{"title": "Final", "_id": {"$oid": "503e4e72cc93742e0d0ccad6"}, "tags": ["sixty", "seventy"]}

What I'd like to do is query so I get a list of unique tags for all of these objects. The result should look something like this:

["ten", "twenty", "thirty", "sixty", "seventy"]

How do I query for this? I'm trying to distinct() it, but the call always fails without even querying.

回答1:

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.

回答2:

You should be able to use this:

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

回答3:

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
}

回答4:

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.

回答5:

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" ] }