mongodb - 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 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.

Answer 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.