Let say we have user and post collection. In post collection, vote store the user name as a key.
db.user.insert({name:'a', age:12});
db.user.insert({name:'b', age:12});
db.user.insert({name:'c', age:22});
db.user.insert({name:'d', age:22});
db.post.insert({Title:'Title1', vote:[a]});
db.post.insert({Title:'Title2', vote:[a,b]});
db.post.insert({Title:'Title3', vote:[a,b,c]});
db.post.insert({Title:'Title4', vote:[a,b,c,d]});
We would like to group by the post.Title and find out the count of vote in different user age.
> {_id:'Title1', value:{ ages:[{age:12, Count:1},{age:22, Count:0}]} }
> {_id:'Title2', value:{ ages:[{age:12, Count:2},{age:22, Count:0}]} }
> {_id:'Title3', value:{ ages:[{age:12, Count:2},{age:22, Count:1}]} }
> {_id:'Title4', value:{ ages:[{age:12, Count:2},{age:22, Count:2}]} }
I have searched through and doesn't find a way to access 2 collection in mongodb mapreduce. Could it be possible to achieve in re-reduce?
I know it is much simple to embedded the user document in post, but it is not a nice way to do as the real user document have many properties. If we include the simplify version of user document, it will limit the dimension of analysis.
{Title:'Title1', vote:[{name:'a', age:12}]}
MongoDB does not have a multi-collection Map / Reduce. MongoDB does not have any JOIN syntax and may not be very good for ad-hoc joins. You will need to denormalize this data in some way.
You have a few options:
Option #1: Embed the age with the vote.
{Title:'Title1', vote:[{name:'a', age:12}]}
Option #2: Keep a counter of the ages
{Title:'Title1', vote:[a, b], age: { "12" : 1, "22" : 1 }}
Option #3: Do a "manual" join
Your last option is to write script/code that does a for loop over both collections and merges the data correctly.
So you would loop over post and output a collection with the title and the list of votes. Then you would loop through the new collection and update the ages by looking up each user.
My suggestion
Go with #1 or #2.