mongodb“独特”的查询？

Question

I am not sure whether "distinct" is the right word for this.

I have a simple messaging system in mongo with "sender","receiver","content".

The problem I have is in the inbox page: say for example I have 3 messages from John and 4 from Jack, I would like to show in my inbox only the last message sent by each of those people.

I have searched online and I thought this http://www.mongodb.org/display/DOCS/Aggregation#Aggregation-Distinct would be the right thing but, it doesn't seem to work or atlest I can't get it to work... meh..

Answer 1

Ciao Enrico,

I didn't really get your question, so I'll try to explain how distinct works:

Collection name: foo

Doc no.1: {'a':1,...}
Doc no.2: {'a':2,...}
Doc no.3: {'a':3,...}

If then you run:

db.runCommand( { distinct: 'foo', key: 'a' } )

What you'll get will be:

[1,2,3]

And I don't think that will be of any use to you. Instead try sorting by date sent and limiting the results to one:

db.collection.find(<your_filters_here>).sort({<date_sent_key> : -1}).limit(1)