How to do build this mongoDB query (mongomapper) - ruby

Question

I have certain documents with a name: String and a version: Integer.
What I need is a list of documents of the highest version per name.
So I think I need to do the equivalent of group by in sql and then a having for max version per name.
I have no idea where to start to do this with mongoDB. If anyone could make this query for the mongo terminal that would be a great start, but an added bonus would be to give the sytnax for MongoMapper specifically.

Answer 1

If you are on Mongodb version 2.2+, you can do your query by using the aggregation framework with the group pipeline operator.
The documentation is here: http://docs.mongodb.org/manual/reference/aggregation/
MongoMapper doesn't have a helper for the aggregation framework but you can use the Ruby driver directly (driver version 1.7.0+ has an aggregate helper method). You would have to get an instance of Mongo::Collection and call the aggregate method on it. For example:
Model.collection.aggregate(["$group" =>
{"_id" => "$name",
"max_version" => {"$max" => "$version"}}
])
I hope that helps!

Answer 2

If you want to do a group by with Mongo DB, check the Aggregation Framework, it is the exact tool for the job !
Here you'll find the equivalent in Aggregation Framework for GROUP BY, HAVING, and more.

Answer 3

Thanks to #Simon I had a look at Map Reduce with MongoMapper. My take on it is probably not perfect, but it does what I want it to do. Here's the implementation:
class ChildTemplate
...
key :name, String
key :version, Integer, :default => 1
...
private
def self.map
<<-JS
function() {
emit(this.name, this);
}
JS
end
private
def self.reduce
<<-JS
function(key, values) {
var res = values[0];
for(var i=1; i<values.length; i++)
{
if(values[i].version > res.version)
{
res = values[i];
}
}
return res;
}
end
def self.latest_versions(opts = {})
results = []
opts[:out] = "ct_latest_versions"
ChildTemplate.collection.map_reduce(map, reduce, opts).find().each do |map_hash|
results << map_hash["value"]
end
return results
end