Mongoid Group By or MongoDb group by in rails - ruby-on-rails

Question

I have a mongo table that has statistical data like the following....
course_id
status which is a string, played or completed
and timestamp information using Mongoid's Timestamping feature
so my class is as follows...
class Statistic
include Mongoid::Document
include Mongoid::Timestamps
include Mongoid::Paranoia
field :course_id, type: Integer
field :status, type: String # currently this is either play or complete
I want to get a daily count of total # of plays for a course. So for example...
8/1/12 had 2 plays, 8/2/12 had 6 plays. Etc. I would therefore be using the created_at timestamp field, with course_id and action. The issue is I don't see a group by method in Mongoid. I believe mongodb has one now, but I'm unsure of how that would be done in rails 3.
I could run through the table using each, and hack together some map or hash in rails with incrementation, but what if the course has 1 million views, retrieving and iterating over a million records could be messy. Is there a clean way to do this?

Answer 1

As mentioned in comments you can use map/reduce for this purpose. So you could define the following method in your model ( http://mongoid.org/en/mongoid/docs/querying.html#map_reduce )
def self.today
map = %Q{
function() {
emit(this.course_id, {count: 1})
}
}
reduce = %Q{
function(key, values) {
var result = {count: 0};
values.forEach(function(value) {
result.count += value.count;
});
return result;
}
}
self.where(:created_at.gt => Date.today, status: "played").
map_reduce(map, reduce).out(inline: true)
end
which would result in following result:
[{"_id"=>1.0, "value"=>{"count"=>2.0}}, {"_id"=>2.0, "value"=>{"count"=>1.0}}]
where _id is the course_id and count is the number of plays.
There is also dedicated group method in MongoDB but I am not sure how to get to the bare mongodb collection in Mongoid 3. I did not have a chance to dive into code that much yet.
You may wonder why I emit a document {count: 1} as it does not matter that much and I could have just emitted empty document or anything and then always add 1 to the result.count for every value. The thing is that reduce is not called if only one emit has been done for particular key (in my example course_id has been played only once) so it is better to emit documents in the same format as result.

Answer 2

Using Mongoid
stages = [{
"$group" => { "_id" => { "date_column_name"=>"$created_at" }},
"plays_count" => { "$sum" => 1 }
}]
#array_of_objects = ModelName.collection.aggregate(stages, {:allow_disk_use => true})
OR
stages = [{
"$group" => {
"_id" => {
"year" => { "$year" => "$created_at" },
"month" => { "$month" => "$created_at" },
"day" => { "$dayOfMonth" => "$created_at" }
}
},
"plays_count" => { "$sum" => 1 }
}]
#array_of_objects = ModelName.collection.aggregate(stages, {:allow_disk_use => true})
Follow the links below to group by using mongoid
https://taimoorchangaizpucitian.wordpress.com/2016/01/08/mongoid-group-by-query/
https://docs.mongodb.org/v3.0/reference/operator/aggregation/group/