MongoDB category tallies of found set - php

Question

I have a product collection. Most products have a category, a sub-category and a subsub-category, some only have 1 or 2 of those. I'm currently storing them in an array field 'category', it could look like ["german", "literature", "novels"], for a product of type "book" (there are about 15 types, each with their own category trees).
What I would like to do is do a search, maybe there's 10K matches, return 100 to the browser, and also present a list of categories with found-counts for the query. I don't know what the categories are in advance, and they can change also.
Different ways I'm looking at:
MapReduce, but I hear this is "slow" and better geared for daily statistics than live searches
One suggestion I got was Aggregation->$group: looked at this but I cannot see how that could count values instead of just summing or averaging them.. am I missing something?
do a second search that just returns the category field, for all products, so I can do the counts in the production code
do a looped search for each category and simply return count() of the cursor. For this to work I will need to know the categories obviously, and it seems like a last-resort..
Basically my question is "what is the best way?", it should be reasonably fast, and scale.
When this works, it's the same after the user clicks on a category - then the results should be tallied for the sub-categories of that category, and so on for the subsub-categories, if any.
Additional info: the collection will have a few million products maybe, as we don't have the data yet it's hard to test against that, only about 50K products currently.. future plans include a sharded setup (there's a lot of other data besides "products").
Am I storing the categories in the right way or should they be separate fields, would that help? There's 3 items in the array right now but this could increase later.
New to MongoDB, only worked lots with MySQL so far..
Clarifying the categories; for an example product of type "book", "german" will be the main category, "literature" a sub-category and "novels" its subsub-category. Other main categories are 5-6 other languages (for books), other subcategories are for example "academic & study", "business" or "travel & languages". Subsub-categories then depend on the sub-category (for that last, the SSC's could be "foreign language study", "sociolinguistics", ..). I am storing all three in one field, as an array, per product.
When someone does a search for "foo" on type "book", it'll find 123 products in English, 456 products in German, 789 products in French. What I want is to show a listing of all those main (language) categories in which products were found, along with the number of found products.
Then when someone selects "German", it will do another query and show the number of found German books, by subcategory (44 in "academic & study", 57 in "business", ...).

Answer 1

I'm currently storing them in an array field 'category', it could look like ["german", "literature", "novels"]
You should not use one array for three different fields, which are "category", "subcategory" and "sub-subcategory".
Also why store language as a category and not as "language"? Add a bit of logic to the "schema" of your database, since it will help you when things become more complicated.
If you do, it will be much easier to use aggregation (which is faster that hadoop and is possible in a sharded cluster), because you won't have to query inside the arrays and you can get more accurate results. Since their values is really small so should the name of the field("c" for category, "sc" for subcategory, "scc" for sub-subcategory), like this:
{ _id : xxxxxxxxxxxx , name : "A novel of german literature" , c : "german", sc : "literature", ssc : "novels" }
What I would like to do is do a search, maybe there's 10K matches, return 100 to the browser, and also present a list of categories with found-counts for the query. I don't know what the categories are in advance, and they can change also.
Since mongo is schema-less you don't have to set all this fields for every record. If you plan to have much different schema between products, maybe you should use different collection for each product, but that is up to you.
What I would like to do is do a search, maybe there's 10K matches, return 100 to the browser, and also present a list of categories with found-counts for the query. I don't know what the categories are in advance, and they can change also.
Make good use of indexes (there are many kinds of indexes and you should probably use more than one) and use aggregation with $group and the $limit to return just 100 records.
When this works, it's the same after the user clicks on a category - then the results should be tallied for the sub-categories of that category, and so on for the subsub-categories, if any.
Here is a sample query to get all subcategories of a category (using the schema described before):
db.products.aggregate([{ $match : { "c" : "german"}},{ $group : { _id : {"c" : "$c"}, $addToSet :{ "subcategories" : "$sc"}}}])
This query will return an array of all the subcategories that exist for the current category.
(Updated query in case your category is an array and not a single string)
db.products.aggregate([{ $match : { "c" : {$elemMatch : {"german" : 1, "english" : 1}}}},{ $group : { _id : {"c" : "$c"}, $addToSet :{ "subcategories" : "$sc"}}}])