Mongodb Sort Collection By Count In Other Collection

I have two collections: collection A and collection B. Collection B 'A_id' will have the _id from Collection A (kinda like a relationship). Collection A { _id, name } Collect

Solution 1:

There are many ways to do what you're asking. You have to know some things about the data first. How many documents will a B document have?

• It looks like a case where you'd want Collection A to have an items field with documents that are B. There is a 16 MB limit for that, but for most cases works just fine.

• Keep a count on A with numbers of B records inserted. (Ie every time you insert a B you'd $inc the field on A). You'd then do an index on the count field for A and do sort({count: -1})

Solution 2:

It is my understanding that this is not strictly possible. However, you could use the aggregation framework group function and $sum accumulator to group by A_id which will give you a mapping between A_id and the total number of documents in collection B with that A_id. Sort that mapping and then you can select things from collection A in the order you have described, although you'd have to select from collection A one at a time since you are enforcing an ordering at selection time.

The other option would be to have some count field in collection A on each document that you include logic to increment every time something is inserted into in collection B with a particular A_id.