CODEWITHSUNDEEP
mongodb
Jonathan Clark
Jonathan Clark

Reputation: 20538

How to make a fake join in MongoDB?

I am developing a webapp using PHP and MongoDB. In this app I keep two collections. One to keep data about files and one to keep track about download events for each file.

I need to get the 10 latest downloaded files but I know joins is not an option. The events document only stores the file id and the other collection stores the thumbnail.

Right now I first get the 10 recently downloaded files and order it by date and the order is fine but then I use this array of files (their ids) and make a where_in query where I look for files whos id is present in the ids array. This also works fine (I get the thumbnails for the selected files) but I cannot keep the order anymore. The most recently downloaded file is not longer on top.

How can I keep the order of the ids without looping through them thus making 10 new queries instead of just one?

I cannot change the schema because I got over 40.000 documents :)

UPDATE

In short this is what I want to do:

  1. Get all the IDs of the 10 recently downloaded files. Sorted by download timestamp.
  2. Use this array of Ids and make a query to the files collection and get the details for each file like thumbnail, decription and so on.

The above steps works fine, BUT I cannot keep the order from the first step thus I cannot get the most recently downloaded file on top. I know I could look trought the id array and get data for each file but that would cost me 10 queries instead of one.

Upvotes: 0

Views: 286

Answers (1)

Sergio Tulentsev
Sergio Tulentsev

Reputation: 230461

I don't really get your problem. Here's some pseudo-code:

// get last N download events
events = get_latest_downloads() 

// retrieve associated file data
file_array = get_all_file_info_by_events(events)

display_data = []
for(e in events) {
  data = find_file_info_in_the_array(file_array, e.file_id)
  display_data.push(data)
}

// now display_data is contains full file info, sorted by download time.

Upvotes: 1

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