MongoDB Geospatial Load More Between HTTP Requests - ruby

Question

AcaniUsers loads the first 20 users in MongoDB (on Heroku via Sinatra) closest to me from my iPhone. I want to add a Load More button that will load the next 20 users closest to me. Keep in mind, my location and the locations of the users on my phone may have changed. I was thinking of switching from Sinatra to Node.js and opening a WebSocket, so I could have realtime updates of the presences & locations of the users on my phone, but think I should save that challenge for a next iteration. Basically, how should I implement the load more functionality?

Answer 1

To paginate queries in MongoDB you can use a combination of limit() and skip().
So, the first query will be:
your_query.limit(20)
Then if you want to load the second 20 (you will have to remember the first query somewhere):
your_query.skip(20).limit(20)
btw I suggest you to execute in the first place the query with a limit higher than 20 and put in the cache the result you don't display. When requested, just get them from the cache (you can store it in the user session). If the position change, restart from scratch and re-query the db invalidating the cache.

Answer 2

think of it more as a client side question: use subscriptions based on the current group - encode the group into a geo-square if possible (more efficient than circle, I think?) - periodically (t) executes an operation that checks the locations of each user and simply sends them out with a group id to match the subscriptions
actually...to build your subscription groups, just use the geonear command on all of your subscribers
- build a hash of your subscribers and their groups
- each subscriber is subscribed to one group and themselves (for targeted communication => indicate that a specific subscriber should change their subscription)
- iterate through the results i number of times where i is the number of individuals in an update group
- execute an action that checks the current value of j, the group number for a specific subscriber, against the new j value - if there is a change, notify the subscriber on the subsriber's private channel
- notifications synchronously follow subscriber adjustments
something like:
var pageSize;
// assign pageSize in method call
var documents = collection.Find(query);
var max = documents.Size();
for (int i = 0; i == max ; i++)
{
var level = i*pageSize;
if (max / level > 1)
{
documents.Skip(pageSize);
}
else
{
documents.Skip(pageSize).Limit(level);
break;
}
}
:)