For a social network site, I have an activity of events from people you follow, and I'd like to group similar types of events made within a short timeframe together, for a more compact activity feed. Imagine how Facebook displays a comma separated list when you 'like' several things in rapid succession: 'Joe likes beer, football and chips.'
I understand using the group_by method on ActiveRecord Enumerable results, but there needs to be some initial work done populating a property that I can group by later. My questions deal with both storing activity data in a way that these groupings can be marked, and then later retrieving them again.
Right now I have an Activity model, which is a join association between the user that committed the activity and the item that that it's linked to (in my example above, assume 'beer', 'football' and 'chips' are records of a Like model). There are other activity types aside from 'likes' too (events, saving favorites, etc). What I'm considering is, as this association is created, a check is made when the last association of that type was done, and if it was made more than a certain time period ago, incrementing an 'activity block' counter that is part of the Activity model. Later, when rendering this activity feed, I can group by user, then type, then this activity block counter.
Example: Let's say 2 blocks of updates are made within the same day. A user likes 2 things at 2:05 and later 3 more things at 5:45. After the third update (the start of the 2nd block) happens at 5:45, the model detects too much time has passed and increments its activity block counter by 1, thus forcing any following updates into a new block when they are rendered via a group_by call:
2:05 Joe likes beer nuts and Hooters.
5:45 Joe likes couches, chips and salsa.
7:00 Joe is attending the Football Viewing Party At Joe's
My first question: What's an efficient way to increment a counter like this? It's no longer auto_increment, so the easiest thing I can think of is looking at the counter for the last record as a reference point. However, this couldn't be from the same query that checked for when the last update of that type was made, since a later update of another type could have already received the next counter value. They don't have to be globally unique, but that would be nice.
The other overall strategy I thought of was another model Called ActivityBlock, that joins groups of similar activities together. In many cases, updates will be isolated by themselves though, so this seems a little inefficient to have one record for each individual activity.
Do either of these seem like a solid strategy?
My final question revolves around pagination. Now that we're dealing with blocks, it's harder to always display exactly a certain amount of entries, before pagination kicks in. Either an individual (isolated) Activity update, or a block of then should count as just 1, so at the lowest layer of my group_by, I can incorporate a counter to track how many rows I've displayed, but this means I can't just make one DB query anymore and simply specify a limit statement. Is there any way I could still do this without repeatedly performing additional SQL queries until I've reached my page limit?
This would be one advantage of the ActivityBlock model approach, since I could easily apply a limit call to that, and blocks could contain an auto increment counter as well.
Check out http://railscasts.com/episodes/406-public-activity He also posted one on how to do it from scratch in episode 407 (it's a Pro episode though).
You could use the epoch time, or a variation of it as the counter since thats semi-unique and deterministic
来源：https://stackoverflow.com/questions/4174995/how-to-group-similar-items-in-an-activity-feed