Prepare following transaction table. We are generating feature_vector for each item_id based on cooccurrence of purchased items, a sort of bucket analysis.
| userid | item_id | purchase_at timestamp | 
|---|---|---|
| 1 | 31231 | 2015-04-9 00:29:02 | 
| 1 | 13212 | 2016-05-24 16:29:02 |