Elasticsearch query date sorting parent-child relation (recurring events)

Question

I'm currently working on an app where we are handling events. So, in Elasticsearch, we do have a document named Event.

Previously, we only had one kind of event (unique event happening the 13 May from 9 AM to 11 AM), the sorting was simple (sort by start_date with an order)

We recently added a new feature that allows us to create recurring events, that means that we now have 2 levels inside Elasticsearch (parent-child relation).

We can have a parent event that is from the 12 May from 2 PM to the 14 May from 6 PM, linked to that event, we have the children that are daily, for example. So we'd have: 12 May 2PM-6PM, 13 May 2PM-6PM, 14 May 2PM-6PM.

The problem with the actual sort is that when we are the 12 May at 10 PM, we'll find the recurring event on top of the list and after that, will come the unique event.

I'd like to have a sorting where the nearest date has a higher priority. In that case, the unique event should have been the first on the list.

To make that happen, I have indexed node children on recurring event parent, in order to have the children start_date. The idea would be to get the nearest date out of the children node for every recurring event and sort that one with the start_date of every unique event.

I do not have a big experience with elasticsearch, so I'm kind of stuck, I saw a lot of information in the documentation (parent-child, nested objects, scripts, etc.) but I don't know how to handle this case.

I hope that I have explained myself correctly if you have any questions, feel free to ask them, I would be happy to provide you with additional information.

Answer 1

For the future googlers, here's how I fixed it.

Had to use scripts and sort with it, here's a partial exemple of the request I'm using

GET /event/_search
{
    "query" : {
      "match_all": {}
    },
    "sort" : {
        "_script" : {
            "type" : "number",
            "script": {
              "lang": "painless",
              "params": {
                "currentDate": 1560230000
              },
              "source": """
                def isRecurrenceParent = params._source.is_recurrence_parent;
                def countChildren = params._source.children.length;
                def currentDate = params.currentDate;

                if (isRecurrenceParent === false) {
                  return params._source.timestamp;
                }

                def nearest = 0;

                def lowestDiff = currentDate;

                for (int i = 0; i < countChildren; i++) {
                  def child = params._source.children[i];

                  def diff = child.timestamp - currentDate;
                  if (diff > 0 && diff < lowestDiff) {
                    lowestDiff = diff;
                    nearest = child.timestamp;
                  }
                }

                return nearest;
"""
            },
            "order" : "asc"
        }
    }
}

Answer 2

First thing you should consider is parent and child docs are saved separately. It means Parent-Event::1 and Child-Event::1 are saved in a same shard (ES routes to shard where parent located by its id hash) but document types are different. So, you should fetch Parent and Children documents separately by query and sort by date. (You can make following queries in php if works)

PS: I have also same situation but I had to implement in Java. So, I made a ES query builder ( https://github.com/mashhur/java-elasticsearch-querybuilder ) which supports parent-child relationship queries too, you can take a look for the reference.

// search child events and sort by date
GET events/_search {
"query": {
"has_parent": {
  "parent_type": "parent-event",
  "query": {
    "match_all": {}
   }
  },
  "sort": [{"start_date": {"desc"}}]
 }
}


// search parent events and sort by date
GET events/_search {
"query": {
"has_child": {
  "type": "child-event",
  "query": {
    "match_all": {}
   }
  },
  "sort": [{"start_date": {"desc"}}]
 }
}