Histogram over fixed range of dates (i.e. fixed number of buckets) even when data is absent - elasticsearch

Question

My goal is to build a histogram between a start and an end dates, the empty dates should appear in the histogram and have zero as a count value.
I am trying the following query to fetch the last 7 days:
POST my_index/_search
{
"size": 0,
"query": {
"range": {
"date": {
"gte": "now-7d/d",
"lte": "now/d"
}
}
},
"aggs" : {
"count_per_day" : {
"date_histogram" : {
"field" : "date",
"interval" : "day",
"order": {"_key": "desc"},
"min_doc_count": 0
}
}
}
}
The issues is that I have data only for the last 3 days, so there is no data at all prior to 3 days ago. In this case, the result contains only the last 3 days and the previous days are not returned at all.
But if there is a gap (i.e. there is data 6 days ago, but no data in the 5th and the 4th day), the empty days will appear with zero as a count.
How can I force to return the absent dates even if there is no data?
In other word, how to fix the number of buckets (to 7 in the example above) even if there is no data?

Answer 1

You have already added "min_doc_count": 0 to include empty buckets. All you need to do is to simply add extended_bounds param as well to force starting and ending buckets. More on it can be found here.
Update your query as below:
{
"size": 0,
"query": {
"range": {
"date": {
"gte": "now-7d/d",
"lte": "now/d"
}
}
},
"aggs": {
"count_per_day": {
"date_histogram": {
"field": "date",
"interval": "day",
"order": {
"_key": "desc"
},
"min_doc_count": 0,
"extended_bounds": {
"min": "now-7d/d",
"max": "now/d"
}
}
}
}
}