Elasticsearch does not support min_children and max_children for nested datatypes by default (see: issue #10043). In case you need this behavior, this gist provides an alternative solution. Please note that I haven't benchmarked it against join queries.
To support min_children and max_children for your nested query, all you have to do is to use a function_score query. To make this more concrete: you have an index called Person with the following mapping:
first_name(text)email(text)children(nested)first_name(text)last_name(text)
To effectively support min_children and max_children, there are multiple queries you need to consider:
- Find all persons who don't have children.
- Find all persons who have more than n children:
- requires: n > 0
- if n is 0, it means you're asking for any Person, regardless if they have or don't have children.
- Find all persons who have between n and m children:
- requires: n > 0
- if n is 0, it means you're asking to find all Person who have less than m children.
- Find all persons who have less than n children.
Depending on the scenario, the request will look different.
function_scoresupports min_score. It filters out any document where the score is lower than themin_score.function_scorehas amax_boost. This doesn't filter documents returned, it simply caps the score to a specific value. For instance: if after calculating the score, you end up with500, and the max_boost is50,50will be returned.- If you don't want this
function_scoreto pollute the overall score of the document, apply a boost of 0.
Explanation: easiest query, you simply have to verify there are no nested documents. It is significantly faster than using the function_score.
{
"query": {
"bool": {
"must_not": {
"nested": {
"path": "children",
"query": {
"exists": {
"field": "children.first_name"
}
}
}
}
}
}
}Explanation: each matching document is boosted by 10 and the nested query sums them. The function_score filters out any document that is less than what is expected.
Example: Find all persons who have a minimum of 2 children: the boost applied here is 10 (you can set any number you want here), as such the min_score is 20 (2 * 10).
Before Elastic 7:
{
"query": {
"function_score": {
"min_score": 20,
"boost": 1,
"query": {
"nested": {
"path": "children",
"query": {
"exists": {
"field": "children.first_name"
}
},
"boost": 10,
"score_mode": "sum"
}
}
}
}
}Elastic 7+:
{
"query": {
"function_score": {
"min_score": 20,
"boost": 1,
"score_mode": "multiply",
"boost_mode": "replace",
"query": {
"nested": {
"path": "children",
"boost": 10,
"score_mode": "sum",
"query": {
"constant_score": {
"boost": 1,
"filter": {
"exists": {
"field": "children.first_name"
}
}
}
}
}
}
}
}
}Explanation: same as above, except here we apply a script to alter the score. This script checks that the sum of all boosts is not exceeding m * boost, if it does, it returns 0 which automatically guarantee the document will be excluded (0 < min_score).
Example: Find all persons who have a minimum of 2 children and a maximum of 5 children (inclusive).
Before Elastic 7:
{
"query": {
"function_score": {
"min_score": 20,
"boost": 1,
"functions": {
"script_score": {
"script": {
"source": "if (_score > 50) { return 0; } return _score;",
"lang": "painless"
}
}
},
"query": {
"nested": {
"path": "children",
"query": {
"exists": {
"field": "children.first_name"
}
},
"boost": 10,
"score_mode": "sum"
}
}
}
}
}Elastic 7+:
{
"query": {
"function_score": {
"min_score": 20,
"boost": 1,
"score_mode": "multiply",
"boost_mode": "replace",
"functions": [
{
"filter": {
"match_all": {
"boost": 1
}
},
"script_score": {
"filter": {
"match_all": {
"boost": 1
}
},
"script": {
"source": "if (_score > 50) { return 0; } return _score;",
"lang": "painless"
}
}
}
],
"query": {
"nested": {
"path": "children",
"boost": 10,
"score_mode": "sum",
"query": {
"constant_score": {
"boost": 1,
"filter": {
"exists": {
"field": "children.first_name"
}
}
}
}
}
}
}
}
}Explanation: this request means you are asking for persons who have no children and persons who have been 1 to n children. Expressing this with elastic can be tricky, so taking the negation makes it easier: you're asking to not find all persons who have more than n + 1 children.
Example: find all persons who have less than 2 children.
Before Elastic 7:
{
"query": {
"bool": {
"must_not": {
"function_score": {
"min_score": 30,
"boost": 1,
"query": {
"nested": {
"path": "children",
"query": {
"exists": {
"field": "children.first_name"
}
},
"boost": 10,
"score_mode": "sum"
}
}
}
}
}
}
}Elastic 7+:
{
"query": {
"bool": {
"must_not": {
"function_score": {
"min_score": 30,
"boost": 1,
"score_mode": "multiply",
"boost_mode": "replace",
"query": {
"nested": {
"path": "children",
"boost": 10,
"score_mode": "sum",
"query": {
"constant_score": {
"boost": 1,
"filter": {
"exists": {
"field": "children.first_name"
}
}
}
}
}
}
}
}
}
}
}Hope this helps.