2019獨角獸企業重金招聘Python工程師標準>>> 
一、背景
最近在做基于宴會廳檔期的商戶搜索推薦時，如果用傳統平鋪式的mapping結構，無法滿足需求場景，于是用到了Elasticsearch支持的Nested(嵌套)查詢。
二、普通對象與嵌套對象的索引異同
如果一個對象不是嵌套類型，那么以如下原數據為例：
PUT /my_index/blogpost/1
{ "title":"Nest eggs", "body": "Making your money work...", "tags": [ "cash", "shares" ], "comments":[ { "name": "John Smith", "comment": "Great article", "age": 28, "stars": 4, "date": "2014-09-01" }, { "name": "Alice White", "comment": "More like this please", "age": 31, "stars": 5, "date": "2014-10-22" } ]
}由于是json格式的結構化文檔，es會平整成索引內的一個簡單鍵值格式，如下：
{ "title": [ eggs, nest ], "body": [ making, money, work, your ], "tags": [ cash, shares ], "comments.name": [ alice, john, smith, white ], "comments.comment": [ article, great, like, more, please, this ], "comments.age": [ 28, 31 ], "comments.stars": [ 4, 5 ], "comments.date": [ 2014-09-01, 2014-10-22 ]
}這樣的話，像這種john/28，Alice/31間的關聯性就丟失了，Nested Object就是為了解決這個問題。
將comments指定為Nested類型，如下mapping:
curl -XPUT 'localhost:9200/my_index' -d '
{ "mappings":{ "blogpost":{ "properties":{ "comments":{ "type":"nested", //聲明為nested類型"properties":{ "name": {"type":"string"}, "comment": { "type": "string"}, "age": { "type": "short"}, "stars": { "type": "short"}, "date": { "type": "date"} } } } } }
}這樣，每一個nested對象將會作為一個隱藏的單獨文本建立索引，進而保持了nested對象的內在關聯關系，如下：
{ ① "comments.name": [ john, smith ], "comments.comment": [ article, great ], "comments.age": [ 28 ], "comments.stars": [ 4 ], "comments.date": [ 2014-09-01 ]
}
{ "comments.name": [ alice, white ], "comments.comment": [ like,more,please,this], "comments.age": [ 31 ],"comments.stars": [ 5 ], "comments.date": [ 2014-10-22 ]
}
{ "title": [ eggs, nest ], "body": [ making, money, work, your ], "tags": [ cash, shares ]
}
①nested object三、嵌套對象的查詢
命令查詢（輸出結果1）：
curl -XGET localhost:9200/yzsshopv1/shop/_search?pretty -d '{"query" : {"bool" : {"filter" : {"nested" : {"path":"hallList","query":{"bool":{"filter":{"term":{"hallList.capacityMin" : "11"}}}}}}}}}'
{"took" : 3,"timed_out" : false,"_shards" : {"total" : 5,"successful" : 5,"failed" : 0},"hits" : {"total" : 1,"max_score" : 0.0,"hits" : [ {"_index" : "yzsshopv1","_type" : "shop","_id" : "89999988","_score" : 0.0,"_source" : {"cityId" : "1","shopName" : "xxxx婚宴(yyyy店)","shopId" : "89999988","categoryId" : [ "55", "165", "2738" ],"hallList" : [ {"hallId" : "20625","schedule" : ["2017-11-10", "2017-11-09"],"capacityMax" : 16,"capacityMin" : 12}, {"hallId" : "21080","schedule" : [ "2017-12-10", "2017-09-09", "2017-02-25"],"capacityMax" : 20,"capacityMin" : 11} ],"wedHotelTagValue" : [ "12087", "9601", "9603", "9602" ],"regionId" : [ "9", "824" ]}} ]}
}java api查詢封裝：
BoolQueryBuilder boolBuilder = new BoolQueryBuilder();
NestedQueryBuilder nestedQuery = new NestedQueryBuilder("hallList", new TermQueryBuilder("hallList.capacityMin","11")); //注意：除path之外，fieldName也要帶上path (hallList)boolBuilder.filter(nestedQuery);
searchRequest.setQuery(boolBuilder); //設置查詢條件java api輸出字段封裝：
searchRequest.addField("shopId");
searchRequest.addField("hallList. schedule");
searchRequest.addField("hallList.capacityMin");
searchRequest.addField("hallList.capacityMax");如果輸出的outputField為searchRequest.addField("hallList")，則會報錯：illegal_argument_exception,reason:field [hallList] isn't a leaf field;
如果輸出的outputField為searchRequest.addField("capacityMin")，則不報錯，但沒有capacityMin字段的值；
正確調用search后的輸出結果（輸出結果2）:
{"took" : 8,"timed_out" : false,"_shards" : {"total" : 5,"successful" : 5,"failed" : 0},"hits" : {"total" : 1,"max_score" : 0.0,"hits" : [{"_index" : "yzsshopv1","_type" : "shop","_id" : "89999988","_score" : 0.0,"fields" : {"shopId" : [ "89999988" ],"hallList.hallId" : [ "20625", "21080"],"hallList.capacityMin" : [12, 11 ],"hallList.capacityMax" : [16, 20 ],"hallList.schedule" : [ "2017-11-10", "2017-11-09", "2017-12-10", "2017-09-09", "2017-02-25"]}}]}
}對比輸出結果1和2發現，命令輸出嵌套對象結果1沒問題，但通過java api輸出結果2時，嵌套對象內部的關系也會打亂，比如hallList.schedule字段，無法區分到底哪些值屬于hallList.hallId-20625，哪些屬于21080。
//============以下更新20170331===========
經過后續調試，發現要讓java api輸出正確結果的嵌套對象，不能通過searchRequest.addField的方式，因為嵌套對象并不是葉子節點，需要通過以下的方式添加輸出字段:
searchRequest.setFetchSource(new String[]{"shopId","hallList"},new String[]{});還有一個不足點是： 嵌套查詢請求返回的是整個文本，而不僅是匹配的nested文本。
四、參考文檔
- https://www.elastic.co/guide/en/elasticsearch/guide/master/nested-objects.html
- http://stackoverflow.com/questions/23562192/unable-to-retrieve-nested-objects-using-elasticsearch-java-api
- http://elasticsearch.cn/book/elasticsearch_definitive_guide_2.x/nested-aggregation.html
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