Spring data mongodb 聚合查詢(aggregation) 之 unwind

unwind：分解一個文檔中的數組字段（例如數組大小爲n） 爲 （n）個文檔

官方案例：

inventory數據：{ "_id" : 1, "item" : "ABC1", sizes: [ "S", "M", "L"] }
執行：db.inventory.aggregate( [ { $unwind : "$sizes" } ] )
結果：{ "_id" : 1, "item" : "ABC1", "sizes" : "S" }
{ "_id" : 1, "item" : "ABC1", "sizes" : "M" }
{ "_id" : 1, "item" : "ABC1", "sizes" : "L" }

注：

1 includeArrayIndex：給每個數組的元素增加序號

db.inventory.aggregate( [ { $unwind: { path: "$sizes", includeArrayIndex: "arrayIndex" } } ] )
{ "_id" : 1, "item" : "ABC", "sizes" : "S", "arrayIndex" : NumberLong(0) }
{ "_id" : 1, "item" : "ABC", "sizes" : "M", "arrayIndex" : NumberLong(1) }
{ "_id" : 1, "item" : "ABC", "sizes" : "L", "arrayIndex" : NumberLong(2) }

2 preserveNullAndEmptyArrays：default：false 如果是true，將包括null，空，或者缺失

數據：

{ "_id" : 1, "item" : "ABC", "sizes": [ "S", "M", "L"] }
{ "_id" : 2, "item" : "EFG", "sizes" : [ ] }
{ "_id" : 3, "item" : "IJK", "sizes": "M" }
{ "_id" : 4, "item" : "LMN" }
{ "_id" : 5, "item" : "XYZ", "sizes" : null }

結果：

db.inventory.aggregate( [
   { $unwind: { path: "$sizes", preserveNullAndEmptyArrays: true } }
] )
{ "_id" : 1, "item" : "ABC", "sizes" : "S" }
{ "_id" : 1, "item" : "ABC", "sizes" : "M" }
{ "_id" : 1, "item" : "ABC", "sizes" : "L" }
{ "_id" : 2, "item" : "EFG" }
{ "_id" : 3, "item" : "IJK", "sizes" : "M" }
{ "_id" : 4, "item" : "LMN" }
{ "_id" : 5, "item" : "XYZ", "sizes" : null }

Spring data mongodb：

 TypedAggregation<Role> agg = Aggregation.newAggregation(Role.class,
                Aggregation.unwind("sizes","arrayIndex",true)
        );
        AggregationResults<Document> result = mongoTemplate.aggregate(agg,Document.class);
        result.getMappedResults().forEach(document -> System.out.println(document));

注意：

返回的數據可以用org.bson.Document 來接收，也可以自定義實體類來接收