我有一個員工記錄，其中包含僅包含 id 的 leads 數組，基本上是他領導的其他員工。我想輸出將每個潛在客戶與員工集合中的名稱連接起來的結果員工集合{ "_id" : ObjectId("5d4dc8635dd32dbcba4ae0ba"), "name" : "John"}{ "_id" : ObjectId("5d4dc8635dd32dbcba4ae0bb"), "name" : "Jane"}{ "_id" : ObjectId("5d4dc8635dd32dbcba4ae0b4"), "name" : "Richard"}employee_leads集合{ "_id" : ObjectId("5d55ac30e533bc76e4581923"), "employee_id" : ObjectId("5d4dc8635dd32dbcba4ae0c5"), "leads" : [ ObjectId("5d4dc8635dd32dbcba4ae0ba"), ObjectId("5d4dc8635dd32dbcba4ae0bb") ]}預期產出{ "_id" : ObjectId("5d4dc8635dd32dbcba4ae0c3"), "leads" : [ { "_id" : ObjectId("5d4dc8635dd32dbcba4ae0ba"), "name" : "John" }, { "_id" : ObjectId("5d4dc8635dd32dbcba4ae0bb"), "name" : "Jane" } ]}試圖：Document match = new Document("$match", new BasicDBObject("_id", new ObjectId("5d55ac30e533bc76e4581923")));Document lookup = new Document("$lookup", new BasicDBObject("from", "employee")) .append("localField", "leads") .append("foreignField", "_id") .append("as", "leads");// Document unwind = new Document("$unwind", "$leads");Document project = new Document("$project", new BasicDBObject("name", "$lead.name"));Document document = database.getCollection("employee_lead") .aggregate(Arrays.asList(match, lookup, unwind, project)) .first();// TODO: iterate through the lead array and display it問題是，是否只有一個連接語句，或者我是否必須對數據庫進行多次調用（天真方式）？
1 回答
忽然笑
TA貢獻1806條經驗 獲得超5個贊
您不必多次調用數據庫，$lookup將完全按照您在聚合框架中的要求進行操作。
嘗試以下查詢：
db.getCollection("employee_leads").aggregate([
{
? ? ? ? $match : {
? ? ? ? ? ? "_id" : new ObjectId("5d55ac30e533bc76e4581923") // This is in case you want to filter anything.?
? ? ? ? }
},
{
? ? ? ? $lookup : {
? ? ? ? ? ? "from": "employee",
? ? ? ? ? ? "localField": "leads",
? ? ? ? ? ? "foreignField": "_id",
? ? ? ? ? ? "as": "leads"
? ? ? ? }
}])
上述查詢的 Java 等效代碼：
示例 1
List<Bson> aggregates = Arrays.asList(
? ? ? ? ? ? ? ? Aggregates.match(Filters.eq("_id", new ObjectId("5d55ac30e533bc76e4581923"))),
? ? ? ? ? ? ? ? Aggregates.lookup("employee", "leads", "_id", "leads"));
? ? ? ? AggregateIterable<Document> iterable = this.collection.aggregate(aggregates);
示例 2
List<Document> aggregates = Arrays.asList(
? ? ? ? new Document("$match", new Document("_id", new ObjectId("5d55ac30e533bc76e4581923"))),
? ? ? ? new Document("$lookup", new Document("from", "employee")
? ? ? ? .append("localField", "leads")
? ? ? ? .append("foreignField", "_id")
? ? ? ? .append("as", "leads")));
AggregateIterable<Document> iterable = collection.aggregate(aggregates);
for (Document row : iterable) {
? ? System.out.println(row.toJson());
}
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