我有一個(gè)員工記錄，其中包含僅包含 id 的 leads 數(shù)組，基本上是他領(lǐng)導(dǎo)的其他員工。我想輸出將每個(gè)潛在客戶與員工集合中的名稱連接起來(lái)的結(jié)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(yù)期產(chǎn)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(wèn)題是，是否只有一個(gè)連接語(yǔ)句，或者我是否必須對(duì)數(shù)據(jù)庫(kù)進(jìn)行多次調(diào)用（天真方式）？
1 回答
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TA貢獻(xiàn)1806條經(jīng)驗(yàn) 獲得超5個(gè)贊
您不必多次調(diào)用數(shù)據(jù)庫(kù)，$lookup將完全按照您在聚合框架中的要求進(jìn)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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