加入子数组中的多个记录

Question

我有一个员工记录，其中包含仅包含 id 的 leads 数组，基本上是他领导的其他员工。我想输出将每个潜在客户与员工集合中的名称连接起来的结果员工集合{    "_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")    ]}预期产出{    "_id" : ObjectId("5d4dc8635dd32dbcba4ae0c3"),    "leads" : [         {            "_id" : ObjectId("5d4dc8635dd32dbcba4ae0ba"),            "name" : "John"        },         {            "_id" : ObjectId("5d4dc8635dd32dbcba4ae0bb"),            "name" : "Jane"        }    ]}试图：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问题是，是否只有一个连接语句，或者我是否必须对数据库进行多次调用（天真方式）？

Answer 1

您不必多次调用数据库，$lookup将完全按照您在聚合框架中的要求进行操作。

尝试以下查询：

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"

        }

}])

上述查询的 Java 等效代码：

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());

}