Java Spring数据mongodb模拟ifNull行为

我有一套表格：

{ "_id" : { "$oid" : "57050f22d5f79d2e6866c6f8" }, "form_name" : "Form 1", "ver" : 0, "createdAt" : { "$date" : 1459950030037 } }
{ "_id" : { "$oid" : "57050fa6d5f79d2e6866c6f9" }, "form_name" : "Form 1", "ver" : 1, "vid" : { "$oid" : "57050f22d5f79d2e6866c6f8" }, "createdAt" : { "$date" : 1459950030037 } }
{ "_id" : { "$oid" : "57050ff2d5f79d2e6866c6fa" }, "form_name" : "Form 1", "ver" : 2, "vid" : { "$oid" : "57050f22d5f79d2e6866c6f8" }, "createdAt" : { "$date" : 1459950030037 } }
{ "_id" : { "$oid" : "570511ced5f79d2e6866c6fb" }, "form_name" : "Form 2", "ver" : 0, "createdAt" : { "$date" : 1459950030037 } }

我得到的表单版本如下：

db.forms.aggregate([
    {"$match":{"deletedAt":{"$exists":false}}},
    {"$sort":{"createdAt":1}},
    {"$group":{
            "_id": {"$ifNull":["$vid", "$_id"]},
            "vid":{"$push":"$$ROOT"}
        }
    }
])

List<DBObject> tmpAggr = new ArrayList<>();
tmpAggr.add(new BasicDBObject("$match", new BasicDBObject("deletedAt", new BasicDBObject("$exists", false))));
tmpAggr.add(new BasicDBObject("$sort",  new BasicDBObject("createdAt", 1)));
tmpAggr.add(new BasicDBObject("$group", BasicDBObjectBuilder.start("_id", new BasicDBObject("$ifNull", new String[]{"$vid", "$_id"}))
                                                        .append("vid", new BasicDBObject("$push", "$$ROOT")).get()));
tmpAggr.add(new BasicDBObject("$sort",  new BasicDBObject("_id", 1)));
        list_with_versions = Collections.unmodifiableList(tmpAggr);  
org.getDB().getCollection("forms").aggregate(list_with_versions).results()

我得到了这样的答案：

[
    {
        "_id" : ObjectId("57050f22d5f79d2e6866c6f8"),
        "vid" : [
            {
                "_id" : ObjectId("57050f22d5f79d2e6866c6f8"),
                "form_name" : "Form 1",
                "ver" : 0,
                "createdAt" : ISODate("2016-04-06T13:29:06.079Z")
            },
            {
                "_id" : ObjectId("57050fa6d5f79d2e6866c6f9"),
                "form_name" : "Form 1",
                "ver" : 1,
                "createdAt" : ISODate("2016-04-06T13:31:18.742Z"),
                "vid" : ObjectId("57050f22d5f79d2e6866c6f8")
            },
            {
                "_id" : ObjectId("57050ff2d5f79d2e6866c6fa"),
                "form_name" : "Form 1",
                "ver" : 2,
                "createdAt" : ISODate("2016-04-06T13:32:34.986Z"),
                "vid" : ObjectId("57050f22d5f79d2e6866c6f8")
            }
        ]
    },
    {
        "_id" : ObjectId("570511ced5f79d2e6866c6fb"),
        "vid" : [
            {
                "_id" : ObjectId("570511ced5f79d2e6866c6fb"),
                "form_name" : "Form 2",
                "ver" : 0,
                "createdAt" : ISODate("2016-04-06T13:40:30.037Z")
            }
        ]
    }
]

在我的Java后端中，我将按如下方式进行操作：

db.forms.aggregate([
    {"$match":{"deletedAt":{"$exists":false}}},
    {"$sort":{"createdAt":1}},
    {"$group":{
            "_id": {"$ifNull":["$vid", "$_id"]},
            "vid":{"$push":"$$ROOT"}
        }
    }
])

List<DBObject> tmpAggr = new ArrayList<>();
tmpAggr.add(new BasicDBObject("$match", new BasicDBObject("deletedAt", new BasicDBObject("$exists", false))));
tmpAggr.add(new BasicDBObject("$sort",  new BasicDBObject("createdAt", 1)));
tmpAggr.add(new BasicDBObject("$group", BasicDBObjectBuilder.start("_id", new BasicDBObject("$ifNull", new String[]{"$vid", "$_id"}))
                                                        .append("vid", new BasicDBObject("$push", "$$ROOT")).get()));
tmpAggr.add(new BasicDBObject("$sort",  new BasicDBObject("_id", 1)));
        list_with_versions = Collections.unmodifiableList(tmpAggr);  
org.getDB().getCollection("forms").aggregate(list_with_versions).results()

List tmpAggr=newarraylist（）；
tmpAggr.add（新BasicDBObject（$match），新BasicDBObject（$deletedAt，新BasicDBObject（$exists，false））；
tmpAggr.add（新的BasicDBObject（$sort），新的BasicDBObject（“createdAt”，1））；
tmpAggr.add（新BasicDBObject（“$group”，BasicDBObject Builder.start（“\u-id”，新BasicDBObject（$ifNull，新字符串[]{“$vid”，“$\u-id”}））
.append（“vid”，新的BasicDBObject（“$push”，“$$ROOT”））.get（））；
tmpAggr.add（新的BasicDBObject（$sort），新的BasicDBObject（'u id'，1））；
list_with_versions=Collections.unmodifiableList（tmpAggr）；
org.getDB（）.getCollection（“forms”）.aggregate（列出\u及其版本）。results（）

如何用Spring Data MongoDB框架重现这种行为？

---

问题是没有ifNull构造。如何管理此问题？

您可以创建一个解决方案，实现接口，以接受一个

DBObject

，该对象表示聚合管道中使用

$ifNull

运算符的单个操作：

public class GroupAggregationOperation implements AggregationOperation {
    private DBObject operation;

    public GroupAggregationOperation (DBObject operation) {
        this.operation = operation;
    }

    @Override
    public DBObject toDBObject(AggregationOperationContext context) {
        return context.getMappedObject(operation);
    }
}

然后将

$group

操作作为聚合管道中的DBObject执行，该对象与您拥有的对象相同：

DBObject operation = (DBObject) new BasicDBObject(
    "$group", BasicDBObjectBuilder.start(
        "_id", new BasicDBObject(
            "$ifNull", new String[]{"$vid", "$_id"}
        )
    )
    .append("vid", new BasicDBObject("$push", "$$ROOT"))
    .get()              
);

然后您可以将其用作：

import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;

GroupAggregationOperation groupOp = new GroupAggregationOperation(operation);
Aggregation agg = newAggregation(
    match(Criteria.where("deletedAt").exists(false)),
    sort(ASC, "deletedAt"),
    groupOp 
);
AggregationResults<Forms> results = mongoTemplate.aggregate(agg, Forms.class); 
List<Forms> forms = results.getMappedResults();

import static org.springframework.data.mongodb.core.aggregation.aggregation.*；
GroupAggregationOperation groupOp=新的GroupAggregationOperation（操作）；
聚合agg=newAggregation(
匹配（条件，其中（“删除数据”）。存在（错误）），
排序（ASC，“删除数据”），
群居
);
AggregationResults=mongoTemplate.aggregate（agg，Forms.class）；
List forms=results.getMappedResults（）；

工作起来很有魅力。非常感谢。