前言
最近学习使用mongodb中间使用关联表查询问题遇到一些问题记录下来分享给大家。
mongodb关联查询
之前使用SQL语法来查询oracle、sqlserver、mysql表之间的关联，但是到mongodb之后完全无从下手，写法完全不一样，于是到网上查询mongodb关联表查询的写法，于是参考代码自己试着写了下，但是发现有好多问题，比如我有两个表user和apple，
user表：
| id | name | age | createDt |
|---|---|---|---|
| ObjectId(1xxxx1) | zhangsan | 32 | |
| ObjectId(2xxxx2) | lishi | 23 | |
| ObjectId(3xxxx3) | wangwu | 19 | |
| ObjectId(4xxxx4) | liuliu | 28 |
apple表：
| id | uid | device | createDt |
|---|---|---|---|
| ObjectId(axxxxa) | 1xxxx1 | 10 | |
| ObjectId(bxxxxb) | 2xxxx2 | 2 | |
| ObjectId(cxxxxc) | 3xxxx3 | 7 | |
| ObjectId(dxxxxd) | 4xxxx4 | 1 |
user实体类：
@Setter
@Getter
public class User {
private String id;
private String name;
private Integer age;
private Date createDt;
}apple实体类：
@Setter
@Getter
public class Apple {
private String id;
private String uid;
private Integer device;
private Date createDt;
}user和apple之间是一对一关系，现在我的查询要求是：按device数量从大到小排序查询用户数据。
查询结果应该是：
| id | name | age | createDt |
|---|---|---|---|
| ObjectId(1xxxx1) | zhangsan | 32 | |
| ObjectId(3xxxx3) | wangwu | 19 | |
| ObjectId(2xxxx2) | lishi | 23 | |
| ObjectId(4xxxx4) | liuliu | 28 |
我们分别使用user表和apple表做主表来查询，那么使用mongodb语法我们试着来写一下。
以user表做主表查询
// 1、添加_id字段类型转换
AddFieldsOperation addFieldsOperation = AddFieldsOperation.addField("_id").withValue(ConvertOperators.ToString.toString("$_id")).build();
// 2、按参数顺序：被关联表apple，主表user.id，被关联表apple.uid，别名
LookupOperation lookupOperation = Aggregation.lookup("apple", "_id", "uid", "apple_as");
// 3、查询哪些字段，类似sql里面的 select 选择器
ProjectionOperation project = Aggregation.project("name","age","createDt").and("apple_as.device").as("device");
// 4、按照apple.device数量排序
// 注意：不能使用apple_as.device，并且 device字段必须出现在project里面，否则查询失败
SortOperation sort = Aggregation.sort(Sort.Direction.DESC, "device");
// 5、添加取List中
List operationList = Lists.newArrayList();
operationList.add(addFieldsOperation);
operationList.add(lookupOperation);
operationList.add(project);
operationList.add(sort);
Aggregation agg = Aggregation.newAggregation(operationList);
// 因为返回的是User实体类，所以不会出现device字段，你可以使用Map.class来返回想要的字段
AggregationResults aggregationResults = mongoTemplate.aggregate(agg, "user", User.class);
// 6、返回关联查询结果
List dataList = aggregationResults.getMappedResults(); 注意：我们在查询之前要把主表的id转换成String类型，因为被关联表的uid就是String类型，否则查询失败
以apple表做主表查询
// 1、添加uid字段类型转换
AddFieldsOperation addFieldsOperation = AddFieldsOperation.addField("uid").withValue(ConvertOperators.ToObjectId.toObjectId("$uid")).build();
// 2、按参数顺序：被关联表user，主表apple.uid，被关联表user.id，别名
LookupOperation lookupOperation = Aggregation.lookup("user", "uid", "_id", "user_as");
// 3、查询哪些字段，类似sql里面的 select 选择器
// 注意：因为user是被关联表，所以要使用user_as别名的方式来获取用户信息字段
ProjectionOperation project = Aggregation.project("user_as.name","user_as.age","user_as.createDt","device");
// 4、按照apple.device数量排序
SortOperation sort = Aggregation.sort(Sort.Direction.DESC, "device");
// 5、添加取List中
List operationList = Lists.newArrayList();
operationList.add(addFieldsOperation);
operationList.add(lookupOperation);
operationList.add(project);
operationList.add(sort);
Aggregation agg = Aggregation.newAggregation(operationList);
// 因为返回的是User实体类，所以不会出现device字段，你可以使用Map.class来返回想要的字段
AggregationResults aggregationResults = mongoTemplate.aggregate(agg, "apple", User.class);
// 6、返回关联查询结果
List dataList = aggregationResults.getMappedResults(); 注意：我们在查询之前要把主表的uid转换ObjectId类型，因为被关联表的_id就是ObjectId类型，否则查询失败
2022-01-14更新
三表关联
List operationList = Lists.newArrayList();
AddFieldsOperation add1FieldsOperation = AddFieldsOperation.addField("categoryId").withValue(ConvertOperators.ToObjectId.toObjectId("$categoryId")).build();
LookupOperation lookupOperation1 = Aggregation.lookup("urlCategory", "categoryId", "_id", "category_as");
LookupOperation lookupOperation2 = Aggregation.lookup("user", "uid", "uid", "user_as");
MatchOperation match = Aggregation.match(Criteria.where("uid").in(uids).and("isDelete").is(false));
ProjectionOperation project = Aggregation.project("user_as.userName","user_as.avatar","uid", "title", "url", "domain", "favicon", "isAttention", "isCopy", "createdDt")
.and("category_as.name").as("categoryName")
.andExpression("{ $dateToString: {date: '$createdDt', format: '%Y-%m-%d %H:%M:%S', timezone: '+08:00'}}").as("date")
.and(ConvertOperators.Convert.convertValueOf("categoryId").to("string")).as("category_id");
// sort里面的clickCount字段必须从Project中取，否则 失败
SortOperation sort = Aggregation.sort(Sort.Direction.DESC, "createdDt");
FacetOperation facet = Aggregation.facet(
count().as("count")).as("total")
.and(skip(pageSize * (pageNum - 1)), limit(pageSize)).as("rows");
UnwindOperation unwindOperation1 = Aggregation.unwind("category_as");
UnwindOperation unwindOperation2 = Aggregation.unwind("user_as");
operationList.add(add1FieldsOperation);
operationList.add(lookupOperation1);
operationList.add(lookupOperation2);
operationList.add(unwindOperation1);
operationList.add(unwindOperation2);
//因为match里面含有url_as，所以lookupOperation要在match上面
operationList.add(match);
operationList.add(project);
operationList.add(sort);
operationList.add(facet);
Aggregation agg = Aggregation.newAggregation(operationList);
// collectionName:主表
AggregationResults aggregationResults = mongoTemplate.aggregate(agg, "url", HashMap.class);
return aggregationResults.getMappedResults(); 2022-12-22更新
三表关联 一对多(多的这边进行条件筛选)
// 1、添加uid字段类型转换
AddFieldsOperation addFieldsOperation1 = AddFieldsOperation.addField("objectId").withValue(ConvertOperators.ToObjectId.toObjectId("$objectId")).build();
AddFieldsOperation addFieldsOperation2 = AddFieldsOperation.addField("objectId").withValue(ConvertOperators.ToString.toString("$objectId")).build();
// 2、按参数顺序：被关联表url，主表userEvent.objectId，被关联表url.id，别名
LookupOperation lookupOperation1 = Aggregation.lookup("url", "objectId", "_id", "url_as");
LookupOperation lookupOperation2 = Aggregation.lookup("userLikeUrl", "objectId", "urlId", "userLikeUrl_as");
LookupOperation lookupOperation3 = Aggregation.lookup("userCopyUrl", "objectId", "fromUrlId", "userCopyUrl_as");
MatchOperation matchOperation1 = Aggregation.match(Criteria.where("url_as.uid").in(uids).and("url_as.isDelete").is(false));
ProjectionOperation projectionOperation1 = Aggregation.project("uid", "userName", "userAvatar", "action", "objectType", "objectOthers",
"createdDt", "url_as", "userCopyUrl_as")
.andExpression("toString(objectId)").as("objectId")
// 先筛选出userLikeUrl_as.valid=1的列表到userLikeUrl_as_list中
.and(filter("userLikeUrl_as").as("item")
.by(ComparisonOperators.Eq.valueOf("item.valid").equalToValue(1))).as("userLikeUrl_as_list");
ProjectionOperation projectionOperation2 = Aggregation.project("uid", "userName", "userAvatar", "action", "objectType", "objectOthers",
"createdDt", "url_as", "objectId", "userCopyUrl_as")
// 再对userLikeUrl_as_list统计数量到userLikeUrl_count
.andExpression("{$size: '$userLikeUrl_as_list'}").as("userLikeUrl_count")
.andExpression("{$size: '$userCopyUrl_as'}").as("userCopyUrl_count");
// 4、按照apple.device数量排序
SortOperation sort = Aggregation.sort(Sort.Direction.DESC, "createdDt");
FacetOperation facet = Aggregation.facet(count().as("count")).as("total").and(skip(pageSize * (pageNum - 1)), limit(pageSize)).as("rows");
UnwindOperation unwindOperation1 = Aggregation.unwind("url_as");
List operationList = Lists.newArrayList();
operationList.add(lookupOperation3);
operationList.add(lookupOperation2);
operationList.add(addFieldsOperation1);
operationList.add(lookupOperation1);
operationList.add(matchOperation1);
operationList.add(unwindOperation1);
operationList.add(projectionOperation1);
operationList.add(projectionOperation2);
operationList.add(sort);
operationList.add(facet);
Aggregation agg = Aggregation.newAggregation(operationList);
// 因为返回的是User实体类，所以不会出现device字段，你可以使用Map.class来返回想要的字段
AggregationResults aggregationResults = mongoTemplate.aggregate(agg, "userEvent", HashMap.class);
return aggregationResults.getMappedResults(); 注意看上面的projectionOperation1和projectionOperation2，并且lookupOperation2中有userLikeUrl_as集合数据，然后我们在projectionOperation1中添加筛选条件过滤出userLikeUrl_as_list列表，如下所示：
.and(filter("userLikeUrl_as").as("item")
.by(ComparisonOperators.Eq.valueOf("item.valid").equalToValue(1))).as("userLikeUrl_as_list");过滤条件是：item.valid=1的数据，接着我们再projectionOperation2中聚合数据统计userLikeUrl_as_list集合中的数量
.andExpression("{$size: '$userLikeUrl_as_list'}").as("userLikeUrl_count")总结
1、mongodb关联查询一定要注意ObjectId类型和String类型之间的转换，否则不成功
ObjectId转换String方法一：
.andExpression("toString(objectId)").as("objectId")ObjectId转换String方法二：
.and(ConvertOperators.Convert.convertValueOf("categoryId").to("string")).as("category_id")ObjectId转换String方法三：
AddFieldsOperation addFieldsOperation2 = AddFieldsOperation.addField("objectId").withValue(ConvertOperators.ToString.toString("$objectId")).build();
String转换ObjectId方法一：
AddFieldsOperation addFieldsOperation1 = AddFieldsOperation.addField("objectId").withValue(ConvertOperators.ToObjectId.toObjectId("$objectId")).build();
2、project中出现的字段会影响 sort排序
3、operationList的添加前后顺序也会影响查询
4、如果查询失败要多试几遍，到底是哪一步出现的影响，我也是不断的试错才总结出来的。
5、如果有条件的话可以加如下代码：
AggregationOperation match = Aggregation.match(Criteria.where("uid").is(uid));
LimitOperation limit = Aggregation.limit(10);
operationList.add(match);
operationList.add(limit);引用
Mongo学习笔记（三） 通过Aggregation和lookup进行多级关联查询
Springboot整合MongoDB系列(五)---LookupOperation关联查询