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1. $count 聚合阶段
计算匹配到的文档数量：
db.collection.aggregate([ // 匹配条件 { $match: { field: "value" } }, // 其他聚合阶段 // ... // 计算匹配到的文档数量 { $count: "total" } ]) 首先使用 $match 阶段来筛选出满足条件的文档。然后可以添加其他的聚合阶段来进行进一步的数据处理。最后，使用 $count 阶段来计算匹配到的文档数量，并将结果存储在一个字段中（在示例中是 “total”）。
请注意，$count 阶段只返回一个文档，其中包含一个字段，表示匹配到的文档数量。如果没有匹配到任何文档，将返回一个文档，该字段的值为 0。
构造测试数据：
db.scores.drop() db.scores.insertMany([ { "_id" : 1, "subject" : "History", "score" : 88 }, { "_id" : 2, "subject" : "History", "score" : 92 }, { "_id" : 3, "subject" : "History", "score" : 97 }, { "_id" : 4, "subject" : "History", "score" : 71 }, { "_id" : 5, "subject" : "History", "score" : 79 }, { "_id" : 6, "subject" : "History", "score" : 83 } ]) $match阶段筛选 score 大于 80 的文档并计算匹配到的文档数量：
db.scores.aggregate( [ // 第一阶段 { $match: { score: { $gt: 80 } } }, // 第二阶段 { $count: "passing_scores" } ] ) { "passing_scores" : 4 } SpringBoot整合MongoDB实现：
// 输入文档实体类 @Data @Document(collection = "scores") public class Score { private int _id; private String subject; private int score; } // 输出文档实体类 @Data public class AggregationResult { private String passing_scores; } // 聚合操作 @SpringBootTest @RunWith(SpringRunner.class) public class BeanLoadServiceTest { @Autowired private MongoTemplate mongoTemplate; @Test public void aggregateTest() { // $match 聚合阶段 Criteria criteria = Criteria.where("score").gt(80); MatchOperation match = Aggregation.match(criteria); // $count 聚合阶段 CountOperation count = Aggregation.count().as("passing_scores"); // 组合聚合阶段 Aggregation aggregation = Aggregation.newAggregation(match,count); // 执行聚合查询 AggregationResults<AggregationResult> results = mongoTemplate.aggregate(aggregation, Score.class, AggregationResult.class); List<AggregationResult> mappedResults = results.getMappedResults(); // 打印结果 mappedResults.forEach(System.out::println); //AggregationResult(passing_scores=4) } } 2. $skip 聚合阶段
$skip 用于跳过指定数量的文档，并将剩余的文档传递给下一个聚合阶段。
{ $skip: <num> } 构造测试数据：
db.scores.drop() db.scores.insertMany([ { "_id" : 1, "subject" : "History", "score" : 88 }, { "_id" : 2, "subject" : "History", "score" : 92 }, { "_id" : 3, "subject" : "History", "score" : 97 }, { "_id" : 4, "subject" : "History", "score" : 71 }, { "_id" : 5, "subject" : "History", "score" : 79 }, { "_id" : 6, "subject" : "History", "score" : 83 } ]) db.scores.aggregate([ { $skip : 3 } ]); // 1 { "_id": 4, "subject": "History", "score": 71 } // 2 { "_id": 5, "subject": "History", "score": 79 } // 3 { "_id": 6, "subject": "History", "score": 83 } $skip 跳过管道传递给它的前 3 个文档，并将剩余的文档传递给下一个聚合阶段。
// 输入文档实体类 @Data @Document(collection = "scores") public class Score { private int _id; private String subject; private int score; } // 输出文档实体类 @Data public class AggregationResult { private int _id; private String subject; private int score; } // 聚合操作 @SpringBootTest @RunWith(SpringRunner.class) public class BeanLoadServiceTest { @Autowired private MongoTemplate mongoTemplate; @Test public void aggregateTest() { // $skip 聚合阶段 SkipOperation skip = Aggregation.skip(3); // 组合聚合阶段 Aggregation aggregation = Aggregation.newAggregation(skip); // 执行聚合查询 AggregationResults<AggregationResult> results = mongoTemplate.aggregate(aggregation, Score.class, AggregationResult.class); List<AggregationResult> mappedResults = results.getMappedResults(); // 打印结果 mappedResults.forEach(System.out::println); //AggregationResult(_id=4, subject=History, score=71) //AggregationResult(_id=5, subject=History, score=79) //AggregationResult(_id=6, subject=History, score=83) } } 3. $project 聚合阶段
将带所请求字段的文档传递至管道中的下个阶段。$project 返回的文档可以指定包含字段、排除 _id 字段、添加新字段以及计算现有字段的值。
$project 阶段具有以下原型形式：
{ $project: { <specification(s)> } } 1. 包含指定字段
默认情况下，_id 字段包含在输出文档中。要在输出文档中包含输入文档中的任何其他字段，必须在 $project指定，如果您指定包含的字段在文档中并不存在，那么 $project 将忽略该字段包含，同时不会将该字段添加到文档中。
构造测试数据：
db.books.drop() db.books.insertOne( { "_id" : 1, title: "abc123", isbn: "0001122223334", author: { last: "zzz", first: "aaa" }, copies: 5 } ) 以下 $project 阶段输出文档中仅包含 _id、title 和 author 字段：
db.books.aggregate( [ { $project: { title: 1 , author: 1 } } ] ) { "_id": 1, "title": "abc123", "author": { "last": "zzz", "first": "aaa" } } SpringBoot 整合 MongoDB实现：
// 输入文档实体类 @Data @Document(collection = "books") public class Book { private int _id; private String title; private String isbn; private Author author; private int copies; @Data public static class Author { private String last; private String first; } } // 输出文档实体类 @Data public class AggregationResult { private int _id; private String title; private Author author; @Data public static class Author { private String last; private String first; } } // 聚合操作 @SpringBootTest @RunWith(SpringRunner.class) public class BeanLoadServiceTest { @Autowired private MongoTemplate mongoTemplate; @Test public void testAggregate(){ // project 阶段 ProjectionOperation project = Aggregation.project("title", "author"); // 组合阶段 Aggregation aggregation = Aggregation.newAggregation(project); // 执行聚合 AggregationResults<AggregationResult> aggregationResults = mongoTemplate.aggregate(aggregation, Book.class, AggregationResult.class); List<AggregationResult> mappedResults = aggregationResults.getMappedResults(); // 打印结果 mappedResults.forEach(System.out::println); // AggregationResult(_id=1, title=abc123, author=AggregationResult.Author(last=zzz, first=aaa)) } } 2. 排除_id字段
构造测试数据：
db.books.drop() db.books.insertOne( { "_id" : 1, title: "abc123", isbn: "0001122223334", author: { last: "zzz", first: "aaa" }, copies: 5 } ) 以下 $project 阶段输出文档中仅包含 title 和 author 字段：
db.books.aggregate( [ { $project: { _id: 0, title: 1, author: 1 } } ] ) { "title": "abc123", "author": { "last": "zzz", "first": "aaa" } } SpringBoot 整合 MongoDB实现：
// 输入文档实体类 @Data @Document(collection = "books") public class Book { private int _id; private String title; private String isbn; private Author author; private int copies; @Data public static class Author { private String last; private String first; } } // 输出文档实体类 @Data public class AggregationResult { private int _id; private String title; private String isbn; private Author author; private int copies; @Data public static class Author { private String last; private String first; } } // 聚合操作 @SpringBootTest @RunWith(SpringRunner.class) public class BeanLoadServiceTest { @Autowired private MongoTemplate mongoTemplate; @Test public void testAggregate(){ // project 阶段 ProjectionOperation project = Aggregation.project("title", "author").andExclude("_id"); // 组合阶段 Aggregation aggregation = Aggregation.newAggregation(project); // 执行聚合 AggregationResults<AggregationResult> aggregationResults = mongoTemplate.aggregate(aggregation, Book.class, AggregationResult.class); List<AggregationResult> mappedResults = aggregationResults.getMappedResults(); // 打印结果 mappedResults.forEach(System.out::println); // AggregationResult(title=abc123, author=AggregationResult.Author(last=zzz, first=aaa)) } } 3. 排除指定字段
构造测试数据：
db.books.drop() db.books.insertOne( { "_id" : 1, title: "abc123", isbn: "0001122223334", author: { last: "zzz", first: "aaa" }, copies: 5, lastModified: "2016-07-28" } ) 在 $project 阶段中输出文档排除 title 和 isbn 字段：
db.books.aggregate( [ { $project: { title: 0, isbn: 0 } } ] ) // 1 { "_id": 1, "author": { "last": "zzz", "first": "aaa" }, "copies": 5, "lastModified": "2016-07-28" } SpringBoot 整合 MongoDB实现：
@SpringBootTest @RunWith(SpringRunner.class) public class BeanLoadServiceTest { @Autowired private MongoTemplate mongoTemplate; @Test public void testAggregate(){ // project 阶段 ProjectionOperation project = Aggregation.project().andExclude("isbn","title"); // 组合阶段 Aggregation aggregation = Aggregation.newAggregation(project); // 执行聚合 AggregationResults<AggregationResult> aggregationResults = mongoTemplate.aggregate(aggregation, Book.class, AggregationResult.class); List<AggregationResult> mappedResults = aggregationResults.getMappedResults(); // 打印结果 mappedResults.forEach(System.out::println); // AggregationResult(_id=1, title=null, isbn=null, author=AggregationResult.Author(last=zzz, first=aaa), copies=5, lastModified=2016-07-28) } } 4. 不能同时指定包含字段和排除字段
构造测试数据：
db.books.drop() db.books.insertOne( { "_id" : 1, title: "abc123", isbn: "0001122223334", author: { last: "zzz", first: "aaa" }, copies: 5, lastModified: "2016-07-28" } ) 在 $project 阶段中输出文档排除 title 和 isbn 字段：
db.books.aggregate( [ { $project: { title: 0, isbn: 1 } } ] ) 报错信息：
[Error] Bad projection specification, cannot include fields or add computed fields during an exclusion projection: { title: 0.0, isbn: 1.0 }
原因分析：在投影规范中，除_id字段外，不要在包含投影规范中使用排除操作。下面这样是可以的：
db.books.aggregate( [ { $project: { _id: 0, isbn: 1 } } ] ) 5. 排除嵌入式文档中的指定字段
构造测试数据：
db.books.drop() db.books.insertOne( { "_id" : 1, title: "abc123", isbn: "0001122223334", author: { last: "zzz", first: "aaa" }, copies: 5, lastModified: "2016-07-28" } ) 以下 $project 阶段中输出文档排除 author.first 和 lastModified 字段：
db.books.aggregate( [ { $project: { "author.first": 0, lastModified: 0 } } ] ) { "_id": 1, "title": "abc123", "isbn": "0001122223334", "author": { "last": "zzz" }, "copies": 5 } 6. 包含嵌入式文档中的指定字段
构造测试数据：
db.books.drop() db.books.insertMany( [ { _id: 1, user: "1234", stop: { title: "book1", author: "xyz", page: 32 } }, { _id: 2, user: "7890", stop: [ { title: "book2", author: "abc", page: 5 }, { title: "book3", author: "ijk", page: 100 } ] } ] ) 以下 $project 阶段仅包含嵌入式文档中的 title 字段：
db.books.aggregate( [ { $project: { "stop.title": 1 } } ] ) // 1 { "_id": 1, "stop": { "title": "book1" } } // 2 { "_id": 2, "stop": [ { "title": "book2" }, { "title": "book3" } ] } 7. 添加新字段
构造测试数据：
db.books.drop() db.books.insertOne( { "_id" : 1, title: "abc123", isbn: "0001122223334", author: { last: "zzz", first: "aaa" }, copies: 5 } ) 以下 $project 阶段添加新字段 isbn、lastName 和 copiesSold：
db.books.aggregate( [ { $project: { title: 1, isbn: { prefix: { $substr: [ "$isbn", 0, 3 ] }, group: { $substr: [ "$isbn", 3, 2 ] }, publisher: { $substr: [ "$isbn", 5, 4 ] }, title: { $substr: [ "$isbn", 9, 3 ] }, checkDigit: { $substr: [ "$isbn", 12, 1] } }, lastName: "$author.last", copiesSold: "$copies" } } ] ) { "_id": 1, "title": "abc123", "isbn": { "prefix": "000", "group": "11", "publisher": "2222", "title": "333", "checkDigit": "4" }, "lastName": "zzz", "copiesSold": 5 } 8. 重命名字段
构造测试数据：
db.books.drop() db.books.insertOne( { "_id" : 1, title: "abc123", isbn: "0001122223334", author: { last: "zzz", first: "aaa" }, copies: 5 } ) 以下 $project 阶段将字段 copies重命名为 copiesSold ：
db.books.aggregate( [ { $project: { title: 1, copiesSold: "$copies" } } ] ) { "_id": 1, "title": "abc123", "copiesSold": 5 } SpringBoot 整合MongoDB实现：
// 输入文档 @Data @Document(collection = "books") public class Book { private int _id; private String title; private String isbn; private Author author; private int copies; private String lastModified; @Data public static class Author { private String last; private String first; } } // 输出文档 @Data public class AggregationResult { private int _id; private String title; private String isbn; private Author author; private int copiesSold; private String lastModified; @Data public static class Author { private String last; private String first; } } // 聚合操作 @SpringBootTest @RunWith(SpringRunner.class) public class BeanLoadServiceTest { @Autowired private MongoTemplate mongoTemplate; @Test public void testAggregate(){ // project 阶段 ProjectionOperation project = Aggregation.project("title").and("copies").as("copiesSold"); // 组合阶段 Aggregation aggregation = Aggregation.newAggregation(project); // 执行聚合 AggregationResults<AggregationResult> aggregationResults = mongoTemplate.aggregate(aggregation, Book.class, AggregationResult.class); List<AggregationResult> mappedResults = aggregationResults.getMappedResults(); // 打印结果 mappedResults.forEach(System.out::println); // AggregationResult(_id=1, title=abc123, isbn=null, author=null, copiesSold=5, lastModified=null) } } 