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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 {@Autowiredprivate MongoTemplate mongoTemplate;@Testpublic 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 {@Autowiredprivate MongoTemplate mongoTemplate;@Testpublic 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;@Datapublic 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;@Datapublic static class Author {private String last;private String first;}
}// 聚合操作
@SpringBootTest
@RunWith(SpringRunner.class)
public class BeanLoadServiceTest {@Autowiredprivate MongoTemplate mongoTemplate;@Testpublic 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;@Datapublic 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;@Datapublic static class Author {private String last;private String first;}
}// 聚合操作
@SpringBootTest
@RunWith(SpringRunner.class)
public class BeanLoadServiceTest {@Autowiredprivate MongoTemplate mongoTemplate;@Testpublic 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 {@Autowiredprivate MongoTemplate mongoTemplate;@Testpublic 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;@Datapublic 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;@Datapublic static class Author {private String last;private String first;}
}// 聚合操作
@SpringBootTest
@RunWith(SpringRunner.class)
public class BeanLoadServiceTest {@Autowiredprivate MongoTemplate mongoTemplate;@Testpublic 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)}
}