Overview
在本指南中，您可以学习；了解如何使用Java Reactive Streams驾驶员中的构建者指定投影。
MongoDB 支持字段投影，可指定在返回查询结果时包含和排除哪些字段。MongoDB 中的投影遵循一些基本规则：
除非明确排除
_id字段，否则该字段将始终包含在内指定要包含的字段将隐式排除
_id字段以外的所有其他字段指定排除字段只会删除查询结果中的该字段
要学习；了解有关投影机制的更多信息，请参阅手册中的“从查询结果投影字段”教程。
Projections类为所有MongoDB投影操作符提供静态工厂方法。每个方法都返回一个BSON类型的实例，您可以将其传递给任何需要投影的方法。
提示
样本文档和示例
以下部分介绍了对名为projection_builders的示例集合运行查询和投影操作的示例。每个部分都使用一个名为 collection 的变量来引用 projection_builders 集合的MongoCollection实例。
该集合包含以下文档，代表了 2018 年和 2019 年的月平均气温（摄氏度）：
{ "year" : 2018, "type" : "even number but not a leap year", "temperatures" : [ { "month" : "January", "avg" : 9.765 }, { "month" : "February", "avg" : 9.675 }, { "month" : "March", "avg" : 10.004 }, { "month" : "April", "avg" : 9.983 }, { "month" : "May", "avg" : 9.747 }, { "month" : "June", "avg" : 9.65 }, { "month" : "July", "avg" : 9.786 }, { "month" : "August", "avg" : 9.617 }, { "month" : "September", "avg" : 9.51 }, { "month" : "October", "avg" : 10.042 }, { "month" : "November", "avg" : 9.452 }, { "month" : "December", "avg" : 9.86 } ] }, { "year" : 2019, "type" : "odd number, can't be a leap year", "temperatures" : [ { "month" : "January", "avg" : 10.023 }, { "month" : "February", "avg" : 9.808 }, { "month" : "March", "avg" : 10.43 }, { "month" : "April", "avg" : 10.175 }, { "month" : "May", "avg" : 9.648 }, { "month" : "June", "avg" : 9.686 }, { "month" : "July", "avg" : 9.794 }, { "month" : "August", "avg" : 9.741 }, { "month" : "September", "avg" : 9.84 }, { "month" : "October", "avg" : 10.15 }, { "month" : "November", "avg" : 9.84 }, { "month" : "December", "avg" : 10.366 } ] }
投影操作
以下各节包含可用投影操作以及使用Projections类构建这些操作的相关信息。
包含
使用 include() 方法指定包含一个或多个字段。
以下示例包括 year 字段和（隐式的）_id 字段：
Bson filter = Filters.empty(); Bson projection = include("year"); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson())) .blockLast();
{"_id": {"$oid": "6042edc9f2b56342164e0790"}, "year": 2018} {"_id": {"$oid": "6042edc9f2b56342164e0791"}, "year": 2019}
以下示例包括 year、type 和隐式 _id 字段：
Bson filter = Filters.empty(); Bson projection = include("year", "type"); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson())) .blockLast();
{"_id": {"$oid": "6042edc9f2b56342164e0790"}, "year": 2018, "type": "even number but not a leap year"} {"_id": {"$oid": "6042edc9f2b56342164e0791"}, "year": 2019, "type": "odd number, can't be a leap year"}
Exclusion
使用 exclude() 方法指定排除一个或多个字段。
以下示例排除了 temperatures（温度）字段：
Bson filter = Filters.empty(); Bson projection = exclude("temperatures"); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson())) .blockLast();
{"_id": {"$oid": "6042edc9f2b56342164e0790"}, "year": 2018, "type": "even number but not a leap year"} {"_id": {"$oid": "6042edc9f2b56342164e0791"}, "year": 2019, "type": "odd number, can't be a leap year"}
以下示例排除了type和temperatures字段：
Bson filter = Filters.empty(); Bson projection = exclude("temperatures", "type"); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson())) .blockLast();
{"_id": {"$oid": "6042edc9f2b56342164e0790"}, "year": 2018} {"_id": {"$oid": "6042edc9f2b56342164e0791"}, "year": 2019}
合并投影
使用 fields() 方法合并多个投影。
以下示例包括 year 和 type 字段，不包括 _id 字段：
Bson filter = Filters.empty(); Bson projection = fields(include("year", "type"), exclude("_id")); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson())) .blockLast();
{"year": 2018, "type": "even number but not a leap year"} {"year": 2019, "type": "odd number, can't be a leap year"}
排除 _id
使用 excludeId() 便捷方法指定排除 _id 字段：
Bson filter = Filters.empty(); Bson projection = fields(include("year", "type"), excludeId()); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson())) .blockLast();
{"year": 2018, "type": "even number but not a leap year"} {"year": 2019, "type": "odd number, can't be a leap year"}
投影数组元素匹配
使用elemMatch(String, Bson)方法变体指定一个数组投影，该投影将包含与所提供的查询筛选器匹配的数组的第一个元素。在检索到与查询筛选器（如提供）匹配的所有文档后，会发生此筛选。
注意
无论有多少个匹配项，都只包含与指定查询筛选器匹配的第一个元素。
以下示例投影 temperatures大量的第一个元素，其中 avg字段大于 10.1：
Bson filter = Filters.empty(); Bson projection = fields(include("year"), elemMatch("temperatures", Filters.gt("avg", 10.1))); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson())) .blockLast();
{"_id": {"$oid": "6042edc9f2b56342164e0790"}, "year": 2018} {"_id": {"$oid": "6042edc9f2b56342164e0791"}, "year": 2019, "temperatures": [{"month": "March", "avg": 10.43}]}
在操作的查询部分指定匹配条件后，使用 elemMatch(String) 方法变体指定位置投影以包含数组的第一个元素。只有与查询筛选条件匹配的文档才会被检索。
重要
在 4.4 之前的MongoDB版本中，指定的大量字段必须出现在查询过滤中。从MongoDB 4.4 开始，您可以对未出现在查询过滤中的大量字段使用位置项目。
以下示例投影 temperatures 数组的第一个元素：
Bson filter = Filters.gt("temperatures.avg", 10.1); Bson projection = fields(include("year"), elemMatch("temperatures")); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson())) .blockLast();
{"_id": {"$oid": "6042edc9f2b56342164e0791"}, "year": 2019, "temperatures": [{"month": "March", "avg": 10.43}]}
投影数组切片
使用 slice() 方法投影数组片段。
以下示例投影 temperatures 数组的前 6 个元素：
Bson filter = Filters.empty(); Bson projection = slice("temperatures", 6); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson(JsonWriterSettings.builder().indent(true).build()))) .blockLast();
{ "_id": { "$oid": "6042f1bc8ee6fa2a84d2be69" }, "year": 2018, "type": "even number but not a leap year", "temperatures": [ ... <January-June temperature nested documents> ] } { "_id": { "$oid": "6042f1bc8ee6fa2a84d2be6a" }, "year": 2019, "type": "odd number, can't be a leap year", "temperatures": [ ... <January-June temperature nested documents> ] }
以下示例跳过 temperatures 数组的前 6 个元素，投影接下来的 6 个元素：
Bson filter = Filters.empty(); Bson projection = slice("temperatures", 6, 6); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson(JsonWriterSettings.builder().indent(true).build()))) .blockLast();
{ "_id": { "$oid": "6042f1bc8ee6fa2a84d2be69" }, "year": 2018, "type": "even number but not a leap year", "temperatures": [ ... <July-December temperature nested documents> ] } { "_id": { "$oid": "6042f1bc8ee6fa2a84d2be6a" }, "year": 2019, "type": "odd number, can't be a leap year", "temperatures": [ ... <July-December temperature nested documents> ] }
投影文本分数
使用metaTextScore()方法指定文本查询分数的投影
以下示例将文本得分投影为 score 字段的值：
Bson filter = Filters.text("even number"); Bson projection = fields(include("year"), metaTextScore("score")); FindPublisher<Document> findPublisher = collection.find(filter).projection(projection); Flux.from(findPublisher) .doOnNext(doc -> System.out.println(doc.toJson())) .blockLast();
{"_id": {"$oid": "6042f1bc8ee6fa2a84d2be69"}, "year": 2018, "score": 1.25} {"_id": {"$oid": "6042f1bc8ee6fa2a84d2be6a"}, "year": 2019, "score": 0.625}