目录
QUESTION：SpringBoot有关@Document(collection = "")与@Document(collation= "")注解的区别？
ANSWER：
一、问题原因
二、分析
2.1 Collection
@Id
@Document
@Indexed
@CompoundIndex
@Field
@Transient
@DBRef
2.2 Collation
2.2.1特性
2.2.2示例
转载博客出处：
QUESTION：SpringBoot有关@Document(collection = "")与@Document(collation= "")注解的区别？
ANSWER：
在启动项目时，报出如下异常:
org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'messageDaoImpl': Unsatisfied dependency expressed through field 'mongoTemplate'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'mongoTemplate' defined in class path resource [org/springframework/boot/autoconfigure/data/mongo/MongoDbFactoryDependentConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.data.mongodb.core.MongoTemplate]: Factory method 'mongoTemplate' threw exception; nested exception is org.springframework.data.mongodb.UncategorizedMongoDbException: Command failed with error 2 (BadValue): 'failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: "status", ns: "testdb.message", collation: { locale: "message" } } :: caused by :: Field 'locale' is invalid in: { locale: "message" }' on server www.youngxy.top:27018. The full response is {"ok": 0.0, "errmsg": "failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: \"status\", ns: \"testdb.message\", collation: { locale: \"message\" } } :: caused by :: Field 'locale' is invalid in: { locale: \"message\" }", "code": 2, "codeName": "BadValue"}; nested exception is com.mongodb.MongoCommandException: Command failed with error 2 (BadValue): 'failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: "status", ns: "testdb.message", collation: { locale: "message" } } :: caused by :: Field 'locale' is invalid in: { locale: "message" }' on server www.youngxy.top:27018. The full response is {"ok": 0.0, "errmsg": "failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: \"status\", ns: \"testdb.message\", collation: { locale: \"message\" } } :: caused by :: Field 'locale' is invalid in: { locale: \"message\" }", "code": 2, "codeName": "BadValue"}at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:643) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:116) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1422) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:594) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:879) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:878) ~[spring-context-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550) ~[spring-context-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) ~[spring-boot-2.2.4.RELEASE.jar:2.2.4.RELEASE]at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) ~[spring-boot-2.2.4.RELEASE.jar:2.2.4.RELEASE]at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) ~[spring-boot-2.2.4.RELEASE.jar:2.2.4.RELEASE]at org.springframework.boot.test.context.SpringBootContextLoader.loadContext(SpringBootContextLoader.java:126) [spring-boot-test-2.2.4.RELEASE.jar:2.2.4.RELEASE]at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:99) [spring-test-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:124) [spring-test-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:123) [spring-test-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.test.context.web.ServletTestExecutionListener.setUpRequestContextIfNecessary(ServletTestExecutionListener.java:190) [spring-test-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.test.context.web.ServletTestExecutionListener.prepareTestInstance(ServletTestExecutionListener.java:132) [spring-test-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:244) [spring-test-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.test.context.junit.jupiter.SpringExtension.postProcessTestInstance(SpringExtension.java:98) [spring-test-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$invokeTestInstancePostProcessors$5(ClassBasedTestDescriptor.java:337) [junit-jupiter-engine-5.5.2.jar:5.5.2]at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.executeAndMaskThrowable(ClassBasedTestDescriptor.java:342) [junit-jupiter-engine-5.5.2.jar:5.5.2]at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$invokeTestInstancePostProcessors$6(ClassBasedTestDescriptor.java:337) [junit-jupiter-engine-5.5.2.jar:5.5.2]at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[na:1.8.0_202]at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175) ~[na:1.8.0_202]at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382) ~[na:1.8.0_202]at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) ~[na:1.8.0_202]at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[na:1.8.0_202]at java.util.stream.StreamSpliterators$WrappingSpliterator.forEachRemaining(StreamSpliterators.java:312) ~[na:1.8.0_202]at java.util.stream.Streams$ConcatSpliterator.forEachRemaining(Streams.java:743) ~[na:1.8.0_202]at java.util.stream.Streams$ConcatSpliterator.forEachRemaining(Streams.java:742) ~[na:1.8.0_202]at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580) ~[na:1.8.0_202]at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.invokeTestInstancePostProcessors(ClassBasedTestDescriptor.java:336) [junit-jupiter-engine-5.5.2.jar:5.5.2]at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.instantiateAndPostProcessTestInstance(ClassBasedTestDescriptor.java:259) [junit-jupiter-engine-5.5.2.jar:5.5.2]at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$testInstancesProvider$2(ClassBasedTestDescriptor.java:252) [junit-jupiter-engine-5.5.2.jar:5.5.2]at java.util.Optional.orElseGet(Optional.java:267) ~[na:1.8.0_202]at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$testInstancesProvider$3(ClassBasedTestDescriptor.java:251) [junit-jupiter-engine-5.5.2.jar:5.5.2]at org.junit.jupiter.engine.execution.TestInstancesProvider.getTestInstances(TestInstancesProvider.java:29) ~[junit-jupiter-engine-5.5.2.jar:5.5.2]at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$prepare$0(TestMethodTestDescriptor.java:106) ~[junit-jupiter-engine-5.5.2.jar:5.5.2]at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.prepare(TestMethodTestDescriptor.java:105) ~[junit-jupiter-engine-5.5.2.jar:5.5.2]at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.prepare(TestMethodTestDescriptor.java:69) ~[junit-jupiter-engine-5.5.2.jar:5.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$prepare$1(NodeTestTask.java:107) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.prepare(NodeTestTask.java:107) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:75) ~[junit-platform-engine-1.5.2.jar:1.5.2]at java.util.ArrayList.forEach(ArrayList.java:1257) ~[na:1.8.0_202]at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80) ~[junit-platform-engine-1.5.2.jar:1.5.2]at java.util.ArrayList.forEach(ArrayList.java:1257) ~[na:1.8.0_202]at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51) ~[junit-platform-engine-1.5.2.jar:1.5.2]at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:229) ~[junit-platform-launcher-1.5.2.jar:1.5.2]at org.junit.platform.launcher.core.DefaultLauncher.lambda$execute$6(DefaultLauncher.java:197) ~[junit-platform-launcher-1.5.2.jar:1.5.2]at org.junit.platform.launcher.core.DefaultLauncher.withInterceptedStreams(DefaultLauncher.java:211) ~[junit-platform-launcher-1.5.2.jar:1.5.2]at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:191) ~[junit-platform-launcher-1.5.2.jar:1.5.2]at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:128) ~[junit-platform-launcher-1.5.2.jar:1.5.2]at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:69) ~[junit5-rt.jar:na]at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33) ~[junit-rt.jar:na]at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230) ~[junit-rt.jar:na]at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58) ~[junit-rt.jar:na]
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'mongoTemplate' defined in class path resource [org/springframework/boot/autoconfigure/data/mongo/MongoDbFactoryDependentConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.data.mongodb.core.MongoTemplate]: Factory method 'mongoTemplate' threw exception; nested exception is org.springframework.data.mongodb.UncategorizedMongoDbException: Command failed with error 2 (BadValue): 'failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: "status", ns: "testdb.message", collation: { locale: "message" } } :: caused by :: Field 'locale' is invalid in: { locale: "message" }' on server www.youngxy.top:27018. The full response is {"ok": 0.0, "errmsg": "failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: \"status\", ns: \"testdb.message\", collation: { locale: \"message\" } } :: caused by :: Field 'locale' is invalid in: { locale: \"message\" }", "code": 2, "codeName": "BadValue"}; nested exception is com.mongodb.MongoCommandException: Command failed with error 2 (BadValue): 'failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: "status", ns: "testdb.message", collation: { locale: "message" } } :: caused by :: Field 'locale' is invalid in: { locale: "message" }' on server www.youngxy.top:27018. The full response is {"ok": 0.0, "errmsg": "failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: \"status\", ns: \"testdb.message\", collation: { locale: \"message\" } } :: caused by :: Field 'locale' is invalid in: { locale: \"message\" }", "code": 2, "codeName": "BadValue"}at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:656) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:636) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1338) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1177) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1287) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1207) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640) ~[spring-beans-5.2.3.RELEASE.jar:5.2.3.RELEASE]... 81 common frames omitted
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.data.mongodb.core.MongoTemplate]: Factory method 'mongoTemplate' threw exception; nested exception is org.springframework.data.mongodb.UncategorizedMongoDbException: Command failed with error 2 (BadValue): 'failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: "status", ns: "testdb.message", collation: { locale: "message" } } :: caused by :: Field 'locale' is invalid in: { locale: "message" }' on server www.youngxy.top:27018. The full response is {"ok": 0.0, "errmsg": "failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: \"status\", ns: \"testdb.message\", collation: { locale: \"message\" } } :: caused by :: Field 'locale' is invalid in: { locale: \"message\" }", "code": 2, "codeName": "BadValue"}; nested exception is com.mongodb.MongoCommandException: Command failed with error 2 (BadValue): 'failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: "status", ns: "testdb.message", collation: { locale: "message" } } :: caused by :: Field 'locale' is invalid in: { locale: "message" }' on server www.youngxy.top:27018. The full response is {"ok": 0.0, "errmsg": "failed to add collation information to index spec for index creation: { v: 2, key: { status: 1 }, name: \"status\", ns: \"testdb.message\", collation: { locale: \"message\" } } :: caused by :: Field 'locale' is invalid in: { locale: \"message\" }", "code": 2, "codeName": "BadValue"}
一、问题原因
经查找，'failed to add collation information to index spec for index creation:是指出了异常的问题。
在一个实体类中使用了:
@Document(collation = "message")这里应该变为:
@Document(collection = "message")
二、分析
这里区分一下两者的区别：
2.1 Collection
@Id
主键，不可重复，自带索引，可以在定义的列名上标注，需要自己生成并维护不重复的约束。如果自己不设置@Id主键，mongo会自动生成一个唯一主键，并且插入时效率远高于自己设置主键。原因可参考上一篇mongo和mysql的性能对比。
在实际业务中不建议自己设置主键，应交给mongo自己生成，自己可以设置一个业务id，如int型字段，用自己设置的业务id来维护相关联的表。
@Document
标注在实体类上，类似于hibernate的entity注解，标明由mongo来维护该表。
org.springframework.data.mongodb.core.mapping.Document.class
把一个java类声明为mongodb的文档，可以通过collection参数指定这个类对应的文档。
@Document(collection="mongodb 对应 collection 名")
// 若未加 @Document ，该 bean save 到 mongo 的 user collection
// 若添加 @Document ，则 save 到 reUser collection
org.springframework.data.mongodb.core.mapping.Document.class
把一个java类声明为mongodb的文档，可以通过collection参数指定这个类对应的文档。
@Document(collection="mongodb 对应 collection 名") // 若未加 @Document ，该 bean save 到 mongo 的 user collection
// 若添加 @Document ，则 save 到 reUser collection
@Document(collection="reUser")
public class User{
}
@Indexed
声明该字段需要加索引，加索引后以该字段为条件检索将大大提高速度。
唯一索引的话是@Indexed(unique = true)。
也可以对数组进行索引，如果被索引的列是数组时，MongoDB会索引这个数组中的每一个元素。
也可以对整个Document进行索引，排序是预定义的按插入BSON数据的先后升序排列。
也可以对关联的对象的字段进行索引，譬如User对关联的address.city进行索引。（注解怎么写还不清楚，待查）
@CompoundIndex
复合索引，加复合索引后通过复合索引字段查询将大大提高速度。
@Document
@CompoundIndexes({@CompoundIndex(name = "age_idx", def = "{'lastName': 1, 'age': -1}")
})
public class Person<T extends Address> {
}
写法如上，lastName和age将作为复合索引，数字参数指定索引的方向，1为正序，-1为倒序。方向对单键索引和随机存不要紧，但如果你要执行分组和排序操作的时候，它就非常重要了。
@Field
代表一个字段，可以不加，不加的话默认以参数名为列名。
给映射存储到 mongodb 的字段取别名
在 java bean 中字段名为 firstName，存储到 mongo 中 key 为 fName
给映射存储到 mongodb 的字段取别名
在 java bean 中字段名为 firstName，存储到 mongo 中 key 为 fName@Field("fName")private String firstName;
@Transient
被该注解标注的，将不会被录入到数据库中。只作为普通的javaBean属性。
@DBRef
关联另一个document对象。类似于mysql的表关联，但并不一样，mongo不会做级联的操作。
先来看一下不加DBRef时，mongo保存数据的情况:
Article类有String title，List pictureList，两个属性，Picture有一个url，一个desc属性。新建数个Picture对象，并赋值给Article的list，执行Article的insert操作，mongo保存的结果如图：
list会作为普通的数据存到article里，并不会为Picture建表，这一点是区别于mysql的级联存储的。
在Article里给list加上DBRef注解后就不同了
再次执行添加Article操作后，看结果
发现就不再是直接显示的Picture的各个属性了，而是只保存了Picture的id和namespace，同时仍然没有创建Picture的collection（等同于mysql的表）。
如此此时查询该Article，会发现list为空，并没有关联上Picture的值。其实上一步已经发现了，系统并没有去创建Picture的表。那即便Article关联了PictureList的id，也是无法查询的。
官方解释：
The mapping framework does not handle cascading saves. If you change an Account object that is referenced by a Person object, you must save the Account object separately. Calling save on the Person object will not automatically save the Account objects in the property accounts.意思就是不会处理级联保存，你必须单独处理关联的对象。
现在修改一下添加Article的代码，先做Picture的持久化操作。
再看结果，发现Picture已经被持久化，再次查询该Article时，相应的Picture也全部查了出来。
如果在Article里删除关联的list，set为null并保存，系统只会删掉Article里关联的list，而Picture对象本身的数据是不会被删除的。
从上面看来，貌似DBRef比较鸡肋，而且甚至有时还会带着误导的性质，譬如Article关联了两个空的Picture时在Article还能看到2个对象的引用，然后2个对象并不存在，是查询不出来的。
那么这个标签存在的意义何在？
官方的说法是：
In short,the best time to use DBRefs are when you’re storing heterogeneous references to documents in different collections.like when you want to take advantage of some additional DBRef-specific
functionality in a driver or tool.
实际使用中，感觉貌似作用是在不同的表做划分吧，有点模拟mysql外键的意思。免得数据都落到一个大表的，不便于做关联的表的查询。
2.2 Collation
2.2.1特性
Collation的3.4版本新特性。
拼写允许用户在字符串比较中使用特殊语言-特殊规则，例如字母拼写和口音标记规则。
你可以为一个收藏或视图、索引或支持拼写的具体操作做特色拼写。
一份拼写文件有以下字段：
{locale: <string>,caseLevel: <boolean>,caseFirst: <string>,strength: <int>,numericOrdering: <boolean>,alternate: <string>,maxVariable: <string>,backwards: <boolean>
}
当具体的拼写时，当地的拼写是强制性的；所有其他拼写是可选的。点击查看官方文档：see collation document.
2.2.2示例
db.createCollection("person")
db.person.insert({name: "张三"})
db.person.insert({name: "李四"})
db.person.insert({name: "王五"})
db.person.insert({name: "马六"})
db.person.insert({name: "张七"})
默认情况下，名字字段会被当做一个普通的二机制字符串来对比，按照name字段排序的结果如下
mongo-9554:PRIMARY> db.person.find().sort({name: 1})
{ "_id" : ObjectId("586b98980cec8d86881cffac"), "name" : "张七" }
{ "_id" : ObjectId("586b98980cec8d86881cffa8"), "name" : "张三" }
{ "_id" : ObjectId("586b98980cec8d86881cffa9"), "name" : "李四" }
{ "_id" : ObjectId("586b98980cec8d86881cffaa"), "name" : "王五" }
{ "_id" : ObjectId("586b98980cec8d86881cffab"), "name" : "马六" }
而对于中文名字，通常有按拼音顺序排序的需求，这时就可以通过collation来搞定
db.createCollection("person", {collation: {locale: "zh"}})
db.person.insert({name: "张三"})
db.person.insert({name: "李四"})
db.person.insert({name: "王五"})
db.person.insert({name: "马六"})
db.person.insert({name: "张七"})
此时再按name字段排序，则会按照locale指定的中文规则来排序
mongo-9554:PRIMARY> db.person.find().sort({name: 1})
{ "_id" : ObjectId("586b995d0cec8d86881cffae"), "name" : "李四" }
{ "_id" : ObjectId("586b995d0cec8d86881cffb0"), "name" : "马六" }
{ "_id" : ObjectId("586b995d0cec8d86881cffaf"), "name" : "王五" }
{ "_id" : ObjectId("586b995d0cec8d86881cffb1"), "name" : "张七" }
{ "_id" : ObjectId("586b995d0cec8d86881cffad"), "name" : "张三" }
MongoDB 3.4里，基本所有设计字符串字段排序的命令，都支持指定collation，比如「创建集合、创建索引、find」等；上述例子里在createCollection的时候指定了collation，则该集合里所有字符串默认都会按指定的collation来排序，如果只想针对某一个字段来指定collation，可以该字段创建指定collation的索引，例如
db.person.createIndex({name: 1}, {collation: {locale: "zh"}})
注意：如果是从3.2版本升级到3.4的，需要先执行如下命令才能使用collation特性
db.adminCommand( { setFeatureCompatibilityVersion: "3.4" } )
转载博客出处：
SpringBoot中MongoDB注解概念及使用
