春季数据MongoDb:MappingMongoConverter删除_class [英] Spring data MongoDb: MappingMongoConverter remove _class

问题描述

默认的 MappingMongoConverter 将自定义类型键("_class")添加到数据库中的每个对象.因此，如果我创建一个Person:

package my.dto;
public class Person {
    String name;
    public Person(String name) {
        this.name = name; 
    }
}

并将其保存到数据库:

MongoOperations ops = new MongoTemplate(new Mongo(), "users");
ops.insert(new Person("Joe"));

在mongo中生成的对象将是:

{ "_id" : ObjectId("4e2ca049744e664eba9d1e11"), "_class" : "my.dto.Person", "name" : "Joe" }

问题:

1. 将Person类移动到另一个名称空间有什么含义?

2. 是否可以不使用"_class"键污染对象?无需为Person类编写唯一的转换器?

解决方案

所以这是故事:默认情况下，我们将类型添加为某种提示，以实际实例化哪个类.由于必须通过管道输入类型以通过MongoTemplate将文档读入，因此有两种可能的选择:

1. 您输入可以分配实际存储类型的类型.在这种情况下，我们考虑存储类型，将其用于对象创建.这里的经典示例是执行多态查询.假设您有一个抽象类Contact和您的Person.然后，您可以查询Contact，我们实际上必须确定要实例化的类型.
2. 另一方面，如果您传入一个完全不同的类型，我们将仅编组为该给定类型，而不是实际上存储在文档中的那个类型.那将涵盖您的问题，如果您移动类型，会发生什么情况.

您可能有兴趣观看此票证，其中涵盖了某种可插拔的类型映射将类型信息转换为实际类型的策略.这可能只是出于节省空间的目的，因为您可能希望将长的合格类名减少为几个字母的哈希.它还将允许更复杂的迁移方案，在这些方案中，您可能会发现由另一个数据存储区客户端产生的完全任意的类型密钥，并将它们绑定到Java类型.

The default MappingMongoConverter adds a custom type key ("_class") to each object in the database. So, if I create a Person:

package my.dto;
public class Person {
    String name;
    public Person(String name) {
        this.name = name; 
    }
}

and save it to the db:

MongoOperations ops = new MongoTemplate(new Mongo(), "users");
ops.insert(new Person("Joe"));

the resulting object in the mongo will be:

{ "_id" : ObjectId("4e2ca049744e664eba9d1e11"), "_class" : "my.dto.Person", "name" : "Joe" }

Questions:

1. What are the implications of moving the Person class into a different namespace?

2. Is it possible not to pollute the object with the "_class" key; without writing a unique converter just for the Person class?

解决方案

So here's the story: we add the type by default as some kind of hint what class to instantiate actually. As you have to pipe in a type to read the document into via MongoTemplate anyway there are two possible options:

1. You hand in a type the actual stored type can be assigned to. In that case we consider the stored type, use that for object creation. Classical example here is doing polymorphic queries. Suppose you have an abstract class Contact and your Person. You could then query for Contacts and we essentially have to determine a type to instantiate.
2. If you - on the other hand - pass in a completely different type we'd simply marshal into that given type, not into the one stored in the document actually. That would cover your question what happens if you move the type.

You might be interested in watching this ticket which covers some kind of pluggable type mapping strategy to turn the type information into an actual type. This can serve simply space saving purposes as you might want to reduce a long qualified class name to a hash of a few letters. It would also allow more complex migration scenarios where you might find completely arbitrary type keys produced by another datastore client and bind those to Java types.

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