MongoEngine _types和_cls字段 [英] MongoEngine _types and _cls fields

问题描述

为什么mongoengine会向集合的每个文档添加 _types 和 _cls 字段。

这两个都是一个（键，值）对并且它们都包含文档的模型类的名称。唯一的区别是 _types value是一个列表，我假设如果涉及到一些继承，它可以有多个模型类名。

Why does mongoengine add _types and _cls fields to every document of a collection.
Both of them are a (key, value) pair and both of them contain the name of the document's model class. The only difference is _types value is a list and I assume it can have multiple model class names if there is involved some inheritance.

但是问题是：为什么我需要它们存在于集合中的每个文档中，当所有文档的两个字段都具有相同的值时？

However the question is: why do I need them to exist in every document within a collection when all the documents will have the same values for both fields?

推荐答案

Mongoengine允许文档继承。定义类时，元属性 allow_inheritance 用于允许对此特定类进行子类化。
_cls 和 _types 字段用于标识对象所属的类。

Mongoengine allows Document Inheritance. When defining a class a meta attribute allow_inheritance is used to allow subclassing this particular class. The _cls and _types fields are used to identify which class the object belongs to.

考虑一个名为用户的用于存储用户信息的文档：

Consider a document called User used to store users' information:

class User(Document):
    meta = {'allow_inheritance': True}
    # stores information regarding a user

现在考虑一个名为 StackOverFlowUser 的文档：本文档继承自用户文档，并为用户保存一些StackOverflow相关信息：

Now consider a document called StackOverFlowUser : this document is inherited from the User document and saves some StackOverflow-related information for a user:

class StackOverFlowUser(User):
    # stores StackOverflow information of a user

对于这两个文档类， mongoengine将使用名为 user 的相同集合。无论您创建哪个文档对象，它将作为文档存储在此集合中。

For both these document classes, mongoengine will use the same collection named user. No matter which document object you create, it will be stored as a document in this collection.

要区分对象所属的类，将使用_cls 和 _types 字段。

To differentiate to which class the object belongs to, _cls and _types fields will be used.

对于用户对象：

{
    ...
    '_cls' = 'User',
    '_types' = ['User', 'User.StackOverFlowUser']
}

对于 StackOverFlowUser 对象：

{
    ...
    '_cls' = 'User.StackOverFlowUser',
    '_types' = ['User', 'User.StackOverFlowUser']
}

如果您确定文档不具有子类文档，则设置 allow_inheritance 到 False ，并且mongoengine将不会保存 _cls 和 _types 该文件的字段。

If you are sure that a document is not going to have a sub-class document, then set allow_inheritance to False and mongoengine will not save _cls and _types fields for that document.

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