限制对db4o中创建对象的孩子重复 [英] Restrict child duplicates on creating object in db4o

问题描述

这是一个很常见的情况，但我使用ORM尤其是在Android的还挺新，所以你的帮助将是真棒。

It's a very common situation, but I'm kinda new using ORM especially in Android, so your help would be awesome.

适用范围：
对象，例如消息有另一个对象的原始字段和字段（子），例如讨论。所以它看起来像：

Scope: Object, e.g. Message has primitive fields and field (child) of another object, e.g. Discussion. So it looks like:

 public class Message {

    private int id;
    private String text;
    private Discussion discussion;

    public Message(int id, String text, int discussionId){
        this.id=id;
        this.text=text;
        discussion = new Discussion (discussionId);
    }
}

public class Discussion {

    private int id;
    private String title;

    public Discussion(int id) {
        this.id = id;
        this.title = "Sample title";
    }
}

的注意::标识是由服务器提供的，这就是为什么它是由手工设置。的

问题：
如你所知多个消息可能属于一个讨论。但是，当我存储信息的名单，我得到表一式两份的讨论（大小相同的消息表）。如何避免呢？

Problem: as you know multiple messages could belong to one discussion. But when I store list of Message I get table with duplicate discussions (the same size as messages table). How to avoid it?

下面我如何保存信息的ArrayList

Here how I store Message ArrayList:

ArrayList<Message> itemsList = new ArrayList<Message>;
itemsList.add(new Message(1, "Message 1", 50));
itemsList.add(new Message(2, "Message 2", 50));

ItemDBProvider dbProvider = new ItemDBProvider();

for (Item item:itemsList) {
    dbProvider.store(item);
}

dbProvider.getDB().commit();
dbProvider.close();

我的意思是，不知何故db4o的应该检查一下讨论的对象是已在DB（由ID字段），并限制创建重复。是真的吗？

I mean, somehow db4o should check if Discussion object is already in db (by "id" field) and restrict creating duplicate. Is it real?

推荐答案

db4o的不知道你想要不同的讨论对象合并，只是因为它们具有相同的ID字段。 db4o的通过他们的身份区分对象（即 == 运营商），而不是对象的任何领域。可以有数百个相等的对象的数据库中的

Db4o does not know that you intend different Discussion objects to be merged, just since they have the same id field. Db4o distinguishes objects by their identity (i.e. the == operator), not any fields of the object. You can have hundreds of equal objects in the database.

有没有理由为每个消息一个新的讨论对象 - 检索现有的，以及设置它作为新的Message对象的讨论字段。

There is no reason to create a new Discussion object for each Message - retrieve the existing one, and set it as the discussion field of your new Message object.

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