blocks|key|327607|text|1)在使用db4o存储对象时，如何管理对象标识？在db4o中，你通常没有id。db4o使用object-identity来区分对象。因此，内存中的相同对象将成为数据库的相同对象。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|327608|只要你不序列化对象，它就能正常工作。然而，一旦对象被序列化/断开，这就不再起作用了。例如，在web场景中:您将数据发送到浏览器。现在，您需要稍后再次通过一些ids来标识对象。|327609|我认为这三个选项是可能的：-使用db4o+internal+id。然而，这个id不是永远的。对数据库进行碎片整理会更改此id。-使用db4o's+UUIDs。但是db4o+UUID是相当大的-您自己创建ids|327610|2)有一个对象管理器工具来查看数据库。然而，在目前的状态下，它是极其有限的。在我看来，这是db4o的一个巨大缺陷。|327611|3)您可以创建aliases,+rename+classes+and+fields+etc。但是，更改继承层次结构并不起作用。然后您需要将旧数据复制到新实例中。|327612|4)是的。您可以使用.NET-非序列化属性或自定义属性将字段标记为transient。|327613|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.db4o.com/Documentation/Reference/db4o-8.0/net35/reference/Content/basics/identity_concept.htm|1|http://developer.db4o.com/Documentation/Reference/db4o-8.0/net35/reference/Content/advanced_topics/ids_and_uuids.htm|2|3|http://developer.db4o.com/Documentation/Reference/db4o-8.0/net35/reference/Content/advanced_topics/refactoring_and_schema_evolution.htm|4|http://developer.db4o.com/Documentation/Reference/db4o-8.0/net35/reference/Content/tuning/selective_persistence/transient_fields_in_.net.htm^0|19|F|0|0|0|G|G|1|1U|C|2|0|0|7|12|3|0|X|9|4|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@$A|13|B|14|1|15]]|C|$]]|$1|D|3|E|5|6|7|16|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|17|8|@]|9|@$A|18|B|19|1|1A]|$A|1B|B|1C|1|1D]]|C|$]]|$1|H|3|I|5|6|7|1E|8|@]|9|@]|C|$]]|$1|J|3|K|5|6|7|1F|8|@]|9|@$A|1G|B|1H|1|1I]]|C|$]]|$1|L|3|M|5|6|7|1J|8|@]|9|@$A|1K|B|1L|1|1M]]|C|$]]|$1|N|3|-4|5|6|7|1N|8|@]|9|@]|C|$]]]|O|$P|$5|Q|R|S|C|$T|U]]|V|$5|Q|R|S|C|$T|W]]|X|$5|Q|R|S|C|$T|W]]|Y|$5|Q|R|S|C|$T|Z]]|10|$5|Q|R|S|C|$T|11]]]]

1) How do you manage object identity when working with db4o stored objects?
In db4o you have normally no id. db4o uses the <a href="http://developer.db4o.com/Documentation/Reference/db4o-8.0/net35/reference/Content/basics/identity_concept.htm" rel="nofollow noreferrer">object-identity</a> to distinguish the object apart. So the same object in memory is going to be the same object for the database. 

As long a you don't serialize object this works fine. However as soon as objects are serialized / disconnected this doesn't work anymore. For example in a web-scenario: You send the data to the browser. Now you need to identify the objects later again by some ids.

I think this three options are possible:
- Use the <a href="http://developer.db4o.com/Documentation/Reference/db4o-8.0/net35/reference/Content/advanced_topics/ids_and_uuids.htm" rel="nofollow noreferrer">db4o internal id</a>. However this id isn't forever. Defragmenting the database changes this id.
- Using <a href="http://developer.db4o.com/Documentation/Reference/db4o-8.0/net35/reference/Content/advanced_topics/ids_and_uuids.htm" rel="nofollow noreferrer">db4o's UUIDs</a>. But db4o UUIDs are quite large
- Creating ids by yourself

2) There's a Object-Manager Tool to look at the database. However it extremely limited in its current state. In my opinion this is a huge drawback for db4o.

3) You can create <a href="http://developer.db4o.com/Documentation/Reference/db4o-8.0/net35/reference/Content/advanced_topics/refactoring_and_schema_evolution.htm" rel="nofollow noreferrer">aliases, rename classes and fields etc</a>. However changing the inheritance-hierarchy doesn't work. Then you need to copy the old data to new instances.

4) Yes. You can mark fields as <a href="http://developer.db4o.com/Documentation/Reference/db4o-8.0/net35/reference/Content/tuning/selective_persistence/transient_fields_in_.net.htm" rel="nofollow noreferrer">transient</a> with the .NET-NonSerialized attribute or custom attributes.

blocks|key|3295515|text|在使用DB4O进行一些工作的同时，我编写了一个简单的DB4O对象浏览器，它的工作性能比附带的浏览器要好得多。试一试，它是开源的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3295516|http://sourceforge.net/projects/db4oviewer/develop|offset|length|3295517|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|1E|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@$D|O|E|P|1|Q]]|A|$]]|$1|F|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|C]]]]

While doing some work with DB4O, I wrote a simple DB4O object browser that works much better than the included one. Give it a try, it's open source.

<a href="http://sourceforge.net/projects/db4oviewer/develop" rel="nofollow">http://sourceforge.net/projects/db4oviewer/develop</a>

blocks|key|3295531|text|在面向对象的数据库(如db4o)中，不应该真正使用对象标识。取而代之的是使用查询和导航。首先执行查询以获取一个/某些对象，然后使用导航获取其他对象。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3295532|“导航”意味着你只需跟随任何已加载对象中的字段/引用。|3295533|使用对象标识符可以被认为是糟糕的风格，大多数应用程序(据我所知)根本不使用它们。|3295534|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

In objects oriented databases (such as db4o) object identity should not really be used.
Instead one used queries and navigation. First perform a query to get one/some objects, then use navigation to get to others.

'Navigation' means that you just follow the fields/references in any loaded object.

Using object identifiers can be considered bad style, most application (that I know) don't use them at all.

I am trying the db4o object databse and so far I quite like what I am seeing, but I also read this post on stackoverflow <a href="https://stackoverflow.com/questions/21207/db4o-experiences/24499#24499">db4o experiences?</a> indicating that not everything that seems so easy is easy. 

Right now, I have some questions regarding on how db4o is used in real world apps. So if you have any experience in working (especially in web app context) with db4o, I would love to hear them.

Here are my questions:

<ol>
<li>How do you manage object identity when working with db4o stored objects?** 
Coming from RDBMS background where you normally always have a primary key / identity column for every table, I cant imagine right now on how to manage object identity in db4o. 

For example, if I was working with NHibernate / mysql and needed to find a User object by id, I would do session.Load(primaryKey) and it will be retrieved by its PK. It is also very common that the PK is defined as auto increment in the table definition. </li>
<li>As there is no such option in db4o, my thought was using a Guid struct in order to identify some objects in the object database. </li>
<li>Any tools to view the stored objects in the db?

Is there something like SQL Server Management Studio (probably less sophisticated) in the db4o world? I would like to view the already stored data / objects in the db file.</li>
<li>Are you screwed when renaming your domain objects?

As far as I know when you rename a class, any previously stored instances in the db cannot be retrieved anymore. Is there a way to work around this issue? How do you deal with updates against a live database which already contains many objects?</li>
<li>Can I exclude properties from being saved to the DB?

If for example one domain object holds a reference to a (stateless) service object, then the service object will also be persisted if the domain object gets persisted, right? </li>
</ol>

It seems a bit odd to have a service instace saved in the database, at least to me. 

Can you exclude the service instance from being saved? If the domain object is retrieved again, how can I make sure that the service is also injected in the instance again?

A few questions about working with db4o

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我正在尝试db4o对象数据库，到目前为止，我非常喜欢我所看到的，但我也读到了stackoverflow 上的这篇文章，它表明并不是所有看起来那么容易的事情都是容易的。现在，我有一些关于如何在现实世界的应用中使用db4o的问题。因此，如果你有任何使用db4o的经验(特别是在web应用环境中)，我很乐意听听他们的意见。以下...

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