保存新实体时，NHibernate如何使用数据库默认值? [英] How can NHibernate use the database-default value when saving a new entity?

问题描述

考虑以下简单的C#类:

Consider the following simple C# class:

public class Entity
{
    public Entity() { }
    public virtual int Id { get; private set; }
    public virtual DateTime DateCreated { get; private set; }
}

映射了以下简单的NHibernate映射:

Mapped with the following simple NHibernate mapping:

<class name="Entity" mutable="false">
    <id name="Id">
        <generator class="native">
    </id>
    <property name="DateCreated"/>
</class>

对于以下简单数据库模式:

To the following simple database schema:

CREATE TABLE Entity (
    Id int IDENTITY(1,1) PRIMARY KEY,
    DateCreated datetime NOT NULL DEFAULT getUtcDate()
)

在创建Entity的新实例并将其保存到数据库时，如果NHibernate的值为null，如何指示NHibernate将数据库的默认值用于DateCreated列?或者，如何指定NHibernate在插入时应使用getUtcDate()函数的结果作为DateCreated字段的值?

When creating a new instance of the Entity and saving to the database, how do you instruct NHibernate to use the database's default value for the DateCreated column if its value is null? Alternatively, how can I specify that NHibernate should use the result of the getUtcDate() function as the value for the DateCreated field upon insertion?

虽然我可以轻松添加

DateCreated = DateTime.Now;

使用实体服务器的本地时钟进入Entity构造函数，当有多个应用程序服务器(每个服务器可能具有不同步的本地时钟)时，我需要使用数据库的本地时钟来确保一致性.

into the Entity constructor, this is using the application server's local clock, and I need to use the database's local clock to ensure consistency when there are multiple application servers each with their potentially non-synchronized local clocks.

推荐答案

您可以指定该属性是由数据库生成的:

You can specify that the property is generated by the database:

NHibernate映射-属性

因此，对于您的情况，您需要指定:

So for your case you would want to specify:

generated="insert"

通过这种方式，NHibernate知道在插入后必须刷新实体，但是更新后DateCreated将不会更改.

This way NHibernate knows after an INSERT it will have to refresh the entity, but after updates DateCreated will not be changed.

您可能还必须指定:

update="false" insert="false"

我从没使用过生成，而且我不确定NHibernate是否推断要设置这些，还是您必须显式地进行设置.

I've never used generated, and I'm not sure if NHibernate infers to set those or you have to do it explicitly.

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