映射nhibernate xml多对一关系

Question

I have the Article and Author objects. An Author can have many articles, but an Article can only have 1 Author:

public class Author
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
}

public class Article
{
    public virtual int Id {get;set;}
    public virtual string Title {get;set;}
    public virtual Author Author {get;set;}
}

Here is what I have so far:

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
               namespace="Core.Domain.Model"
               assembly="Core">

  <class name="Author" table="Author" dynamic-update="true">
    <cache usage="read-write"/>
    <id name="Id" column="Id" type="int">
      <generator class="native"/>
    </id>
    <property name="Name" column="Name" type="string"/>
  </class>
</hibernate-mapping>



<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
               namespace="Core.Domain.Model"
               assembly="Core">

  <class name="Article" table="Article" dynamic-update="true">
    <cache usage="read-write"/>
    <id name="Id" column="Id" type="int">
      <generator class="native"/>
    </id>
    <property name="Title" column="Title" type="string"/>
    <one-to-one name="Author"
        class="Author"
        constrained="true"/>
  </class>
</hibernate-mapping>

The error that this current code is throwing is:

Cannot insert the value NULL into column 'AuthorId', table 'NHibernate101.dbo.Article'; column does not allow nulls. INSERT fails.

How to I map these correctly?

Answer 1

You need many-to-one in this case, not one-to-one. Your Author can have many Articles, but an Article can have only one Author. See NHibernate Reference Documentation, 5.1.10. many-to-one

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   namespace="Core.Domain.Model"
                   assembly="Core">

    <class name="Author" table="Author" dynamic-update="true">
        <cache usage="read-write" />
        <id name="Id" column="Id" type="int">
            <generator class="native" />
        </id>
        <property name="Name" column="Name" type="string" />
    </class>
</hibernate-mapping>

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
               namespace="Core.Domain.Model"
               assembly="Core">

    <class name="Article" table="Article" dynamic-update="true">
        <cache usage="read-write"/>
        <id name="Id" column="Id" type="int">
            <generator class="native"/>
        </id>
        <property name="Title" column="Title" type="string"/>
        <many-to-one name="Author" column="Id" not-null="true" class="Author" />
    </class>
</hibernate-mapping>

Answer 2

The above use-case is very simliar to documented example:

• 22.2. NHibernate Mappings

The most typical mapping (without any but) I would use is:

C#

public class Author // new collection mapping
{
    public virtual int Id { get; set; }
    public virtual IList<Article> Articles { get; set; }
    ...
}

public class Article // as is
{
    public virtual int Id {get;set;}
    public virtual Author Author {get;set;}
    ...
}

xml

<class name="Author" table="Author" dynamic-update="true" batch-size="25">
  ...
  <bag 
        name="Articles" 
        inverse="true" 
        lazy="true"
        batch-size="25"
        cascade="all">            
        <key column="Author_ID"/>
        <one-to-many class="Article"/>            
    </bag> 

...
<class name="Article" table="Article" dynamic-update="true" batch-size="25">
  ...
  <many-to-one name="Author" column="Author_ID" not-null="true" />

What we have is mapping of both relation ends, where every new Article must have properly set reference to Author (to make all WRITE operations working). So this should be the assignment

var author = ...
var article = ...
author.Articles.Add(article);
article.Author = author;

This is must during creation. Later NHibernate will load all these settings for us (lazily if needed)