Creating one-to-many relationship in NHibernate

We have recently brought in NHibernate in our environment and I was the first one in the team to do something real with it. It was going perfect until this one-to-many relationship and then I had to consult the architect in UK to get it going Infact I successfully implemented many-to-many but could not do one-to-many . *&#%&*#W(*(*&%#

Anyways, we had a class SearchCritera having a list of SearchCriterion. The mapping file looked like:

<class name="BusinessObjects.Search.SearchDocumentCriteria" table="searchdocumentcriteria">
<id name="Id" column="id">
<generator class="sequence">
<param name="sequence">pc_searchdocumentcriteria</param>
</generator>
</id>
<bag name="Criteria" lazy="false" cascade="all" inverse="true">
<key column="criteriaid"/>
<one-to-many class="BusinessObjects.Search.SearchDocumentCriterion"/>
</bag>
</class>
<class name="BusinessObjects.Search.SearchDocumentCriterion" table="searchdocumentcriterion">
<id name="Id" column="id">
<generator class="sequence">
<param name="sequence">pc_searchdocumentcriterion</param>
</generator>
</id>
<property name="FieldName" column="fieldname" type="string"/>
<many-to-one name="Id" class="BusinessObjects.Search.SearchDocumentCriteria" column="criteriaid" not-null="true"/>
<bag name="Expression" lazy="false" cascade="all" inverse="true">
<key column="criterionid"/>
<one-to-many class="Search.SearchDocumentCriterionOperatorValue"/>
</bag>
</class>

Look at the red line, it maps back to the parent class’s column Id. This is what I found in the manual and an example on the internet as well. But we were getting the error:

Unknown entity class: System.Int32

Later we found that we need to have a parent reference in the child class as well which make the relationship 2-way. So we added the following property in the SearchDocumentCriterion class:

public virtual SearchDocumentCriteria Parent { get; set; }

And the following change in the mapping file:

<many-to-one name="Parent" class="BusinessObjects.Search.SearchDocumentCriteria" column="criteriaid" not-null="true"/>

Note the Parent property instead of Id property. This solved the problem Anyone having a better solution for one-to-many relationship in NHibernate? Yes, I don’t like the two-way relationship.

 

Technorati Tags: database,database mapping,many-to-one,nhibernate,one-to-many,ORM,persist,relationship

 

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Creating one-to-one relationship in NHibernate

A one-to-one relationship between two tables can be created by:

• Adding a column to the parent table with some unique ID of the child table
• Without any extra column: just inserting the same PK value in both tables

NHibernate supports the second method (correct me) and you can create this relationship using the “foreign” type ID generator and the <one-to-one> tags in both mappings. Today I created the following mappings for two classes named Document and DocumentStatus.

<class name="Document" table="documents">
    <id name="Id" column="id">
          <generator class="foreign">
               <param name="property">Status</param>
          </generator>
    </id>
    <one-to-one name="Status" class="DocumentStatus" constrained="true"/>
    ... other properties ...
</class>

The trick here is just the foreign ID generator class. Now the mapping for DocumentStatus would be fairly simple.

<class name="DocumentStatus" table="documentstatus">
    <id name="Id" column="id">
       ... any generator class ...
    </id>
    <one-to-one name="Document" class="Document"/>
    ... other properties ...
</class>

The classes will have a property of the corresponding type to map this relationship so you can navigate both ways.

class Document {
... other stuff ...
DocumentStatus Status;
}

class DocumentStatus {
... other stuff ...
Document Document;
}

Whenver you insert a new object in Document, the corresponding DocumentStatus object will be created and its PK will be assigned to Document object as well.

Great Stuff!