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Entries tagged as ‘many-to-one’

Creating one-to-many relationship in NHibernate

March 31, 2009 · 9 Comments

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.