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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View SQL generated by NHibernate

NHibernate has a built-in mechanism to log all SQL statements generated. It has a configuration parameter “show_sql” which should be set to true. NHibernate will throw all statements to Console.out if this is true. So a console application can easily display all statements but what about the web application?

An easy way is to use the NUnit console which has a separate tab for all Console.out output. If you do not use NUnit work out with log4net.

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!

Persisting Custom Data Types with NHibernate

NHibernate provides native support for primitive types/enumerations only. What if I want to save something like a custom data type, say for example a System.Uri object? NHibernate provides a fairly simple way to do that which is an interface actually called IUserType. It has got a whole bunch of methods which should be implemented by your class.

    public interface IUserType
    {
        SqlType[] SqlTypes { get; }
        System.Type ReturnedType { get; }
        bool Equals( object x, object y );
        object NullSafeGet( IDataReader rs, string[] names, object owner );
        void NullSafeSet( IDbCommand cmd, object value, int index );
        object DeepCopy( object value );
        bool IsMutable { get; }
    }

The following method of the interface tells the data base type:

public SqlType[] SqlTypes
{
get
{
// We store our Uri in a single column in the database that can contain a string
SqlType[] types = new SqlType[1];
types[0] = new SqlType(DbType.String);
return types;
}
}

And this method tells the .NET type to be built for the value.

public System.Type ReturnedType
{
get { return typeof(Uri); }
}

This is how you build the .NET datatype from the value:

public object NullSafeGet(IDataReader rs, string[] names, object owner)
{
// We get the string from the database using the NullSafeGet used to get strings
string uriString = (string)NHibernateUtil.String.NullSafeGet(rs, names[0]);

// And save it in the Uri object. This would be the place to make sure that your string
// is valid for use with the System.Uri class, but i will leave that to you
Uri result = new Uri(uriString);
return result;
}

This is how you translate the value back to database type.

public void NullSafeSet(IDbCommand cmd, object value, int index)
{
// Set the value using the NullSafeSet implementation for string from NHibernateUtil
if (value == null)
{
NHibernateUtil.String.NullSafeSet(cmd, null, index);
return;
}

value = value.ToString(); // ToString called on Uri instance
NHibernateUtil.String.NullSafeSet(cmd, value, index);
}

The rest of the method are simple, and you can find them on the internet as well (where I found them )