R – Many-To-One relationships in NHibernate

asp.netnhibernate

I am still trying to learn NHibernate best practices, so please be easy on me… 🙂

I have a Product class with a Many-To-One relationship set up to a Category class. On my Add A Product page, I am loading a DropDownList with the Categories. Now when the user enters the product information and clicks submit, I have to pull down the Category by ID (DropDownList SelectedValue) from the database to populate the Category property of my Product object so I can save it.

It seems like the Category by ID lookup is a waste. Is there a way I can simply use the ID value that was retrieved from the DropDownList?

Thanks for any advice!

Best Answer

Use ISession.Load() to create a proxy object. eg:

myProduct.Category = session.Load<Category>(userSuppliedCategoryId);

Note that the Load() method doesn't actually hit the database unless you try accessing a property besides the primary key, so there is no extra DB hit.

Ayende has a good post about this

Related Solutions

C# – Removing many to many associations in NHibernate

Have you tried using the ISession.Delete(query) method? It takes a HQL query, loads the objects then deletes them.

session.Delete("from Product p join p.Categories c where c.id = :category", category, NHibernateUtil.Entity(typeof(Category)));

My HQL is a but rusty, so excuse me if the query is not quite right.

Another option if you're not keen on the idea of preloading all the objects, is to use session.CreateSQLQuery and just pass in a straight SQL statement to do the delete. NHibernate will execute that against the server no questions asked.

The correct way to define many-to-many relationships in NHibernate to allow deletes but avoiding duplicate records

What you need to do in order to have your mappings work as you would expect them to, is to move the inverse="true" from the Category.Items collection to the Item.Categories collection. By doing that you will make NHibernate understand which one is the owning side of the association and that would be the "Category" side.

If you do that, by deleting a Category object it would delete the matching record from the lookup table as you want it to as it is allowed to do so because it is the owning side of the association.

In order to NOT delete the Items that are assigned to a Category object that is to be deleted you need to leave have the cascade attribe as: cascade="save-update".

cascade="all" will delete the items that are associated with the deleted Category object.

A side effect though would be that deleting the entity on the side where the inverse=tru exists will thow a foreign key violation exception as the entry in the association table is not cleared.

A solution that will have your mappings work exactly as you want them to work (by the description you provided in your question) would be to explicitly map the association table. Your mappings should look like that:

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

  <class name="Category" lazy="true">

    <id name="CategoryId" unsaved-value="0">
      <generator class="native" />
    </id>
    <property name="Name" />

    <bag name="ItemCategories" generic="true" inverse="true" lazy="true" cascade="none">
        <key column="CategoryId"/>
        <one-to-many class="ItemCategory"/>
    </bag>

    <bag name="Items" table="ItemCategory" cascade="save-update" generic="true">
      <key column="CategoryId"></key>
      <many-to-many class="Item" column="ItemId"></many-to-many>
    </bag>

  </class>

</hibernate-mapping>

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

  <class name="Item" table="Item" lazy="true">

    <id name="ItemId" unsaved-value="0">
      <generator class="native" />
    </id>
    <property name="Name" />

    <bag name="ItemCategories" generic="true" inverse="true" lazy="true" cascade="all-delete-orphan">
        <key column="ItemId"/>
    <one-to-many class="ItemCategory"/>
    </bag>

    <bag name="Categories" table="ItemCategory" inverse="true" cascade="save-update" generic="true">
      <key column="ItemId"></key>
      <many-to-many class="Category" column="CategoryId"></many-to-many>
    </bag>

  </class>

</hibernate-mapping>

As it is above it allows you the following:

Delete a Category and only delete the entry in the association table without deleting any of the Items
Delete an Item and only delete the entry in the association table without deleting any of the Categories
Save with Cascades from only the Category side by populating the Category.Items collection and saving the Category.
Since the inverse=true is necessary in the Item.Categories there isn't a way to do cascading save from this side. By populating the Item.Categories collection and then saving the Item objec you will get an insert to the Item table and an insert to the Category table but no insert to the association table. I guess this is how NHibernate works and I haven't yet found a way around it.

All the above are tested with unit tests. You will need to create the ItemCategory class mapping file and class for the above to work.

Best Answer

Related Solutions

C# – Removing many to many associations in NHibernate

The correct way to define many-to-many relationships in NHibernate to allow deletes but avoiding duplicate records

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