I got following tables :

Items
itemId

Materials
materialId

ItemMaterials
itemId
materialId

So item to material is many-to-many relation.

I am going to have more than 5000-6000 items in database and about 20-30 materials.

According to my nhibernate knowledge i have to use following classes and mappings :

class Item
{
int id;
IList<Materials>
}

class Material
{
int id;
IList<Items>
}

and following mappings :

<class name="Item" table="`Items`" >
<bag name="Materials" table="`Materials`" lazy="true" inverse="true">
<key column="`itemId`"/>
<many-to-many class="Material" column="`materialId`"/>
</bag>
</class>

<class name="Material" table="`Materials`" >
<bag name="Items" table="`Items`" lazy="true">
<key column="`materialId`"/>
<many-to-many class="Item" column="`itemId`"/>
</bag>
</class>

To be honest i do not need the reference from Material to Items but i believe this is necessary for this to work.

Now i have one problem.

To add relation between item nad Material i believe i have to do something like that

Item it1 = new Item();
Material mat1 = new Material();

it1.materials.add(mat1);
mat1.items.add(it1);

My problem is that if this mat1 has 5000 more items connected to it, would it fetch all these items from database during the add command when i add this new it1 item ? This would be very very inefficient.

Thank you in anticipation for reply.
Regards,
Piotr

Piotr,

-The table "ItemMaterials" is mentioned nowhere in your mapping.
-if you do not declare the relationship as bidirectional, (and you are saying that you don't need it) you don't need to write both ends of the relationship, In other words, you don't need to retrieve your Items given a Material, nor set the parent Item on the new Material.
-It is a good practice to add a method "addMaterial" to the item, which would add the material to the internal list of the item, and (if you want to keep it bidirectional) add the parent Item on the new material's list with an "addItem".
I am not sure how Hibernate manages that internally, but the list wont get initialized until you don't call the getter

Thank you gonzao_diaz,
your reply was very helpful.