we are using Hibernate and came up with a model like this.
Each Vessel
---- has a Set of Positions
----------Each Position has a Set of Hatches.

So three tables to represent this . This looks fine.
However the the data to be stored differs on cargo type.
If it is a bulk cargo we have hatches , for petrol we have no hatches.

VESSEL
|
|
CARGODETAIL
^ ^
| |
BULK TANKER
|
HATCH

BULK and TANKER are 2 types of cargo and HATCH is associated only to bulk
Should we go for table per class hierarchy to store both BULK AND TANKER using a discriminator(cargo type) or go for table per subclass.

For as much as i've understood, both BULK and TANKER have the same attributes. Their only difference would be an hatcher Set inside the BULK object.
Due to this, i see no point in using a table per subclass, which could be useful when you have many different attributes among the subclasses. I would instead using a table per class hierarchy approach.
The Hatch set will then be mapped using a many-to-one relationship inside the BULK specific mapping.

thanks

The BULK object has a set of POSITON and TANKER also has a set.
for a BULK object all the info about a position goes into the hatches table.
However for tanker the info goes directly into the hierarchy table.
Do you think the object model looks good in this scenario