hello everyone,
I have some difficulties trying to map a bidirectional zero-or-one-to-one association when inheritance is involved.
I would need some hints and pointers on the matter because I am not able to find out a solution by myself (and I wasn't able to find similar examples or experiences in the forums.)
Here's the setup:
I have a class Cow and a class Sheep, each inherits from a class Mammal which itself inherits from Animal. The inheritance is handled through a one-table-per-subclass strategy (joined-subclass).
On the other end, I have a Farm that can only house, at most, one Mammal of each species (e.g. zero or one Cow and zero or one Sheep, but no Duck for example.)
Code:
Mammal[0..*]<-->[1..1]Farm
Cow is_a Mammal
Sheep is_a Mammal
Cow[0..1]<-->[1..1]Farm
Sheep[0..1]<-->[1..1]Farm
Only zero or one member of each mammal species can be associated with a specific farm
Tables:
Mammal:
Code:
id int(11) NOT NULL,
PRIMARY KEY (id),
CONSTRAINT xxx FOREIGN KEY (id) REFERENCES animal (id)
Cow or Sheep:
Code:
id int(11) NOT NULL,
farm_id id(11) NOT NULL UNIQUE,
PRIMARY KEY (id),
CONSTRAINT yyy FOREIGN KEY (id) REFERENCES mammal (id),
CONSTRAINT zzz FOREIGN KEY (farm_id) REFERENCES farm (id)
Farm:
Code:
id int(11) NOT NULL,
PRIMARY KEY (id)
Now here's the part where I have some real difficulties using Hibernate to map the association: I need to be able to gather all mammals (a property called getMammals() in my farm class) in a farm without having to individually fetch them by species (no properties getCow() and getSheep() in my farm class).
It works as expected if I have a seperate field for each different kind of mammals in the farm class:
Farm object:
Code:
...
<one-to-one cascade="all" class="Sheep" name="sheep" property-ref="farm"/>
<one-to-one cascade="all" class="Cow" name="cow" property-ref="farm"/>
...
Animal object:
Code:
...
<joined-subclass abstract="true" name="Mammal" table="mammal">
<key column="id"/>
<joined-subclass name="Sheep" table="sheep">
<key column="id"/>
<many-to-one cascade="none" class="Farm" column="farm_id" name="farm" not-null="true" unique="true"/>
</joined-subclass>
<joined-subclass name="Cow" table="cow">
<key column="id"/>
<many-to-one cascade="none" class="Farm" column="farm_id" name="farm" not-null="true" unique="true"/>
</joined-subclass>
</joined-subclass>
...
Now what if I want to collect all mammals linked to a farm via a getMammals() in the farm class? The problem is that I can't change the database structure and I can't come up with a proper solution. Can you help me please?
TIA
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