Hi,

Assume we have two entities A and B and many to many relationship.

A
long aID
string aaa;

B
long bID;
string bbb;

AB
long aID (pfK)
long bID (pfK)
string addInfo1;
string addInfo2;
long something; (fk)

And there are corresponding c# classes with exactly the same public properties and names (i.e. table A is class A).

Now should I add a new column or this is OK:

<composite-id class="AB"> //name = ???
<key-many-to-one name="aID" class="A" column="aID"/>
<key-many-to-one name="bID" class="B" column="bID"/>
//do i need key-property?
</composite-id>

Hibernate is able to define a many-to-many relationship through a 3rd table, only if that 3rd table contains just the PKs of both tables.
In a case like yours, on the contrary, where there is more information on the "relationship table", I would advise trying to think that "many-to-many" relationship into two different relationship.

1) A "one to many" relationship between A and AB
2) A "many to one" relationship between AB and B

I would further advice, in order to make things simpler for Hibernate, to create an additional, single-field PK for the AB table.

well, isn't it exactly what i showed above?



1) A "one to many" relationship between A and AB
2) A "many to one" relationship between AB and B

As AB has additional info apart from foreign keys it has to ba an entity...
So:
A is one to many AB
B is one to many AB

AB is many to one A
AB is many to one B

And the single PK for AB os rather out of the question, that's why i asked is it OK:

<composite-id class="AB"> //name = ???
<key-many-to-one name="aID" class="A" column="aID"/>
<key-many-to-one name="bID" class="B" column="bID"/>
//do i need key-property?
</composite-id>

is it?

I am rather not asking about relationships as is it simple, but hibernate id which is required for every entity..

No, it isn't. You are creating a many-to-many table with a composite id.
I am telling you to create one with a single id, and then relate the other 2 tables to it in the manner I previously described.

Ok I can do that (add additional column with PK) but I still don't understand why my approach is wrong, you are telling that what I am doing is wrong but not telling why.
I overrided hashcode and equals methods and did small nunit test saving AB class to database with no problems so where is the problem?

And mappings are:
A
<id name="id">
<generator class="native" />
</id>
<set name="ABList" table="AB">
<key column="aID" />
<one-to-many class="AB" />
</set>

and for B:

B
<id name="id">
<generator class="native" />
</id>
<set name="ABList" table="AB">
<key column="bID" />
<one-to-many class="AB" />
</set>

and for AB there is no many-to-one collection mapping, only:

<composite-id class="AB">
<key-many-to-one name="aID" class="A" column="aID"/>
<key-many-to-one name="bID" class="B" column="bID"/>
</composite-id>

and some string propersies