Hello Bojan,
This may cause you problems, as the both relationships will be writable,
causing the potential for duplicates to be added. For instance, if you
add a new Node, and add it to an existing Node's collection, that
existing Node also needs to be added to the new one's collection. This
will cause writes from both sides, causing two entries in the relation
table for the same relationship. You can mark one side as read-only to
prevent this, but the better way is to mark one side using the mappedby
tag I mentioned in the previous email. This is similar to making it
read-only in that the owning relationship controls the relation table,
but will allow changes to be merged into the cache instead of just being
ignored.
Best Regards,
Chris
On 13/08/2010 2:51 AM, reljicb wrote:
> Thank you both a lot for the feedback.
> Chris, your answer is a hit right into the target.
>
> The way I decided to proceed with this is to create two members in my entity
> class (instead only one):
>
> @JoinTable(name = "link",
> joinColumns = {_at_JoinColumn(name = "fromNode",
> referencedColumnName = "id", nullable = false)},
> inverseJoinColumns = {_at_JoinColumn(name = "toNode",
> referencedColumnName = "id", nullable = false)})
> @ManyToMany
> private Collection<Node> outgoingLinks;
>
> @JoinTable(name = "link",
> joinColumns = {_at_JoinColumn(name = "toNode",
> referencedColumnName = "id", nullable = false)},
> inverseJoinColumns = {_at_JoinColumn(name = "fromNode",
> referencedColumnName = "id", nullable = false)}
> )
> @ManyToMany
> private Collection<Node> incomingLinks;
>
>
> There will be getter functions to retrieve either only incoming/outgoing,
> and one to retrieve all of them, by joining two collections into a single
> set.
>
>
> Thanks once more!
> Bojan
>
>
>
>
> christopher delahunt wrote:
>
>> The relationship is unidirectional, otherwise there would be no way to
>> add a new reference to the list. Without a direction being specified,
>> there would be no consistency to which 'side' of the relation table a
>> node gets added to, and it will lead to duplicates when being read in.
>> And in most relationships, nodeId1->nodeId2 does not imply that nodeId2
>> references nodeId1. Think of a person having a collection of children
>> (who are legally also persons). You would not want a parent in its own
>> list of children.
>>
>> In JPA, when you want to access the relationships in the other direction
>> you need to create a mapping for it. Since you wish to use the same
>> relationship table and just represent the NodeId2->nodeId1 relationship,
>> create a new collection and mark it with
>> @ManyToMany(mappedBy="linkedNodes").
>>
>> Best Regards,
>> Chris
>>
>>
>>
>>
>> On 12/08/2010 11:26 AM, reljicb wrote:
>>
>>> Hi!
>>>
>>> I'm not very experienced with persistence, so if anybody of persistence
>>> experts have idea how to solve this, please assist.
>>>
>>> I have database that models bi-directional graphs.
>>> There is a Node table:
>>>
>>> Table node
>>> ==========
>>> id INT
>>> name VARCHAR
>>> nodeType INT
>>>
>>> Any Node can be linked to many other nodes. I use Link table as
>>> association
>>> table to link them.
>>>
>>> Table link
>>> ==========
>>> nodeId1 INT
>>> nodeId2 INT
>>>
>>> When I generate persistence entities (NetBeans), I get a class member
>>> "linkedNodes" defined as following:
>>>
>>> @JoinTable(name = "link",
>>> joinColumns = {_at_JoinColumn(name = "nodeId1",
>>> referencedColumnName = "id", nullable = false)},
>>> verseJoinColumns = {_at_JoinColumn(name = "nodeId2",
>>> referencedColumnName = "id", nullable = false)})
>>> @ManyToMany
>>> private Collection<Node> linkedNodes;
>>>
>>> The problem is that this JPA mapping behaves as if the graph is
>>> unidirectional - linkedNodes collection contains ONLY those nodes which
>>> are
>>> linked to this one in nodeId1->nodeId2 direction manner. In other words,
>>> table Link is queried for only those records where Id of the current node
>>> is
>>> in nodeId1 filed, and Ids of other (linked) nodes are in nodeId2 column,
>>> and
>>> not vice-verse.
>>>
>>> This perfectly makes seanse when ManyToMany relationship is defined
>>> between
>>> two different tables.
>>>
>>> But here I want to obtain all the nodes which are linked to the current
>>> one.
>>> In other words, when Id of current node is in EITHER nodeId1 OR nodeId2
>>> column of the Link table.
>>>
>>> Thanks in advance.
>>> Your help is very much appreciated.
>>>
>>> Bojan
>>>
>>>
>>>
>>
>
>