Hi Taylor,
It is possible to make sure only one relationship of a specific type
gets created between two nodes. The easiest way to achieve this is to
make sure a write lock is grabbed on the node (or nodes if the
relationship could start from either n1 or n2). Grabbing a write lock
can be done using the internal lock manager of Neo4j but even easier
is to just perform a write operation on the node then check for
existing relationship. Here is some sample code how to make sure node1
only has one relationship of a specific type to node2:
// grab write lock on nodes by setting a dummy property
node1.setProperty( "dummy", true );
node2.setProperty( "dummy", true );
boolean relationshipAlreadyExist = false;
for ( Relationship rel : node1.getRelationships( type, Direction.BOTH ) )
{
if ( rel.getOtherNode( node1 ).equals( node2 ) )
{
relationshipAlreadyExist = true;
break;
}
}
if ( !relationshipAlreadyExist )
{
node1.createRelationshipTo( node2, type );
}
// remove dummy property
node1.removeProperty( "dummy" );
node2.removeProperty( "dummy" );
Once the transaction is committed all other threads that are blocked
on the node.setProperty (that grabs the write lock) will see the
relationship created and not create a duplicate one.
It is possible to do this a bit more efficient using the internal lock
manager (see this thread:
http://www.mail-archive.com/[email protected]/msg01870.html on how
to do that).
Regards,
-Johan
On Wed, Dec 2, 2009 at 8:28 PM, Taylor Cowan <[email protected]> wrote:
> I'm struggling with choosing the best pattern to keep relationships singular
> regarding a particular target/object, (neo allows many relations of the same
> type between n(1) and n(2)
>
> Within a transaction, a relationship can be checked as missing, and added,
> and simultaneously another thread ads the relationship. I'm now considering
> just allowing duplicates and periodically walking the graph offline to remove
> duplicate relationships between the same two nodes.
>
> Are there any best practices, patterns, tips that might be helpful regarding
> pruning or preventing redundant relationships?
>
> Taylor
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