Relation between VendPackingSlipTrans and VendInoiceTRans Dynamics Ax 2012

Today I got chance to find a relationship between vendor packing list and vendor Invoice.  I was expecting that it there was direct relationship between VendPackingtrans and VendInvocietrans. If you expand VendInvoceTrans, there will be a relation but that relation will never use in Dynamics ax 2012. After searching I found VendPackingSlipQuantityMatch table which has relation between VendPackingTrans and VendInvoiceTrans.

VendPackingSlipTrans _PackingTrans;

VendPackingSlipQuantityMatch _ VendPackingSlipQuantityMatch;

vendInvoicetrans _ vendInvoicetrans;

I got the VendInvoiceTrans and VendPackingTrans as follow.

select firstOnly * from _PackingTrans

join _VendPackingSlipQuantityMatch

where _VendPackingSlipQuantityMatch.InvoiceSourceDocumentLIne ==vendInvoiceTrans.SourceDocumentLine

&& _PackingTrans.SourceDocumentLine == _VendPackingSlipQuantityMatch.PackingSlipSourceDocumentLine;