data binding problem?

See more:

C#

data-binding

entity-framework

*I have data base that contain four tables (Product,ProductUnits,Invoice,InvoiceDetails) .
*Relations between tables as following
1- one-to-many (Product-ProductUnits)
2- one-to-many (Invoice-InvoiceDetails)
3- one-to-many (InvoiceDetails-Product)
4- one-to-many (InvoiceDetails-ProductUnits)
*This data base is mapped to entity model.
*Now InvoiceDetails table contain ProductID as foreign key and UnitID also as foreign key.
*I desigen Invoice form where InvoiceDetails is bound to data grid control ,product and units appear as drop down combo-box.
*All units appear when I drop down Units combobox.
*Question??
*I want units related to current product in InvoiceDetails entry only appear to user in combo-box.
*How can I bind InvoiceDetails.Units to current selected product in InvoiceDetails row.