I have some problem while updating using discriminator

the problem goes like this
i have a table and in that table i have 2 primary key and one column
of which one of them i have made as the discriminator coloumn

the fields cd and flag are the primary key and flag is the discriminator

i have made 3 subclasses for this table

i will get an row not found error when the following type of data is updated

cd flag desc
2 C test1
2 D test2
2 R test3

where C, D and R are discriminator-values for the there subclasses

ie if the cd contain same value for differen subclasses it is giving error

why could this be
thanks in advance
zeeta