django,edit form with unique field, form.is_valid return false

class modelA(..):
    fieldA = ...     unique
    fieldB = ...     unique

    class Meta:
        unique_together = [['fieldA','fieldB']]


class formA(modelform):

    class Meta:
        model = modelA
        field = ['fieldA','fieldB']


def viewA(request):
    if request.method == 'POST':
        form = formA(request.POST)
        if form.is_valid():          here return false !!!!!
            ......

In edit form, when click submit button to update the database, the form.is_valid() return false because the unique_together check , how to solve the issue ?