django_test_stuff.py

# Essentially, the reason you're finding this hard to test is because the
# business logic is baked into the save_model method - this is effectively a
# "fat controller". You want to pull that logic out and give it names. This has
# several outcomes:

#  * it makes your code easier to unit test because you're
#    not coupled to the form and the admin, and in fact you don't even
#    need a real expense object here, it could be just a stub.
#  * it makes code reuse easier - it's rarely a good idea to have
#    business logic living *only* in an admin method.
#  * it's more legible - if you want to know what happens when the state
#    changes for an expense, it lives in one place.

# in finance/utils.py

class ExpenseHandler:
    """
    You give me an expense and a new state, I tell you what happens.
    """
    def __init__(self, expense, new_state):
        # dispatch on state
        mapping = {
            ExpenseStatuses.DRAFT:     'handle_draft_expense_creation',
            ExpenseStatuses.SUBMITTED: 'handle_expense_submitted',
            ExpenseStatuses.REJECT:    'handle_expense_rejection',
            ExpenseStatuses.PAID:      'handle_expense_success',
        }
        return getattr(self, mapping[new_state])(expense)

    def handle_draft_expense_creation(self, expense):
        expense.remove_all_authorisations()
        return expense

    def handle_expense_submitted(self, expense):
        expense.notify_next_authoriser()
        return expense

    def handle_expense_rejection(self, expense):
        expense.send_rejection_email()
        return expense

    def handle_expense_success(self, expense):
        expense.status = ExpenseStatuses.PAID
        expense.reimbursed_by = editor
        expense.reimbursed_on = datetime.now()
        expense.send_reimbursed_email()
        return expense

# Usage:
def save_model(self, request, obj, form, change):
    # This bit should just handle the form parsing and basically defer all the
    # business logic to something more clever:
    expense = obj
    if change:
        if 'reimbursed' in form.changed_data and form.cleaned_data['reimbursed']:
            expense = ExpenseHandler(expense, new_state=ExpenseStatuses.PAID)
            expense.save()
        if phase:
            # Re-save all expense items to ensure the details text contains
            # the correct phase ID:
            for ei in expense.items.all():
                ei.save()
        if 'advance_status_to' in form.changed_data:
            advance_status_to = form.cleaned_data['advance_status_to']
            expense = ExpenseHandler(expense, new_state=advance_status_to)
            expense.save()
        else:
            # more stuff