Copying ManyToMany fields from one model instance to another

django, django-1.4, django-models, python

Solution

The simpliest method I could come up with:

e = Expense(description=self.description,
            amount=self.amount,
            category=self.category,
            date=expense_date)
e.save()
e.tags = self.tags.all()

Problem

I'm new to django, and as a learning app, I'm building an expense logging application. In my models I have three classes that look like this (I simplified them slightly for brevity): ``` class AbstractExpense(models.Model): description = models.CharField(max_length=100) amount = models.IntegerField() category = models.ForeignKey('Category') tags = models.ManyToManyField('Tag') insert_date = models.DateTimeField(auto_now=True) class Meta(object): abstract = True class Expense(AbstractExpense): date = models.DateField('Date') class RecurringExpense(AbstractExpense): FREQUENCY_CHOICES = (('D', 'daily'), ('W', 'weekly'), ('M', 'monthly'), ('Y', 'yearly')) start_date = models.DateField(blank=False) end_date = models.DateField(blank=True, null=True) last_check = models.DateField(blank=True, null=True) frequency = models.CharField(blank=False, max_length=1, choices=FREQUENCY_CHOICES) ``` `RecurringExpense` is just a template: when the system realises that the time to insert a recurring expense (e.g.: the rent) it should take the info in the template and copy them in a new instance of the `Expense` class. Here's the relevant bit from the `RecurringExpense` method in charge of the work: ``` Expense(description=self.description, amount=self.amount, category=self.category, # tags=self.tags, date=expense_date).save() ``` The above works flawlessly, but if I uncomment the `tags=self.tags` line, django complains and throw the following error: ``` Exception Type: TypeError Exception Value: 'tags' is an invalid keyword argument for this function Exception Location: <snip>/django/db/models/base.py in __init__, line 367 ``` I know I could create a loop to work around this problem, but I wonder if there is a more elegant way that would allow me to perform the same at once....

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