How do I remove an item from db.ListProperty?

In a Google App Engine solution (Python), I've used the db.ListProperty as a way to describe a many-to-many relation, like so:

class Department(db.Model):
  name = db.StringProperty()

  @property
  def employees(self):
    return Employee.all().filter('departments', self.key())

class Employee(db.Model):
  name = db.StringProperty()
  departments = db.ListProperty(db.Key)

I create many-to-many relations by simply appending the Department 开发者_如何转开发key to the db.ListProperty like so:

employee.departments.append(department.key())

The problem is that I don't know how to actually remove this relationship again, when it is no longer needed.

I've tried Googling it, but I can't seem to find any documentation that describes the db.ListProperty in details.

Any ideas or references?

The ListProperty is just a Python list with some helper methods to make it work with GAE, so anything that applies to a list applies to a ListProperty.

employee.departments.remove(department.key())
employee.put()

Keep in mind that the data must be deserialized/reserialized every time a change is made, so if you are looking for speed when adding or removing single values you may want to go with another method of modelling the relationship like the one in the Relationship Model section of this page.

The ListProperty method also has the disadvantage of sometimes producing very large indexes if you want to search through the lists in a datastore request. This may not not be a problem for you since your Lists should be relatively small, but it's something to keep in mind for future projects.

Found it via trial and error:

employee.departments.remove(department.key())