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Checks and Validation

When defining the data structure for your application, it is common to want to define rules about your data and what happens when it changes. These include:

  • Validation, e.g. “This column must be a number between 1 and 10”

  • Permissions, e.g. “Only administrators may change the value of this column”

  • Side effects, e.g. “Every time we update a row from this table, we set its last_updated column to the current time”

Model classes allow you to express these constraints in code.

Low-level API

Anvil curently only supports this low-level validation API. We intend to release a higher-level API to reduce code duplication, once we have gathered feedback from how people are using models in practice.

A model class may override the methods _do_update(), _do_delete() and/or _do_create() to intercept updates, deletion and draft creation. To complete the action, call the superclass method.

_do_update()

The _do_update() method is called when column values are updated by assigning to a column, calling the update() method, or saving buffered changes to a row.

It takes two positional arguments:

  1. updates: A dictionary of column names to new values, representing the requested changes

  2. from_client: A boolean value, true if the request was initiated by the client. (This is useful when creating client-writable models, in which case _do_update() will run on the server even if the change was initiated by client code.)

_do_delete()

The _do_delete() method is called when row deletion is requested with the delete() method. It takes one positional argument:

  1. from_client: A boolean value, true if the request was initiated from the client.

_do_create()

The _do_create() class method is called when saving a draft row. It takes two positional arguments:

  1. values: A dictionary of column names to values representing the requested changes

  2. from_client: A boolean value, true if the request was initiated from the client.

Note: _do_create() should be a @classmethod, as it does not refer to an existing instance.

Example

This code extends the previous example to add validation, add permission checks and automatically update the last_created column with the current time whenever the row is changed:

class TodoItem(app_tables.todos.Row):
  def _do_delete(self, from_client):
    me = anvil.users.get_user()
    if not me:
      raise Exception("Must be logged in to delete records")
    if self["assigned_user"] != me:
      raise Exception("Only the assigned user can delete a record")
    super()._do_delete(from_client)
  
  def _do_update(self, updates, from_client):
    me = anvil.users.get_user()
    if me is None:
      raise Exception("Must be logged in to edit records")
    if self["assigned_user"] != me:
      raise Exception("Must be the owner of this item to update it")
    if "title" in updates and not update["titles"]:
      raise Exception("Must specify a title")

    updates['last_updated'] = datetime.now()

    super()._do_update(updates, from_client)
  
  @classmethod
  def _do_create(cls, values, from_client):
    me = anvil.users.get_user()
    if me is None:
      raise Exception("Must be logged in to do this")
    if not values.get("title", ""):
      raise Exception("Must specify a title")

    values["assigned_user"] = me
    values['last_updated'] = datetime.now()

    return super()._do_create(values, from_client)

Do you still have questions?

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Creating Model Classes Client-Writable Models

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