Allow to import orders (#1516)

* Allow to import orders

* seats, subevents

* Plugin support

* Add docs

* Warn about lack of quota handling

* Control interface test

* Test skeleton

* First tests for the impotr columns

* Add tests for all columns

* Fix question validation

doc/development/api/import.rst

@@ -0,0 +1,112 @@
+.. highlight:: python
+   :linenothreshold: 5
+
+.. _`importcol`:
+
+Extending the order import process
+==================================
+
+It's possible through the backend to import orders into pretix, for example from a legacy ticketing system. If your
+plugins defines additional data structures around orders, it might be useful to make it possible to import them as well.
+
+Import process
+--------------
+
+Here's a short description of pretix' import process to show you where the system will need to interact with your plugin.
+You can find more detailed descriptions of the attributes and methods further below.
+
+1. The user uploads a CSV file. The system tries to parse the CSV file and understand its column headers.
+
+2. A preview of the file is shown to the user and the user is asked to assign the various different input parameters to
+   columns of the file or static values. For example, the user either needs to manually select a product or specify a
+   column that contains a product. For this purpose, a select field is rendered for every possible input column,
+   allowing the user to choose between a default/empty value (defined by your ``default_value``/``default_label``)
+   attributes, the columns of the uploaded file, or a static value (defined by your ``static_choices`` method).
+
+3. The user submits its assignment and the system uses the ``resolve`` method of all columns to get the raw value for
+   all columns.
+
+4. The system uses the ``clean`` method of all columns to verify that all input fields are valid and transformed to the
+   correct data type.
+
+5. The system prepares internal model objects (``Order`` etc) and uses the ``assign`` method of all columns to assign
+   these objects with actual values.
+
+6. The system saves all of these model objects to the database in a database transaction. Plugins can create additional
+   objects in this stage through their ``save`` method.
+
+Column registration
+-------------------
+
+The import API does not make a lot of usage from signals, however, it
+does use a signal to get a list of all available import columns. Your plugin
+should listen for this signal and return the subclass of ``pretix.base.orderimport.ImportColumn``
+that we'll provide in this plugin:
+
+.. sourcecode:: python
+
+    from django.dispatch import receiver
+
+    from pretix.base.signals import order_import_columns
+
+
+    @receiver(order_import_columns, dispatch_uid="custom_columns")
+    def register_column(sender, **kwargs):
+        return [
+            EmailColumn(sender),
+        ]
+
+The column class API
+--------------------
+
+.. class:: pretix.base.orderimport.ImportColumn
+
+   The central object of each import extension is the subclass of ``ImportColumn``.
+
+   .. py:attribute:: ImportColumn.event
+
+      The default constructor sets this property to the event we are currently
+      working for.
+
+   .. autoattribute:: identifier
+
+      This is an abstract attribute, you **must** override this!
+
+   .. autoattribute:: verbose_name
+
+      This is an abstract attribute, you **must** override this!
+
+   .. autoattribute:: default_value
+
+   .. autoattribute:: default_label
+
+   .. autoattribute:: initial
+
+   .. automethod:: static_choices
+
+   .. automethod:: resolve
+
+   .. automethod:: clean
+
+   .. automethod:: assign
+
+   .. automethod:: save
+
+Example
+-------
+
+For example, the import column responsible for assigning email addresses looks like this:
+
+.. sourcecode:: python
+
+   class EmailColumn(ImportColumn):
+       identifier = 'email'
+       verbose_name = _('E-mail address')
+
+       def clean(self, value, previous_values):
+           if value:
+               EmailValidator()(value)
+           return value
+
+       def assign(self, value, order, position, invoice_address, **kwargs):
+           order.email = value

doc/development/api/index.rst

@@ -15,6 +15,7 @@ Contents:
    placeholder
    invoice
    shredder
+   import
    customview
    auth
    general