Queueing and mapping utilities for outbound data sync (#4881)

Add a registry for datasync providers and an associated sync queue, to be used by 
plugins that transfer data from pretix orders to external systems. 
Additionally, provide a generic data mapping interface to be used in settings pages 
of such plugins, to let users configure which information from pretix to fill into
which data fields of the external system.

---------

Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/control/forms/mapping.py

@@ -0,0 +1,127 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020 Raphael Michel and contributors
+# Copyright (C) 2020-2021 rami.io GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+import json
+
+from django import forms
+from django.forms import formset_factory
+from django.utils.translation import gettext_lazy as _
+
+from pretix.base.models import Question
+from pretix.base.models.datasync import (
+    MODE_APPEND_LIST, MODE_OVERWRITE, MODE_SET_IF_EMPTY, MODE_SET_IF_NEW,
+)
+
+
+class PropertyMappingForm(forms.Form):
+    pretix_field = forms.CharField()
+    external_field = forms.CharField()
+    value_map = forms.CharField(required=False)
+    overwrite = forms.ChoiceField(
+        choices=[
+            (MODE_OVERWRITE, _("Overwrite")),
+            (MODE_SET_IF_NEW, _("Fill if new")),
+            (MODE_SET_IF_EMPTY, _("Fill if empty")),
+            (MODE_APPEND_LIST, _("Add to list")),
+        ]
+    )
+
+    def __init__(self, pretix_fields, external_fields_id, available_modes, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.fields["pretix_field"] = forms.ChoiceField(
+            label=_("pretix field"),
+            choices=pretix_fields_choices(pretix_fields, kwargs.get("initial", {}).get("pretix_field")),
+            required=False,
+        )
+        if external_fields_id:
+            self.fields["external_field"] = forms.ChoiceField(
+                widget=forms.Select(
+                    attrs={
+                        "data-model-select2": "json_script",
+                        "data-select2-src": "#" + external_fields_id,
+                    },
+                ),
+            )
+            self.fields["external_field"].choices = [
+                (self["external_field"].value(), self["external_field"].value()),
+            ]
+        self.fields["overwrite"].choices = [
+            (key, label) for (key, label) in self.fields["overwrite"].choices if key in available_modes
+        ]
+
+
+class PropertyMappingFormSet(formset_factory(
+    PropertyMappingForm,
+    can_order=True,
+    can_delete=True,
+    extra=0,
+)):
+    template_name = "pretixcontrol/datasync/property_mappings_formset.html"
+
+    def __init__(self, pretix_fields, external_fields, available_modes, prefix, *args, initial_json=None, **kwargs):
+        if initial_json:
+            kwargs["initial"] = json.loads(initial_json)
+        super().__init__(
+            form_kwargs={
+                "pretix_fields": pretix_fields,
+                "external_fields_id": prefix + "external-fields" if external_fields else None,
+                "available_modes": available_modes,
+            },
+            prefix=prefix,
+            *args, **kwargs)
+        self.external_fields = external_fields
+
+    def get_context(self):
+        ctx = super().get_context()
+        ctx["external_fields"] = self.external_fields
+        ctx["external_fields_id"] = self.prefix + "external-fields"
+        return ctx
+
+    def to_property_mappings_json(self):
+        """
+        Returns a property mapping configuration as a JSON-serialized list of dictionaries.
+
+        Each entry specifies how to transfer data from one pretix field to one field in the external system:
+
+          - `pretix_field`: Name of a pretix data source field as declared in `pretix.base.datasync.sourcefields.get_data_fields`.
+          - `external_field`: Name of the target field in the external system. Implementation-defined by the sync provider.
+          - `value_map`: Dictionary mapping pretix value to external value. Only used for enumeration-type fields.
+          - `overwrite`: Mode of operation if the object already exists in the target system.
+
+            - `MODE_OVERWRITE` (`"overwrite"`) to always overwrite existing value.
+            - `MODE_SET_IF_NEW` (`"if_new"`) to only set the value if object does not exist in target system yet.
+            - `MODE_SET_IF_EMPTY` (`"if_empty"`) to only set the value if object does not exist in target system,
+              or the field is currently empty in target system.
+            - `MODE_APPEND_LIST` (`"append"`) if the field is an array or a multi-select: add the value to the list.
+        """
+        mappings = [f.cleaned_data for f in self.ordered_forms]
+        return json.dumps(mappings)
+
+
+QUESTION_TYPE_LABELS = dict(Question.TYPE_CHOICES)
+
+
+def pretix_fields_choices(pretix_fields, initial_choice):
+    return [
+        (f.key, f.label + " [" + QUESTION_TYPE_LABELS[f.type] + "]")
+        for f in pretix_fields
+        if not f.deprecated or f.key == initial_choice
+    ]