AttributionForm in bda admin

- New attribution form issue less queries
- Spectacle and Participant are readonly if updating an attribution.

ReadOnlyMixin allows to set readonly fields only while updating an
object.

bda/admin.py

@@ -94,6 +94,20 @@ class ParticipantAdmin(admin.ModelAdmin):
 
 
 class AttributionAdminForm(forms.ModelForm):
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        if 'spectacle' in self.fields:
+            self.fields['spectacle'].queryset = (
+                Spectacle.objects
+                .select_related('location')
+            )
+        if 'participant' in self.fields:
+            self.fields['participant'].queryset = (
+                Participant.objects
+                .select_related('user', 'tirage')
+            )
+
     def clean(self):
         cleaned_data = super(AttributionAdminForm, self).clean()
         participant = cleaned_data.get("participant")
@@ -106,7 +120,18 @@ class AttributionAdminForm(forms.ModelForm):
         return cleaned_data
 
 
-class AttributionAdmin(admin.ModelAdmin):
+class ReadOnlyMixin(object):
+    readonly_fields_update = ()
+
+    def get_readonly_fields(self, request, obj=None):
+        readonly_fields = super().get_readonly_fields(request, obj)
+        if obj is None:
+            return readonly_fields
+        else:
+            return readonly_fields + self.readonly_fields_update
+
+
+class AttributionAdmin(ReadOnlyMixin, admin.ModelAdmin):
     def paid(self, obj):
         return obj.participant.paid
     paid.short_description = 'A payé'
@@ -116,6 +141,7 @@ class AttributionAdmin(admin.ModelAdmin):
                      'participant__user__first_name',
                      'participant__user__last_name')
     form = AttributionAdminForm
+    readonly_fields_update = ('spectacle', 'participant')
 
 
 class ChoixSpectacleAdmin(admin.ModelAdmin):