Restore names of uploaded files on submit/draft restore (#65)

This involved turning the list of file IDs stored in the hidden
text field into JSON.

apps/map/widgets.py

@@ -1,4 +1,7 @@
+import json
+
 from django.forms import widgets
+from apps.files.models import File
 
 
 class CommaSeparatedTextInput(widgets.HiddenInput):
@@ -19,3 +22,34 @@ class CommaSeparatedTextInput(widgets.HiddenInput):
             return value.split(',')
         except AttributeError:
             return None
+
+
+def idToDict(id):
+    file = File.objects.get(pk=id)
+    return dict(id=id, name=file.name())
+
+
+# For clarity:
+# the canonical format is a list of integers (IDs of files)
+# the input/display format is a string containing a JSON list of {id=x, name=y} objects
+class JSONFileListWidget(widgets.HiddenInput):
+
+    def format_value(self, value):
+        try:
+            value = [ idToDict(id) for id in value ]
+            value = json.dumps(value)
+        except TypeError:
+            value = ''
+        return super().format_value(value)
+
+    def value_from_datadict(self, data, files, name):
+        value = super().value_from_datadict(data, files, name)
+
+        if value == '':
+            return None
+
+        try:
+            filelist = json.loads(value)
+            return [ file['id'] for file in filelist ]
+        except JSONDecodeError:
+            return None