Metadata: Use generator to prepare lookup choices

Change the use of the list/zip combinarion to generate
the full list of metadata lookup choices to a generator.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

HISTORY.rst

@@ -163,7 +163,9 @@
   into a Mayan app. With change adds the new settings:
   "COMMON_AUTOADMIN_EMAIL", "AUTOADMIN_PASSWORD", and
   "AUTOADMIN_USERNAME".
-
+- Changed the use of the list/zip combinarion to generate
+  the full list of metadata lookup choices to a generator.
+  
 3.1.9 (2018-11-01)
 ==================
 - Convert the furl instance to text to allow serializing it into

mayan/apps/metadata/forms.py

@@ -62,11 +62,15 @@ class DocumentMetadataForm(forms.Form):
                     self.fields['value'] = forms.ChoiceField(
                         label=self.fields['value'].label
                     )
-                    choices = self.metadata_type.get_lookup_values()
-                    choices = list(zip(choices, choices))
+
                     if not required:
-                        choices.insert(0, ('', '------'))
-                    self.fields['value'].choices = choices
+                        first_choice=('', '------')
+                    else:
+                        first_choice=None
+
+                    self.fields['value'].choices = self.metadata_type.get_lookup_choices(
+                        first_choice=first_choice
+                    )
                     self.fields['value'].required = required
                     self.fields['value'].widget.attrs.update(
                         {'class': 'metadata-value'}

mayan/apps/metadata/models.py

@@ -123,6 +123,16 @@ class MetadataType(models.Model):
         template = Template(self.default)
         return template.render()
 
+    def get_lookup_choices(self, first_choice=None):
+        template = Template(self.lookup)
+        context = MetadataLookup.get_as_context()
+
+        if first_choice:
+            yield first_choice
+
+        for value in MetadataType.comma_splitter(template.render(**context)):
+            yield (value, value)
+
     def get_lookup_values(self):
         template = Template(self.lookup)
         context = MetadataLookup.get_as_context()