Merge pull request #71 from openlibhums/fix_contributors

Handle instances where a book has no contributors.

Author: ajrbyers

models.py

@@ -162,27 +162,30 @@ def citation(self):
                 title=self.full_title(),
             )
         )
-
+      
     def contributors_citation(self):
         contributors = self.contributor_set.all()
+        contributors_count = contributors.count()
 
-        if not contributors.exists():
+        if contributors_count == 0:
             return ''
 
-        if contributors.count() == 1:
+        if contributors_count == 1:
             return '{contributor} '.format(
                 contributor=contributors[0].citation_name(),
             )
-        elif contributors.count() == 2:
-            return '{contributor_one} & {contributor_two} '.format(
-                contributor_one=contributors[0].citation_name(),
-                contributor_two=contributors[1].citation_name(),
-            )
-        else:
-            return '{contributor} et al. '.format(
-                contributor=contributors[0].citation_name(),
+
+        if contributors_count == 2:
+            return '{one} & {two} '.format(
+                one=contributors[0].citation_name(),
+                two=contributors[1].citation_name(),
             )
 
+        return '{contributor} et al. '.format(
+            contributor=contributors[0].citation_name(),
+        )
+
+
     def full_title(self):
         if self.prefix and self.subtitle:
             return "{0} {1}: {2}".format(self.prefix, self.title, self.subtitle)