Fix selections that include task with no tags

app/src/androidTest/java/org/tasks/data/TagDataDaoTest.java

@@ -6,7 +6,6 @@ import static java.util.Collections.singletonList;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 import static org.tasks.makers.TagDataMaker.NAME;
-import static org.tasks.makers.TagDataMaker.UID;
 import static org.tasks.makers.TagDataMaker.newTagData;
 import static org.tasks.makers.TagMaker.TAGDATA;
 import static org.tasks.makers.TagMaker.TAGUID;
@@ -105,7 +104,6 @@ public class TagDataDaoTest extends InjectingTestCase {
     newTag(1, "tag1", "tag2");
     newTag(2, "tag2", "tag3");
 
-
     assertEquals(
         newHashSet("tag1", "tag3"), tagDataDao.getTagSelections(ImmutableList.of(1L, 2L)).first);
   }
@@ -123,8 +121,7 @@ public class TagDataDaoTest extends InjectingTestCase {
     newTag(1, "tag1", "tag2");
     newTag(2, "tag2", "tag3");
 
-    assertEquals(
-        newHashSet("tag2"), tagDataDao.getTagSelections(ImmutableList.of(1L, 2L)).second);
+    assertEquals(newHashSet("tag2"), tagDataDao.getTagSelections(ImmutableList.of(1L, 2L)).second);
   }
 
   @Test
@@ -135,6 +132,26 @@ public class TagDataDaoTest extends InjectingTestCase {
     assertTrue(tagDataDao.getTagSelections(ImmutableList.of(1L, 2L)).second.isEmpty());
   }
 
+  @Test
+  public void getSelectionsWithNoTags() {
+    newTag(1);
+
+    Pair<Set<String>, Set<String>> selections = tagDataDao.getTagSelections(ImmutableList.of(1L));
+    assertTrue(selections.first.isEmpty());
+    assertTrue(selections.second.isEmpty());
+  }
+
+  @Test
+  public void noCommonSelectionsWhenOneTaskHasNoTags() {
+    newTag(1, "tag1");
+    newTag(2);
+
+    Pair<Set<String>, Set<String>> selections =
+        tagDataDao.getTagSelections(ImmutableList.of(1L, 2L));
+    assertEquals(newHashSet("tag1"), selections.first);
+    assertTrue(selections.second.isEmpty());
+  }
+
   private void newTag(long taskId, String... tags) {
     Task task = newTask(with(ID, taskId));
     taskDao.createNew(task);

app/src/main/java/org/tasks/data/TagDataDao.java

@@ -5,6 +5,7 @@ import static com.google.common.collect.Iterables.transform;
 import static com.google.common.collect.Lists.newArrayList;
 import static com.google.common.collect.Sets.difference;
 import static com.google.common.collect.Sets.newHashSet;
+import static java.util.Collections.emptySet;
 import static org.tasks.db.DbUtils.MAX_SQLITE_ARGS;
 import static org.tasks.db.DbUtils.batch;
 
@@ -76,11 +77,12 @@ public abstract class TagDataDao {
 
   public Pair<Set<String>, Set<String>> getTagSelections(List<Long> tasks) {
     List<String> allTags = getAllTags(tasks);
-    Collection<Set<String>> tags = Collections2.transform(allTags, t -> newHashSet(t.split(",")));
+    Collection<Set<String>> tags =
+        Collections2.transform(allTags, t -> t == null ? emptySet() : newHashSet(t.split(",")));
     Set<String> partialTags = newHashSet(concat(tags));
     Set<String> commonTags = null;
     if (tags.isEmpty()) {
-      commonTags = Collections.emptySet();
+      commonTags = emptySet();
     } else {
       for (Set<String> s : tags) {
         if (commonTags == null) {
@@ -95,8 +97,8 @@ public abstract class TagDataDao {
   }
 
   @Query(
-      "SELECT IFNULL(GROUP_CONCAT(DISTINCT(tag_uid)), '') FROM tasks"
-          + " INNER JOIN tags ON tags.task = tasks._id"
+      "SELECT GROUP_CONCAT(DISTINCT(tag_uid)) FROM tasks"
+          + " LEFT JOIN tags ON tags.task = tasks._id"
           + " WHERE tasks._id IN (:tasks)"
           + " GROUP BY tasks._id")
   abstract List<String> getAllTags(List<Long> tasks);