Caldav subtasks requires lollipop

app/src/main/java/com/todoroo/astrid/adapter/TaskAdapterProvider.java

@@ -1,5 +1,7 @@
 package com.todoroo.astrid.adapter;
 
+import static com.todoroo.andlib.utility.AndroidUtilities.atLeastLollipop;
+
 import android.content.Context;
 import android.text.TextUtils;
 import com.todoroo.astrid.api.CaldavFilter;
@@ -78,7 +80,7 @@ public class TaskAdapterProvider {
       CaldavFilter caldavFilter = (CaldavFilter) filter;
       CaldavCalendar calendar = caldavDao.getCalendarByUuid(caldavFilter.getUuid());
       if (calendar != null) {
-        return new CaldavTaskAdapter(taskDao, caldavDao);
+        return atLeastLollipop() ? new CaldavTaskAdapter(taskDao, caldavDao) : new TaskAdapter();
       }
     } else {
       return subtasksHelper.shouldUseSubtasksFragmentForFilter(filter)

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

@@ -1,5 +1,7 @@
 package org.tasks.data;
 
+import static com.todoroo.andlib.utility.AndroidUtilities.atLeastLollipop;
+
 import androidx.lifecycle.LiveData;
 import androidx.room.Dao;
 import androidx.room.Delete;
@@ -7,6 +9,7 @@ import androidx.room.Insert;
 import androidx.room.Query;
 import androidx.room.Update;
 import io.reactivex.Single;
+import java.util.Collections;
 import java.util.List;
 import org.tasks.filters.CaldavFilters;
 
@@ -114,6 +117,12 @@ public abstract class CaldavDao {
   @Query("UPDATE caldav_tasks SET cd_parent = IFNULL((SELECT cd_task FROM caldav_tasks AS p WHERE p.cd_remote_id = caldav_tasks.cd_remote_parent), cd_parent) WHERE cd_calendar = :calendar AND cd_remote_parent IS NOT NULL and cd_remote_parent != ''")
   public abstract void updateParents(String calendar);
 
+  public List<Long> getChildren(List<Long> ids) {
+    return atLeastLollipop()
+        ? getChildrenRecursive(ids)
+        : Collections.emptyList();
+  }
+
   @Query("WITH RECURSIVE "
           + " recursive_caldav (cd_task) AS ( "
           + " SELECT cd_task "
@@ -132,5 +141,5 @@ public abstract class CaldavDao {
           + " WHERE tasks.deleted = 0 "
           + " ) "
           + "SELECT cd_task FROM recursive_caldav")
-  public abstract List<Long> getChildren(List<Long> ids);
+  abstract List<Long> getChildrenRecursive(List<Long> ids);
 }

app/src/main/java/org/tasks/ui/TaskListViewModel.java

@@ -2,6 +2,7 @@ package org.tasks.ui;
 
 import static com.todoroo.andlib.sql.Field.field;
 import static com.todoroo.andlib.utility.AndroidUtilities.assertMainThread;
+import static com.todoroo.andlib.utility.AndroidUtilities.atLeastLollipop;
 import static com.todoroo.astrid.activity.TaskListFragment.CALDAV_METADATA_JOIN;
 import static com.todoroo.astrid.activity.TaskListFragment.GTASK_METADATA_JOIN;
 import static com.todoroo.astrid.activity.TaskListFragment.TAGS_METADATA_JOIN;
@@ -126,7 +127,7 @@ public class TaskListViewModel extends ViewModel implements Observer<PagedList<T
           Criterion.and(caldavJoinCriterion, field(CALDAV_METADATA_JOIN + ".cd_calendar").eq(uuid));
     }
 
-    if (filter instanceof CaldavFilter) {
+    if (filter instanceof CaldavFilter && atLeastLollipop()) {
       // TODO This is in some ways a proof of concept demonstrating a recursive query used to pull
       //      in CalDAV tasks providing parenting across different sort modes. Tags are implemented
       //      as a subquery, which is ugly, but aggregate recursive queries aren't supported. The