Adding relative date range functionality and tests

We need to be able to generate start_at and end_at pairs from requests for relative amounts of time.

backdrop/read/query.py

@@ -2,7 +2,7 @@
 
 import pytz
 from backdrop.core.timeseries import parse_period
-from backdrop.core.timeutils import parse_time_as_utc
+from backdrop.core.timeutils import now, parse_time_as_utc
 from backdrop.read.response import *
 
 
@@ -19,11 +19,32 @@ def if_present(func, value):
 def parse_request_args(request_args):
     args = dict()
 
-    args['start_at'] = if_present(parse_time_as_utc,
-                                  request_args.get('start_at'))
+    args['period'] = if_present(parse_period,
+                                request_args.get('period'))
+
+    if request_args.get('delta'):
+        # relative time range requested
+        delta = request_args['delta']
+
+        date = if_present(parse_time_as_utc, request_args.get('date')) or now()
+
+        period_delta = args['period'].delta
+        duration = period_delta * delta
+        other_date = date + duration
 
-    args['end_at'] = if_present(parse_time_as_utc,
-                                request_args.get('end_at'))
+        if delta > 0:
+            args['start_at'] = date
+            args['end_at'] = other_date
+        else:
+            args['start_at'] = other_date
+            args['end_at'] = date
+    else:
+        # absolute time range requested
+        args['start_at'] = if_present(parse_time_as_utc,
+                                      request_args.get('start_at'))
+
+        args['end_at'] = if_present(parse_time_as_utc,
+                                    request_args.get('end_at'))
 
     def boolify(value):
         return {
@@ -38,9 +59,6 @@ def parse_filter_by(filter_by):
 
     args['filter_by'] = map(parse_filter_by, request_args.getlist('filter_by'))
 
-    args['period'] = if_present(parse_period,
-                                request_args.get('period'))
-
     args['group_by'] = request_args.get('group_by')
 
     args['sort_by'] = if_present(lambda sort_by: sort_by.split(':', 1),