Get preferred external event from em_type

CHANGES.md

@@ -17,6 +17,9 @@
 -   Get p_astro and em_bright files from the superevent instead of the
     preferred event in order to accomodate PE-based results.
 
+-   Get the preferred external event from 'em_type' field in superevent,
+    if no value grab first external event.
+
 ## 1.1.2 (2019-09-25)
 
 -   Remove check that sky map has `public` and `sky_loc` tags in order to

ligo/followup_advocate/__init__.py

@@ -199,15 +199,18 @@ def compose_raven(gracedb_id, authors=(),
     superevent_far = preferred_event['far']
     gpstime = float(preferred_event['gpstime'])
 
-    for em_event_id in event['em_events']:
-        em_event = client.event(em_event_id).json()
-        em_event_gpstime = float(em_event['gpstime'])
-        external_pipeline = em_event['pipeline']
-        # FIXME in GraceDb: Even SNEWS triggers have an extra attribute GRB.
-        external_trigger_id = em_event['extra_attributes']['GRB']['trigger_id']
-        snews = (em_event['search'] == 'Supernova')
-        grb = em_event['search'] in ['GRB', 'SubGRB']
-        subthreshold = (em_event['search'] == 'SubGRB')
+    try:
+        em_event_id = event['em_type']
+    except KeyError:
+        em_event_id = event['em_events'][0]
+    em_event = client.event(em_event_id).json()
+    em_event_gpstime = float(em_event['gpstime'])
+    external_pipeline = em_event['pipeline']
+    # FIXME in GraceDb: Even SNEWS triggers have an extra attribute GRB.
+    external_trigger_id = em_event['extra_attributes']['GRB']['trigger_id']
+    snews = (em_event['search'] == 'Supernova')
+    grb = em_event['search'] in ['GRB', 'SubGRB']
+    subthreshold = (em_event['search'] == 'SubGRB')
 
     o = urllib.parse.urlparse(client.service_url)
     kwargs = dict(