add content to old broadcast messages with no content

new broadcast messages will have content filled whether they have a
tempalte or not, but old ones won't so populate.

Stole the session constructor from 0044_jos_to_notification_hist.py

migrations/versions/0336_broadcast_msg_content_2.py

@@ -0,0 +1,35 @@
+"""
+
+Revision ID: 0336_broadcast_msg_content_2
+Revises: 0335_broadcast_msg_content
+Create Date: 2020-12-04 15:06:22.544803
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+from sqlalchemy.orm.session import Session
+
+from app.models import BroadcastMessage
+
+revision = '0336_broadcast_msg_content_2'
+down_revision = '0335_broadcast_msg_content'
+
+
+def upgrade():
+    session = Session(bind=op.get_bind())
+
+    broadcast_messages = session.query(BroadcastMessage).filter(BroadcastMessage.content == None)
+
+    for broadcast_message in broadcast_messages:
+        broadcast_message.content = broadcast_message.template._as_utils_template_with_personalisation(
+            broadcast_message.personalisation
+        ).content_with_placeholders_filled_in
+
+    session.commit()
+
+    op.alter_column('broadcast_message', 'content', nullable=False)
+
+
+def downgrade():
+    op.alter_column('broadcast_message', 'content', nullable=True)