Rewrite previous migration in raw SQL

We shouldn’t import models into migrations because if the model changes
later down the line then the migration can’t be re-run at a later date
(for example to rebuild a database from scratch).

We don’t need to encode the content before storing it (we’ll always do
that before rendering/sending) so we don’t need to use
`BroadcastMessageTemplate`.

And given that no past broadcasts will have personalisation, we don’t
need to replace the personalisation in the template before rendering it.
So we can just copy the raw content from the templates table.

migrations/versions/0336_broadcast_msg_content_2.py

@@ -7,6 +7,7 @@ Create Date: 2020-12-04 15:06:22.544803
 """
 from alembic import op
 import sqlalchemy as sa
+from notifications_utils.template import BroadcastMessageTemplate
 from sqlalchemy.dialects import postgresql
 from sqlalchemy.orm.session import Session
 
@@ -17,18 +18,22 @@ down_revision = '0335_broadcast_msg_content'
 
 
 def upgrade():
-    session = Session(bind=op.get_bind())
 
-    broadcast_messages = session.query(BroadcastMessage).filter(BroadcastMessage.content == None)
+    conn = op.get_bind()
 
-    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)
+    results = conn.execute(sa.text("""
+        UPDATE
+            broadcast_message
+        SET
+            content = templates_history.content
+        FROM
+            templates_history
+        WHERE
+            broadcast_message.content is NULL and
+            broadcast_message.template_id = templates_history.id and
+            broadcast_message.template_version = templates_history.version
+        ;
+    """))
 
 
 def downgrade():