Create new column in notifications and notification_history to store the client_reference because I remembered that reference is used for email providers.

migrations/versions/0061_add_client_reference.py

@@ -0,0 +1,24 @@
+"""empty message
+
+Revision ID: 0061_add_client_reference
+Revises: 0060_add_letter_template_type
+Create Date: 2016-11-17 13:19:25.820617
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '0061_add_client_reference'
+down_revision = '0060_add_letter_template_type'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.add_column('notifications', sa.Column('client_reference', sa.String(), index=True, nullable=True))
+    op.add_column('notification_history', sa.Column('client_reference', sa.String(), nullable=True))
+
+
+def downgrade():
+    op.drop_column('notifications', 'client_reference')
+    op.drop_column('notification_history', 'client_reference')