set client.referral_id=None when a referral is deleted

app/models.py

@@ -987,7 +987,9 @@ class Client(Base, ModelMixin):
 
     # a referral can be attached to a client
     # so all users who sign up via the authorize screen are counted towards this referral
-    referral_id = sa.Column(sa.ForeignKey("referral.id"), nullable=True)
+    referral_id = sa.Column(
+        sa.ForeignKey("referral.id", ondelete="SET NULL"), nullable=True
+    )
 
     icon = orm.relationship(File)
     user = orm.relationship(User)

migrations/versions/2021_102612_bbedc353f90c_.py

@@ -0,0 +1,31 @@
+"""empty message
+
+Revision ID: bbedc353f90c
+Revises: d67eab226ecd
+Create Date: 2021-10-26 12:05:38.840492
+
+"""
+import sqlalchemy_utils
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'bbedc353f90c'
+down_revision = 'd67eab226ecd'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_constraint('client_referral_id_fkey', 'client', type_='foreignkey')
+    op.create_foreign_key(None, 'client', 'referral', ['referral_id'], ['id'], ondelete='SET NULL')
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_constraint(None, 'client', type_='foreignkey')
+    op.create_foreign_key('client_referral_id_fkey', 'client', 'referral', ['referral_id'], ['id'])
+    # ### end Alembic commands ###