Use NULL for non-existant passwords

migrations/versions/3f5836a3df5c_.py

@@ -0,0 +1,37 @@
+"""empty message
+
+Revision ID: 3f5836a3df5c
+Revises: b3c7ff6655af
+Create Date: 2020-12-04 22:30:33.420071
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = '3f5836a3df5c'
+down_revision = 'b3c7ff6655af'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.alter_column('user', 'password',
+               existing_type=sa.VARCHAR(length=255),
+               nullable=True,
+               existing_server_default=sa.text("''::character varying"))
+
+    op.execute("""
+        UPDATE "user" SET password=NULL WHERE password=''
+    """)
+    op.create_check_constraint("CK_password", "user",
+            "password IS NULL OR password != ''")
+
+
+def downgrade():
+    op.drop_constraint("CK_password", "user", type_="check")
+    op.alter_column('user', 'password',
+               existing_type=sa.VARCHAR(length=255),
+               nullable=False,
+               existing_server_default=sa.text("''::character varying"))