Fix todo priority migration script to add columns before updating values

migrations/versions/004_add_todo_priority_and_due_date.py

@@ -18,16 +18,18 @@ depends_on = None
 
 def upgrade():
     # Create priority_level enum type
+    # Step 1: Add the priority column
     with op.batch_alter_table('todos', schema=None) as batch_op:
-        # Add priority column
         batch_op.add_column(sa.Column('priority', sa.Enum('low', 'medium', 'high', name='prioritylevel'), nullable=True))
-        # Set default values for existing records
-        batch_op.execute("UPDATE todos SET priority = 'medium' WHERE priority IS NULL")
-        # Make priority not nullable with default
-        batch_op.alter_column('priority', nullable=False, server_default='medium')
-        
         # Add due_date column
         batch_op.add_column(sa.Column('due_date', sa.DateTime(), nullable=True))
+    
+    # Step 2: Update existing records with default priority value
+    op.execute("UPDATE todos SET priority = 'medium' WHERE priority IS NULL")
+    
+    # Step 3: Add constraints to make priority non-nullable with default
+    with op.batch_alter_table('todos', schema=None) as batch_op:
+        batch_op.alter_column('priority', nullable=False, server_default='medium')
 
 
 def downgrade():