Rename SUBMIT_METHODS to SUBMISSION_METHODS.
Add convert_ids_to_models as a private method.
Update convert_model_to_id method for many-to-many association.
Change the association between sales_order model and payment model
from one-to-many to many-to-many
Remove the references column.
Create the join table.
Update the pages.
Add the association between sales_order and shipment.
Create the join table.
Remove shipment columns from sales_orders.
Update pages.
Create invoice scaffold and move it to admin.
Add the association between invoice model and sales_order model.
Create the join table.