You need to declare the BoM before you create the Manufacturing Order. Have you tried this?

Edit:

The products must be declared before as well.

What we do is: 

a) Create parent product (the one containing the BOM) [product.template]. 

b) Create child products (the ones inside the BOM) [product.template]. 

c) Create the BOM [mrp.bom]. 

d) Create the BOM lines [mrp.bom.line].

Hope that clarifies something.

And about the on change part. You should call it like this:

@api.onchange('partner_id')

def your_func():

'partner_id' is just an example, you need to find your object. I think you need 'product_tmpl_id' from mrp.bom.