There's a newer version of this post here: https://odootricks.tips/about/automated-action-to-create-a-bom/
This is a great example of the power of Automated Actions to do something useful.
Credit to Jake Robinson for this one, which automatically creates a BOM for new items:
if record.categ_id.id == 6:
env['mrp.bom'].browse(1).copy({'product_tmpl_id': record.id})
Notes:
The "apply on" means that it will not create a BOM for service items.
Selection by product category is done in the Python code. You need to know the record IDs of the product category and the BOM you will be copying (in this case the product category id is 6 and the BOM we are copying is record 1).
You could easily have different BOM templates for different product categories
if record.categ_id.id == 7:
env['mrp.bom'].browse(2).copy({'product_tmpl_id': record.id})