Added tools to track which fields_to_update after mapping.

[specialunderwear]

It would be nice if we could find out for mapper what the exact data it is that they are mapping to.

The util get_mapped_fields can be used to check which field the mapper will actually update on the to_obj

products_to_db accepts the fields_to_update parameter to do partial updates.

We can now use a combination of get_mapped_fields and ProductToModel.get_fields_impacted_by_mapping to check which fields are actually updated on the product model through a chain of mappings.

>>>     mapper = SomeMapperMappingToProductResource
>>>     mapped_fields = get_mapped_fields(mapper)
>>>     mapped_fields
{'upc', 'vat_rate', 'product_class', 'structure', 'price', 'title', 'partner'}
>>>     ProductToModel.get_fields_impacted_by_mapping(*mapped_fields)
['Product.upc', 'StockRecord.partner_sku', 'StockRecord.partner', 'ProductClass.slug', 'StockRecord.price', 'StockRecord.price_currency', 'Product.structure', 'Product.title']

The output can be directly passed to products_to_db as the fields_to_update parameter

[samar-hassan]

Do we have priority field for product model?

[specialunderwear]

yup

[samar-hassan]

Do we need to remove parent field from this list?

[specialunderwear]

It is in this list twice

[samar-hassan]

Maybe define this list in a variable

[specialunderwear]

Well this is a test so ...