Draft: fix handling through table

[stefano-tilt]

Hi @toluaina,
can you take a look at this draft PR? I am trying to fix the IndexError: list index out of range error when trying to do an insert in a through table

There's already a PR raised from @mpaolino here, but it's not resolving for me. This is how my data model looks like

my_customer_groups:
     fk my_customer_id   (to id in my_customers)
     fk my_group_id         (to id in my_groups)

my_customers
     pk id
     name

my_groups 
     pk id
     group_name

Can you shed some light what would be the preferred approach?

Thanks

[Shankar-khati]

#413 should fix this issue