support reference id which is not `id` for many-to-many association

[sio4]

Hello, how about make many-to-many more flexible by support some more customizing tags?

pop/associations/many_to_many_association.go

Line 76 in 5fd1842

return fmt.Sprintf("id in (%s)", subQuery), []interface{}{modelIDValue}

I have some tables for example stores and table consumers, which is related each others as many to many. But in some reason, consumers has field card_id and mapping table for this relationship should be connected by table cards_stores or transactions, using card_id and store_id.
In this case, if I can override keys id to card_id and/or consumer_id to card_id, it is possible that I make relationship between this two models directly. (without make model card and two step relationships)

for example,

SELECT name FROM stores WHERE id IN (
    SELECT store_id FROM cards_stores WHERE card_id = "0000-0000-0000-0000"
)

If I can override modelColumnID like columnFieldID, or

SELECT name FROM consumers WHERE card_id IN (
    SELECT card_id FROM cards_stores WHERE store_id = "000-00-000000"
)

If I can override id in the code reference above.

How do you think about this idea? If you feel good, I will try to make a PR for it.

For the more, In my opinion, we can consider about custom raw query per the relationship. It will be helpful for more cases.

[sio4]

It is not question but enhencement :-)

[petar-dambovaliev]

It's related to this #55 .
It's WIP