Generate Related for many-to-many tables with extra columns

[limjiayi]

This merged pull request describes the current logic for determining many-to-many relations. PR #1075

I updated to logic to only require the intermediate table of a m-to-n relation to be with

• Exactly 2 foreign keys
• A primary key formed by exactly 2 columns
• Number of primary key equals to number of column

But it sometimes makes sense for these junction tables to contain more than 2 columns. For example, a CustomerProduct table that links the Customer and Product tables based on the products that each customer has bought, may have the following columns to record extra information about the relationship.

• customer_id (foreign key)
• product_id (foreign key)
• price
• quantity
• purchase_date
• promo_code

So, the requirement that the number of primary keys be equal to the number of columns should be removed.