In the User class, I have:

```
has_many :posts, foreign_key: "by_user_id"

```

This generates the following sql (when I call `user.posts`)

```
SELECT "posts".* FROM "posts" 
WHERE "posts"."by_user_id" = 2

```

How do I define an association that generates the following sql ?

```
SELECT "posts".* FROM "posts" 
WHERE "posts"."by_user_id" = 2 or "posts"."group_user_id" = 2

```

The Posts table contains both columns (`by_user_id` and `group_user_id`) and `group_user_id` is also an id of a User.