Let's consider this table:

```
    +----------+----------+
    | user_id  | store_id |
    +----------+----------+
    |    39    |    8     |
    |    8     |    39    |
    |    5     |    11    |
    +----------+----------+

```

I mapped it with slick with (dropping indexes & foreign keys for simplifying it):

```
case class FavoriteStore(userId: Long, storeId: Long)

class FavoriteStoreTable(tag: Tag) 
     extends Table[FavoriteStore](tag, "FAVORITE_STORE") {
  def userId = column[Long]("USER_ID")
  def storeId = column[Long]("STORE_ID")
  override def * = (userId, storeId) <> 
              (FavoriteStore.tupled, FavoriteStore.unapply)
}

```

I would like to be able to write the following SQL query with Slick:

```
INSERT INTO favorite_store (user_id, store_id) VALUES (39, 8)
WHERE NOT EXISTS 
    (SELECT * FROM favorite_store WHERE user_id = 39 AND store_id = 8)

```

Any idea?