sequelize-cross-db-deeper-nesting.js

await UsersDb.models.user.findOne({
  where: { id: 1 },
  include: {
    model: UsersDb.models.order,
    include: {
      model: ProductsDb.models.product,
      on: {
        order_id: Sequelize.literal("`orders`.`id` = `orders->products`.`order_id`") 
        // you can always refer the generated query to get  
        // the exact column names for the literal string
      }
    }
  }
});

// output
{
  "id": 1,
  "name": "Jamie Lannister",
  "orders": [
    {
      "id": 1,
      "user_id": 1,
      "order_date": "2019-05-10T07:00:00.000Z",
      "products": [
        {
          "id": 1,
          "order_id": 1,
          "name": "oathkeeper"
        },
        {
          "id": 2,
          "order_id": 1,
          "name": "longclaw"
        },
        {
          "id": 3,
          "order_id": 1,
          "name": "needle"
        }
      ]
    },
    {
      "id": 2,
      "user_id": 1,
      "order_date": "2019-05-11T07:00:00.000Z",
      "products": []
    },
    {
      "id": 3,
      "user_id": 1,
      "order_date": "2019-05-12T07:00:00.000Z",
      "products": []
    },
    {
      "id": 4,
      "user_id": 1,
      "order_date": "2019-05-13T07:00:00.000Z",
      "products": []
    },
    {
      "id": 5,
      "user_id": 1,
      "order_date": "2019-06-01T07:00:00.000Z",
      "products": []
    },
    {
      "id": 6,
      "user_id": 1,
      "order_date": "2019-06-03T07:00:00.000Z",
      "products": []
    }
  ]
}