Bugfix for :preload, and support ORDER BY query

[soichiro-nishizawa]

Hello.
This PR is bugfix for :preload, and support ORDER BY query.

About bugfix

The behavior of Ecto's preload is that when you try to preload a table that has_many associations like Shipper and Order, Ecto will add the order_by of the associated key in the query to retrieve the Order table.

Ecto expects the list to be sorted by this order_by, but if an unsorted list is returned, the association does not seem to work correctly.
Specifically, it seems to be associating the first association key found in the list up to the point where the key is contiguous.

example

Model.Shipper
|> preload([_x], :orders)
|> Repo.all()

unsorted result

[
  %Northwind.Model.Shipper{
    __meta__: #Ecto.Schema.Metadata<:loaded, "shippers">,
    company_name: "Speedy Express",
    orders: [
      %Northwind.Model.Order{ ... },
      %Northwind.Model.Order{ ... }
    ],
    phone: "(503) 555-9831",
    shipper_id: 1
  },
  %Northwind.Model.Shipper{ ... },
  %Northwind.Model.Shipper{ ... }
]

sorted result

[
  %Northwind.Model.Shipper{
    __meta__: #Ecto.Schema.Metadata<:loaded, "shippers">,
    company_name: "Speedy Express",
    orders: [
      %Northwind.Model.Order{ ... },
      %Northwind.Model.Order{ ... },
      %Northwind.Model.Order{ ... },
      %Northwind.Model.Order{ ... },
      %Northwind.Model.Order{ ... },
      ...
      ...
      ...
    ],
    phone: "(503) 555-9831",
    shipper_id: 1
  },
  %Northwind.Model.Shipper{ ... },
  %Northwind.Model.Shipper{ ... }
]

[evadne]

Thank you very much for this patch 🧠

[rupurt]

@evadne is there anything else required to get this patch merged? I've noticed that order by doesn't currently work on develop.

[evadne]

This is fixed in develop