Improvements for Ecto

[andrewtimberlake]

I was trying to migrate from bigint ids to TypeID
I came across a few problems because I need to have TypeID working during interim steps.

1. You can’t use TypeID as a standard, non primary-key field (this would fail to pick up the prefix)

schema "table" do
  field :uuid, TypeID, prefix: "user", type: :binary_id
end

2. You can’t set the foreign key type aside from using @foreign_key_type TypeID

belongs_to :model, Model, type: TypeID

This pull request addresses both usages
I didn’t see any Ecto tests, so I haven’t provided tests.

[sloanelybutsurely]

hi! thanks for taking the time to contribute. i'll need a moment to review this more thoroughly but i did have some immediate feedback for you:

2. You can’t set the foreign key type aside from using @foreign_key_type TypeID

you can do this using define_field: false on the belongs_to and defining the foreign key field manually. See #20 (comment) for a more complete example.