Possible belongs_to improvement

[PatrikStenmark]

I just started to work with adding TypeIDs to my app and immediately ran into issues with belongs_to. I've looked at the other issues and I did get it to work by using define_field: false and a separate field :thing_id. It did however feel like it should work to just do

schema "posts" do
  has_many :comments, Comment
end

schema "comments" do
  belongs_to :post, Post, type: TypeID 
end

but when I tried that I got an error about type was only allowed to be :uuid or :string. Without using type: TypeID I got an error about value #TypeID<"aaa_01j95zhepsf75rfymj6egjqg9m"> in where cannot be cast to type :integer in query.

Digging further I realized that when using type param to belongs_to, the value for type is included in the call to TypeID.init. This means when using type: TypeID, dump will try to see what database type it should convert itself to, but the type is TypeID, which is the Ecto custom type, not the database type.

I have a solution to this which changes the name of the type parameter to column_type, so the primary key definitions become

@primary_key {:id, TypeID, autogenerate: true, prefix: "post", column_type: :uuid}

and the belongs_to definitions becomes

belongs_to :wallet, Wallet, type: TypeID, column_type: :uuid

With a default of :uuid, the column_type can also be dropped.

I'm guessing this is something around the differences of how Ecto handles a ParameterizedType when used as a normal field and an association. Would you be interested in a PR for this if I clean it up? Or am I missing something here that makes it a bad idea?