Hi, I’ve been scratching my head around this for a while and trying different combinations, but I’m just stuck whilst using build_assoc which does not recognise binary id UUIDs in my code. Here’s my problem and question.

I’m working on a banking app, so I have a user and wallet. user was created with Pow if that makes any difference. There is a has_many relationship between user and wallet. Here are the schemas:

use MyApp.Schema

schema "users" do
  field :role, :string, null: false, default: "user"
  has_many :wallets, MyApp.Accounts.Wallet
  pow_user_fields()
  timestamps()
end

and

use MyApp.Schema
...
schema "wallets" do
  field :description, :string
  field :balance, Money.Ecto.Composite.Type
  belongs_to :user_id, MyApp.Users.User, references: :uuid, type: :binary_id
  timestamps()
end

where MyApp.Schema is set up for UUIDs:

defmodule MyApp.Schema do
  defmacro __using__(_) do
    quote do
      use Ecto.Schema
      @primary_key {:uuid, :binary_id, autogenerate: true}
      @foreign_key_type :binary_id
      @derive {Phoenix.Param, key: :uuid}
      @timestamps_opts
    end
  end
end

Then in my CreateWallet migration:

defmodule MyApp.Repo.Migrations.CreateWallets do
  use Ecto.Migration

  def change do
    create table(:wallets) do
      add :description, :string, null: false
      add :balance, :money_with_currency, null: false
      add :user_id, references(:users, column: :uuid, type: :binary_id, on_delete: :nothing)
      timestamps()
    end

    create index(:wallets, [:user_id])
  end
end

All standard I think so far. However, if I want to implement a create_user_wallet function:

def create_user_wallet(%User{} = user, %{} = wallet_params) do
  wallet = Ecto.build_assoc(user, :wallets, wallet_params)
  Repo.insert(wallet)
end

and then try to run it, there is an error:

** (KeyError) key :user_uuid not found
    (ecto 3.7.1) lib/ecto/association.ex:754: Ecto.Association.Has.build/3

This is fine if it’s referring to the uuid key in the user map, as I don’t see user_uuid there, it’s just %User{uuid: "1234..."} - or is it referring to user_id in the wallet database? However, why should the build_assoc function expect that and not function as is given the Schema implementation and how can I override it, or set up my schemas and/or own Schema implementation and/or migration correctly?

Thanks for reading this far!