Creating a unique constraint on two columns together in Ecto

Hello Fellas,

I am trying to enforce uniquness in a table using to combined fields I originally followed this instructions Stackoverflow thread which seems to work for everybody apart from my self and I do not seem to get why.

So my migration code looks like this :

def change do
    create table(:inventories) do
      add :operation_account_id, references(:operation_accounts, on_delete: :nothing)
      add :currency_id, references(:currencies, on_delete: :nothing)

      timestamps()
    end

    create index(:inventories, [:operation_account_id])
    create index(:inventories, [:currency_id])
    create unique_index(:inventories, [:currency_id, :operation_account_id], name: :operation_currency_index)
  end

and this is my changeset

  @doc false
  def changeset(inventory, attrs) do
    inventory
    |> cast(attrs, [:balance, :currency_id, :operation_account_id])
    |> validate_required([:balance, :currency_id, :operation_account_id])
    |> unique_constraint(:currency_per_account, name: :operation_currency_index)
  end 

The problem is that during testing when I try to insert two Inventory entries with same currency_id and operation_account_id it inserts them properly withour and exception or an error and I do not understand what I am missing.

Thanks a lot