This is the result:

form.html.drab

<%= form_for @changeset, @action, fn f -> %>
<%= inspect(@changeset) %>
<p>---------------------</p>
<%= inspect(@action) %>
  <div class="row">
    <div class="col-md-12">
      <%= render "_invoices.html", conn: @conn, invoices: @invoices, parent: @parent %>
    </div>
  </div>
    ............

result printed out in the browser page

#Ecto.Changeset<action: nil, changes: %{}, errors: [date: {"can't be blank", [validation: :required]}, concept: {"can't be blank", [validation: :required]}, check_number: {"can't be blank", [validation: :required]}, bank_account_id: {"can't be blank", [validation: :required]}], data: #Epr.Payments.PaymentOrder<>, valid?: false>

---------------------

"/contracts/2/payment_orders"

This is the Google Chrome Console output for _drab

1. assigns:

  1. ge4tcmbtge2dkmzz:

    1. invoices: {document: "QTEyOEdDTQ.rkO2qTvBKpBncBjBOUNNhhYj7hrcS__s-bMZkYU…GoDtSb4T2CJn5yhopxEb6dPUAV.1JZQG4HEUiGSDOPx0Vmdug"}
    2. parent: {document: "QTEyOEdDTQ.Ar-AaS6DIYytU2srD6pveNYKX862GekZZYZ0PAl…g8kxTBr81V7k25jWLuZzcHPZHQ.bftVt5kyWJS067Z4ncibRA"}
    3. __proto__: Object

  2. giytanzzha4tgnjs:

    1. conn: {document: "QTEyOEdDTQ.GFC8HqSwajb9jUthihlg2dc5L5gJj7axH4U1O0H…00URxfOPRo2I9jyO5HmrEEKm8Q.KZim6LitfLrAs4VTTWBtLA"}
    2. parent: {document: "QTEyOEdDTQ.p-wIFXp5VELvZx5v1ub82Zrs3yOt24EWpbsy43s…8h1GOrusAo55428capZCxrT3xs.Yvk7mc9jwyNE0DfhHue51A"}
    3. type: {document: "QTEyOEdDTQ.ckW9WiB7ZPC-4dQtE52IBTBAyHmXC6Gxb40MFNL…5FjoWh.AoPxonuHyD4HuDwBsSc.Ni0NIn9tyrkNjlDlzKJ2qg"}
    4. __proto__: Object

  3. giytkojrgu2tcmbw:

    1. currency: {document: "QTEyOEdDTQ.hQz0cCd0ngRDTKs6mqO8qqBnyDJgOvrM792Xs3y…v3SlJSxT1p6aC.m9923cGIFixp.wQ55dmFJyFIhIiQI7LgGhQ"}
    2. invoice: {document: "QTEyOEdDTQ.NCrMDyUyPxAuW_b0gEcF1BxWubdSCXEIdMVmhxt…DZZriJfjT2QdNJbjMpZh3dpguj.IBxdtmoBRUt844rZx-2ttA"}
    3. __proto__: Object

  4. gqzdqojugi4tknbv:

    1. action: {document: "QTEyOEdDTQ.EYSw4Vj-VlIEDvRB6FO3lWAyzk_bmw_97LPI18i…Kjjap6YXne1M4gXbBZ92AbJqBh.ffIV6VcHyLsgCXHQsUZyVw"}
    2. changeset: {document: "QTEyOEdDTQ.Ff44_w87BhuG5viQ2m75GF9vFpkx4Nge4xFUfxw…6F4LgcLXehjmUh7u9yjk1oo6QK.Pak2S9KU8oJBipZk5mDaJg"}
    3. conn: {document: "QTEyOEdDTQ.i7LNqEzjsvqFXlyKZLmWE0Jj-6JHSiJPuQYHCdw…kHWWuSVMYIh7Yr79iuVfwPwayA.8c6o3Ke86NdW1ldEgWoKEw"}
    4. currency_us: {document: "QTEyOEdDTQ.Np39iu7OtdlSepYuQsNqHNgCEXaWukWJZ59_4Mu…3U50h-KYB2yqq.c9H97HU6nQYf.5WWwumklVLAT6GO6sPrJQA"}
    5. invoices: {document: "QTEyOEdDTQ.1MJXW0GvMPZ4w30_-9zAw15UMJTYyqJwi3s7c33…YHQbELwTuFssGlWn0gBrvcK_io.IGOBBM2E2PTNeDfWbAzKTQ"}
    6. parent: {document: "QTEyOEdDTQ.J5hkxodrx40FchLomo7Gmdtvbhzt5nIokzYhTT0…7UZuYxQERun0fAuX7pfaqn6EFw.d0FlWlDgsNLVJgTIXZs2fg"}
    7. po_grantotal: {document: "QTEyOEdDTQ.7pdcRGyALNNERy61CMSA6KUTnlagQOxbSymK-a0…0xlk.ZtVS4b1qlgTPZakD.V2Gt.msowySouBcC2oeeqL8ZbFg"}
    8. __proto__: Object

Thanks again for the help.

I have no idea what can be wrong then. All looks good, the data given to form_for looks exactly the same, I have no idea where the nil may come from

@grych,

So I finally identified the problem. Turns out that I use a Enum function inside the form.html.drab. This is the code:

<%= form_for @changeset, @action, fn f -> %>
  <div class="row">
    <div class="col-md-12">
      <%= render "_invoices.html", conn: @conn, invoices: @invoices, parent: @parent %>
    </div>
  </div>
.......
.......
      <div class="form-group">
        <%= label f, "Cuenta de banco", class: "control-label" %>
        <%= select f,
            :bank_account_id,
            Enum.map(assigns[:bank_accounts], &{&1.num, &1.id} ),
            class: "form-control",
            prompt: "Seleccione un número de cuenta"
         %>
        <%= error_tag f, :bank_account %>
      </div>

Please note that is use assigns[:bank_accounts] instead of @bank_accounts.

The first time this form.html.drab is rendered, this assign is there because I added it in the controller and the form renders correctly with the bank_accounts information:

  def new(conn, _params, %Epr.Debts.Contract{} = document) do
    invoices = Epr.Debts.list_unpaid_invoices(document)

      changeset =
        %PaymentOrder{}
        |> Payments.new_payment_order()

      render(
        conn,
        "new.html",
        changeset: changeset,
        parent: document,
        invoices: invoices,
        bank_accounts: Epr.Payments.list_all_bank_accounts(),
        currency_us: document.currency_us,
        po_grantotal: 0,
        type: "contract"
      )
  end

But when I check the drab assigns, it is not there:

iex(25)> socket |> assigns("form.html")
[:action, :changeset, :conn, :currency_us, :invoices, :parent, :po_grantotal]
iex(26)>

And when you poke the assign and re-render the form, the assign[:bank_accounts] is nil.

So simple solution was to change the assign[:bank_accounts] to @bank_accounts as I should and now it all works fine.

iex(29)> socket |> assigns("form.html")
[:po_grantotal, :conn, :invoices, :parent, :currency_us, :action,
 :bank_accounts, :changeset]

Thanks for your help.

Best regards,