New to Phoenix, need some help please! (form_for @conn VS form_for @changeset)

AkberKhan · September 16, 2019, 4:03pm

Hi All!

I’m new to Phoenix and Elixir, and both of these tools are great. Previously I created websites using only PHP, and played a little bit with Ruby on Rails.
Currently I am reading the Mastering Phoenix book (free) at the writer’s website, which someone shared here in the forum.
What I’m not getting is the following;
In some cases like in registeration/new.html.eex he uses something like,
<%= form_for @changeset, registration_path(@conn, :create), [as: :registration, id: "registration-form"], fn f -> %>

and in other cases he uses,
<%= form_for @conn, session_path(@conn, :create), [as: :session, id: "session-form"], fn f -> %>

Why in one place he uses <%= form_for @changeset and in another place he uses <%= form_for @conn ?

Note, I know the idea of changeset and conn upto some extent.

Thank you in advance!, and I’m sorry if this question sounds stupid!

idi527 · September 16, 2019, 4:13pm

https://hexdocs.pm/phoenix_html/Phoenix.HTML.Form.html might be relevant.

The author probably uses changesets when there exists some underlying entity for the action, like a new user for the registration form. And conn where there isn’t one, or it is relatively ephemeral, like a session for the login action.

peerreynders · September 16, 2019, 4:21pm

Welcome to the forum!

The why is entirely dependent on the specifics of the application.

The mechanism by which this is possible is easier to explain:

Phoenix.HTML.form_for/4 relies on the “Phoenix.HTML.FormData” protocol.

For changesets Phoenix implements the protocol via phoenix_ecto:

github.com

phoenixframework/phoenix_ecto/blob/9e7aba5ff001138c20ae600d3ce29226797d75d3/lib/phoenix_ecto/html.ex#L2-L19


      
          defimpl Phoenix.HTML.FormData, for: Ecto.Changeset do
            def to_form(changeset, opts) do
              %{params: params, data: data} = changeset
              {name, opts} = Keyword.pop(opts, :as)
              name = to_string(name || form_for_name(data))
          
              %Phoenix.HTML.Form{
                source: changeset,
                impl: __MODULE__,
                id: name,
                name: name,
                errors: form_for_errors(changeset),
                data: data,
                params: params || %{},
                hidden: form_for_hidden(data),
                options: Keyword.put_new(opts, :method, form_for_method(data))
              }
            end

For Plug.Conn it is implemented right in phoenix_html:

github.com

phoenixframework/phoenix_html/blob/main/lib/phoenix_html/form_data.ex#L51-L162


      
            * `:prepend` - the values to prepend when rendering. This only
              applies if the field value is a list and no parameters were
              sent through the form.
          
            * `:append` - the values to append when rendering. This only
              applies if the field value is a list and no parameters were
              sent through the form.
          
          """
          @spec to_form(t, Phoenix.HTML.Form.t(), Phoenix.HTML.Form.field(), Keyword.t()) ::
                  [Phoenix.HTML.Form.t()]
          def to_form(data, form, field, options)
          
          @doc """
          Returns the value for the given field.
          """
          @spec input_value(t, Phoenix.HTML.Form.t(), Phoenix.HTML.Form.field()) :: term
          def input_value(data, form, field)
          
          @doc """

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DevotionGeo · September 16, 2019, 4:42pm

Welcome to the Elixir community!

On the page where the login form’s template is displayed, he mentions as,