How to get query string value on html.exx file?

Question

I am new on phoenix, elixir. I am trying to get params[:task_id] on text_field on new.html.exx , something like below rails code.

<%= f.text_field :task_id, value: params[:task_id] %>

I found below info on iex shell

[info] GET /tasks/1/comments/new
[debug] Processing by HelloWorld.CommentController.new/2
Parameters: %{"task_id" => "1"}

I tried with IEX.pry and got following result, but i am not able to apply it on text_input value.

pry(3)> conn.params["task_id"]
"1" 

Also tried below code but no luck.

<%= text_input f, :task_id, value: @conn.params["task_id"] %>
Got Error: assign @conn not available in eex template.

Any help would be appreciated. Thanks.

Answer 1

You can use the copy of params available in @conn (which is available in all templates rendered directly using Phoenix.Controller.render).

# new.html.eex
<%= @conn.params["task_id"] %>

If you want to use @conn in a template rendered using Phoenix.View.render inside the main template, you need to explicitly pass it to the new template:

# new.html.eex
<%= render "form.html", ..., conn: @conn %>

You can also just pass in params:

# new.html.eex
<%= render "form.html", ..., params: @conn.params %>

and use @params:

# form.html.eex
<%= @params["task_id"] %>

Answer 2

Based on comments, you appear to be using Ecto and Changesets.

Given some schema

schema "foo" do
  field :name, :string
  field :age, :integer
end

You can have something like this in your controller

def new(conn, _params) do
  changeset = Foo.changeset(%Foo{})
  render conn, "new.html", changeset: changeset
end

Which will allow you to have something like this in your html view file.

= form_for @changeset, foo_path(@conn, :create), [as: :foo], fn f ->
  = text_input f, :name
  = number_input f, :age
  = submit "Submit"
- end

Then back to your controller for the create method

def create(conn, %{"foo" => foo_params}) do
  foo = Foo.changeset(%Foo{}, foo_params)

  case Repo.insert(foo) do
    {:ok, foo} -> redirect conn, to: foo_path(:show, foo)
    {:error, changeset} -> render conn, "new.html", changeset: changeset
  end
end

Depending on your needs, the logic may be different, but you can take advantage of the changeset to fill the form in for you.