Confusion around LiveView initial render/using a map in assigns

To get to the crux of this question right away: Does LiveView always do a first render without any assigns?

So I’m using a struct as one of my assigns. Imagine I’m the url being something like

defmodule MyAppWeb.HomeLive do
  use MyAppWeb, :live_view

  @default_assigns [
    record: nil, # The default here will never be used, hence `nil`.  I tried giving it a default and it doesn't solve my problem.
    another_key: "that has a dynamic value but needs a default"

  def mount(%{"record_id" => record_id}, _session, socket) do
    record = Records.find_record(record_id)
    {:ok, assign(socket, @default_assigns |> Keyword.replace!(:record, record))}

Then in my template:

<div><%= @record.some_key %></div>

No matter what, even though my page loads seemingly without a hitch, I always get an error that "nil.some_key is not a function".

I can, of course, fix this with:

<div<%= @record && @record.some_key %></div>

but I’m confused about the lifecycle here… does the template always render once without been passed any assigns? OR am I just doing something completely wrong?



The first param for assign should be a socket.

{:ok, assign(socket, @default_assigns |> Keyword.replace!(:record, record))}

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It’s been a few months, but IIRC the liveview lifecycle is as such:

  1. perform render and emit static html. This is for fast load and SEO purposes.
  2. spawn a proper live_view process, and let this live_view process redo assigns from scratch, and then sub in templated values over the live_socket.

However, with the code you have here, it seems like it should always get something out of the find_record.

I’m not sure about your assign function call. unless am I wrong, that you’re supposed to call assign(socket, key, value) and it will put it in the right place in the socket.assign field. I would drop IO.inspects after Records.find_record(...) and after the assigns(...) call to check up on what datastructures are being pushed around. If you use VScode this could help:

Sorry, that was a typo! I’m properly calling assign(socket, ...) in my actual code.

You also defined @defaul_assigns (missing t) and are using @default_assigns

No, the first render uses the data in assigns that is initialized in mount

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Thank you both for your responses. I realize it’s because I’m doing something pretty cavalier that isn’t well represented in my sample code. It’s because my route matches on any root path, so /:record_id. This of course means that requests for favicon.ico is matching :face_with_hand_over_mouth: