Recently I’ve been trying my hand at building a small project with Phoenix. Most of my time learning Elixir was just spent on reading about GenServer lol. It’s nice to spend some quality time on personal projects for a change.

I’m still trying to shake the Rails bias, I suppose Rails does spoil us quite a bit. Modeling is a bit different with Phoenix. Data schemas, changesets, and repositories are still pretty exotic but I’ve hardly scraped the surface.

My first roadblock was actually validating a belongs_to association on the child record. Below is a summary of my findings:

An Ecto schema simply maps a data source into an Elixir struct

Setting up a belongs_to association in the child schema does not automatically validate its presence at the application level. However this information is used by assoc_constraint/3 to infer the foreign key constraint name.

assoc_constraint/3 and foreign_key_contraint/3 do not validate presence of the foreign key

These two constraints come into play once the actual database query is made. I only assumed it’d suffice because the Hexdocs explicitly state not to use validate_required/3 to validate associations.

My rationale is I don’t even want to attempt the database query if the foreign key column is nil. At the database level, the foreign key is non-nullable. I ended going against the documentation and included the foreign key in validate_required/3.

Using changesets is not mandatory

It was kinda shocking to be able to easily insert records into the database. I got wrecked while following along in the book I was using, since it used Kernel.struct/2 and chained that into a repository insert before changesets were introduced. Without using changesets, Phoenix will throw an error…

defmodule Disc.Dawg.Release do
  use Ecto.Schema
  import Ecto.Changeset

  schema "releases" do
    field(:title, :string)
    field(:cover_url, :string)
    field(:description, :string)
    field(:released_at, :date)
    timestamps()
    belongs_to :artist, Disc.Dawg.Artist
  end

  def changeset(release, params \\ %{}) do
    release
    |> cast(params, [:artist_id, :title, :cover_url, :description, :released_at])
    |> validate_required([:title, :artist_id])
    |> assoc_constraint(:artist)
  end
end

defmodule Disc.Dawg.Grid do
  alias Disc.Dawg.{Artist, Release, Repo}

  @repo Repo

  def insert_record(module, attrs) do
    model = module |> struct(attrs)
    apply(module, :changeset, [model, attrs])
    |> @repo.insert()
  end
end