Phoenix & Ecto Many-to-Many m2m Relation

form.html.eex

<%= form_for @changeset, @action, [multipart: true], fn f -> %>
  
  <!-- ... other fields ... -->
    
  <!-- `selected` accepts only a list of IDs -->
  <div class="form-group">
    <%= label f, :categories, class: "control-label" %>
    <%= multiple_select f, :categories, @categories, selected: Enum.map(@changeset.data.categories, &(&1.id)), class: "form-control"  %>
  </div>

<% end %>

post.ex

defmodule Blog.Post do
  use Ecto.Schema
  import Ecto.Changeset

  @required_fields [
    :title
  ]
  @optional_fields [
    :content
  ]

  schema "posts" do
    field(:title, :string)
    field(:content, :text)

    many_to_many(
      :categories,
      Blog.Category,
      join_through: "post_categories",
      on_replace: :delete
    )
    
    timestamps(type: :utc_datetime)
  end

  def changeset(post, attrs) do
    post
    |> Blog.Repo.preload(:categories)
    |> cast(attrs, @required_fields ++ @optional_fields)
    |> validate_required(@required_fields)
    |> put_assoc(:categories, parse_categories(attrs))
  end

  defp parse_categories(attrs) do
    (attrs["categories"] || [])
    |> Enum.map(&Blog.get_category!/1)
  end
end