I am trying to build a sales territory table based off of a combination of states. I believe this constitutes a many_to_many relationship if I am trying to build associations between the territories and the states tables in my database. I am trying to work through the Ecto many_to_many associations section found here. However after several attempts at putting my own associations within the contexts of my territories and states tables I kept getting this error:

** (RuntimeError) attempting to cast or change association `tags` from `Gimli.Prospector.Assignment.AssociationTest.Post` that was not loaded. Please preload your associations before manipulating them through changesets (ecto 3.4.6) lib/ecto/changeset/relation.ex:80: Ecto.Changeset.Relation.load!/2 (ecto 3.4.6) lib/ecto/changeset.ex:1199: Ecto.Changeset.put_change/7 (ecto 3.4.6) lib/ecto/changeset.ex:1409: Ecto.Changeset.put_relation/5

As a way of troubleshooting I tried building a posts and a tags table and duplicating the code exactly as it appears in the many_to_many docs and it still gives the same error which leads me to think something might have been deprecated that I don’t know about. I am running elixir 1.11.3.

For quick reference here are the migrations for the posts and tags tables:

  use Ecto.Migration

  def change do
    create table(:posts) do
      add :header, :string
      add :body, :string
    end
  end
end```

```defmodule Gimli.Repo.Migrations.CreateTag do
  use Ecto.Migration

  def change do
    create table(:tags) do
      add :name, :string
    end
  end
end```

Here are the schemas for each respectively:

```defmodule Gimli.Prospector.Assignment.AssociationTest.Post do
    use Ecto.Schema

    schema "posts" do
        field :header, :string
        field :body, :string
        
        many_to_many :tags, Gimli.Prospector.Assignment.AssociationTest.Tag, join_through: "post_tags"
    end
end```

```defmodule Gimli.Prospector.Assignment.AssociationTest.Tag do
    use Ecto.Schema

    schema "tags" do
      field :name, :string
      # the following line was added
      many_to_many :posts, Gimli.Prospector.Assignment.AssociationTest.Post, join_through: "posts_tags"
    end
  end```

and here is the migration for the creation of the helper table per the documentation:

```defmodule Gimli.Repo.Migrations.CreatePostsTags do
  use Ecto.Migration

  def change do
    create table(:posts_tags) do
      add :tag_id, references(:tags)
      add :post_id, references(:posts)
    end

    create unique_index(:posts_tags, [:tag_id, :post_id])
  end
end```

and here is the code from the shell (condensed):

```alias Gimli.Prospector.Assignment.AssociationTest.Post
  alias Gimli.Prospector.Assignment.AssociationTest.Tag
  alias alias Gimli.Repo
  clickbait_tag = Repo.insert! %Tag{name: "clickbait"}
  misc_tag = Repo.insert! %Tag{name: "misc"}
  ecto_tag = Repo.insert! %Tag{name: "ecto"}
  post = %Post{header: "Clickbait header", body: "No real content"}
  post = Repo.insert!(post)
  post_changeset = Ecto.Changeset.change(post)
  post_with_tags = Ecto.Changeset.put_assoc(post_changeset, :tags, [clickbait_tag, misc_tag])```

This is the point that the `RuntimeError` happens. Again this is just copied line for line from the Ecto docs. Any help figuring this out would be very appreciated! Thanks!