Many to many third table foreign keys are not populated

Phoenix Forum Questions / Help
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Hello everyone,

I’m asking for help or guide to properly insert or populate foreign keys in the third table in a many_to_many association.

I have Posts Entity and Tags Entity which are join through third table (Post_Tags)

//post_tags_migration 
def change do
    create table(:post_tags) do
      add :post_id, references(:posts, on_delete: :delete_all)
      add :tag_id, references(:tags, on_delete: :delete_all)

      timestamps()
    end

    create index(:post_tags, [:post_id])
    create index(:post_tags, [:tag_id])
    create unique_index(:post_tags, [:post_id, :tag_id])
  end

Post Schema

schema "posts" do
    field :content, :string
    field :title, :string

    many_to_many :tags, Tags, join_through: PostTags
    timestamps()
  end

  @doc false
  def changeset(posts, attrs \\ %{}) do
    posts
    |> cast(attrs, [:title, :content])
    |> validate_required([:title, :content])
    |> put_assoc(:tags, parse_tags(attrs))
  end

  defp parse_tags(attrs) do
   (attrs["tags"] || attrs[:tags] || "")
   |> String.split(",")
   |> Enum.map(&String.trim/1)
   |> Enum.reject(& &1 == "")
   |> insert_and_get_all()
 end

 defp insert_and_get_all([]), do: []

 defp insert_and_get_all(names) do
   now = NaiveDateTime.utc_now |> NaiveDateTime.truncate(:second)
   maps = Enum.map(names, &%{name: &1, inserted_at: now, updated_at: now})
   Repo.insert_all Tags, maps, on_conflict: :nothing
   Repo.all from t in Tags, where: t.name in ^names
 end

Post Tags Schema

  schema "post_tags" do
    belongs_to :post, Posts, primary_key: true
    belongs_to :tag, Tags, primary_key: true
    timestamps()
  end

  @required_fields ~w(post_id tag_id)
  @doc false
  def changeset(post_tags, attrs) do
    post_tags
    |> cast(attrs, @required_fields)
    |> validate_required(@required_fields)
  end

My issue is Post_Tags table only populate the timestamps fields:

[debug] QUERY OK db=0.8ms
INSERT INTO "post_tags" ("inserted_at","updated_at") VALUES ($1,$2) RETURNING "id" [~N[2018-11-28 15:12:34], ~N[2018-11-28 15:12:34]]
[debug] QUERY OK db=0.2ms
INSERT INTO "post_tags" ("inserted_at","updated_at") VALUES ($1,$2) RETURNING "id" [~N[2018-11-28 15:12:34], ~N[2018-11-28 15:12:34]]
[debug] QUERY OK db=0.2ms
INSERT INTO "post_tags" ("inserted_at","updated_at") VALUES ($1,$2) RETURNING "id" [~N[2018-11-28 15:12:34], ~N[2018-11-28 15:12:34]]

Given I have this param:

%{ 
    "content" => "This is a sample text to demonstrate multiple insertions of tags",
    "tags" => "Erlang,Elixir,Phoenix",
    "title" => "Dapps"
 }

When I insert this post and tag are created based on the params given but not the third table.
I’m not best in explaining in detail as much as possible, feel free to ask if the information is not sufficient.

Thanks.