phoenix framework - Elixir ecto 2 create many_to_many association

Question

How can I make a many to many relation with ecto 2? As an example app I want to create a Post which can be in multiple categories. The categories already exist. For example:

[%Category{id: "1", name: "elixir"}, %Category{id: "2", name: "erlang"}]

Im using Ecto 2 beta 0. The example project is called Ecto2.

I defined two models:

defmodule Ecto2.Post do
  use Ecto2.Web, :model
  use Ecto.Schema

  schema "posts" do
    field :title, :string
    many_to_many :categories, Ecto2.Category, join_through: "posts_categories", on_replace: :delete
    timestamps
  end

  @required_fields ~w(title)
  @optional_fields ~w()
  def changeset(model, params \ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
    |> cast_assoc(:categories)  # not suitable?
  end
end

defmodule Ecto2.Category do
  use Ecto2.Web, :model

  schema "categories" do
    field :name, :string

    timestamps
  end

  @required_fields ~w(name)
  @optional_fields ~w()
  def changeset(model, params \ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
  end
end

I tried doing it like this:

post = Repo.get!(Post, 1) |> Repo.preload(:categories)
changeset = Post.changeset(post, %{"title"=> "bla", "categories"=> [%{id: "1"}]})
Repo.update!(changeset)

But cast_assoc in Post.changeset is not suitable for this task, it wants to create a whole new Category instead of associate one. What should I use instead? build_assoc? But build_assoc docs do not mention it is useful with many_to_many. How do I use it? Should I put build_assoc in Post.changeset then, or should i use it in a phoenix controller.

Answer 1

You can join through a table by passing a string like "posts_categories", or through a schema by passing through a schema like MyApp.PostCategory. I prefer joining through schema as timestamps can be included. Let say you choose join through a schema instead of a table:

1. You need to create a separate table (e.g. :posts_categories) for the many_to_many relationships to join to.

```

def change do
  create table(:posts_categories) do
    add :post_id, references(:posts)

    add :category_id, references(:categories)
    timestamps
  end
end

2. Create a schema for the table you created in step 1. In your webmodels folder, create a file post_category.ex:

```

defmodule Ecto2.PostCategory do
use Ecto2.Web, :model

schema "posts_categories" do
  belongs_to :post, Ecto2.Post
  belongs_to :category, Ecto2.Category
  timestamps
end

def changeset(model, params \ %{}) do
  model
  |> cast(params, [])
end
end

Ecto beta 2 has changed :empty to empty map and change cast4 to cast 3. Check changelog.

3. Add this line to your post schema:

   many_to_many :categories, Ecto2.Category, join_through: Ecto2.PostCategory

4. Add this line to your category schema:

many_to_many :posts, Ecto2.Post, join_through: Ecto2.PostCategory

That's it! Now you can update like ```

post1 = Repo.get!(Post, 1)
category1 = Repo.get!(Category, 1)

post1
|> Repo.preload(:categories)
|> Post.changeset(%{})
|> put_assoc(:categories, [category1])
|> Repo.update!

```