Manage many-to-many association

Question

Say, I have Post model which belongs to many Tags:

defmodule MyApp.Post do
  use MyApp.Web, :model

  schema "tours" do
    field :title, :string
    field :description, :string
    has_many :tags, {"tags_posts", MyApp.Tag}
  end

  # …
end

When saving a Post I get tags_ids list from multiselect field like that:

tags_ids[]=1&tags_ids[]=2

The question is how to link Tags to the Post on save in Phoenix?

Answer 1

Nested changesets are not supported yet in ecto: https://github.com/elixir-lang/ecto/issues/618 You must be save the tags by yourself.

In the following code snippets I will pick the tag_ids and insert them into the join table if the Post.changeset/2 give me a valid result. For hold the selected tags in Form I added an virtual field that we can read in form and setup a default. It's not the finest solution but it works for me.

PostController

def create(conn, %{"post" => post_params}) do
  post_changeset = Post.changeset(%Post{}, post_params)

  if post_changeset.valid? do
    post = Repo.insert!(post_changeset)

    case Dict.fetch(post_params, "tag_ids") do
      {:ok, tag_ids} ->

        for tag_id <- tag_ids do
          post_tag_changeset = PostTag.changeset(%PostTag{}, %{"tag_id" => tag_id, "post_id" => post.id})
          Repo.insert(post_tag_changeset)
        end
      :error ->
        # No tags selected
    end

    conn
    |> put_flash(:info, "Success!")
    |> redirect(to: post_path(conn, :new))
  else
    render(conn, "new.html", changeset: post_changeset)
  end
end

PostModel

schema "posts" do
  has_many :post_tags, Stackoverflow.PostTag
  field :title, :string
  field :tag_ids, {:array, :integer}, virtual: true

  timestamps
end

@required_fields ["title"]
@optional_fields ["tag_ids"]

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

PostTagModel (JoinTable for create many to many association)

schema "post_tags" do
  belongs_to :post, Stackoverflow.Post
  belongs_to :tag, Stackoverflow.Tag

  timestamps
end

@required_fields ["post_id", "tag_id"]
@optional_fields []

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

PostForm

<%= form_for @changeset, @action, fn f -> %>

  <%= if f.errors != [] do %>
    <div class="alert alert-danger">
      <p>Oops, something went wrong! Please check the errors below:</p>
      <ul>
      <%= for {attr, message} <- f.errors do %>
        <li><%= humanize(attr) %> <%= message %></li>
        <% end %>
      </ul>
    </div>
  <% end %>

  <div class="form-group">
    <%= label f, :title, "Title" %>
    <%= text_input f, :title, class: "form-control" %>
  </div>

  <div class="form-group">
    <%= label f, :tag_ids, "Tags" %>
    <!-- Tags in this case are static, load available tags from controller in your case -->
    <%= multiple_select f, :tag_ids, ["Tag 1": 1, "Tag 2": 2], value: (if @changeset.params, do: @changeset.params["tag_ids"], else: @changeset.model.tag_ids) %>
  </div>

  <div class="form-group">
    <%= submit "Save", class: "btn btn-primary" %>
  </div>

<% end %>

If you want to update tags, you have two options.

1. Delete all and insert new entries
2. Look for changes, and keep the existing entries

I hope it helps.

Answer 2

The first thing you want to do is to fix up the models. Ecto provides the has_many through: syntax for many-to-many relationships. Here are the docs.

A many-to-many relationship requires a join table since neither tags nor posts can have foreign keys pointing directly at one another (that would create a one-to-many relationship).

Ecto requires that you define the one-to-many join table relationship using has_many prior to the many-to-many relationship that uses has_many through:.

With your example, it would look like:

defmodule MyApp.Post do

  use MyApp.Web, :model

  schema "posts" do
    has_many :tag_posts, MyApp.TagPost
    has_many :tags, through: [:tag_posts, :tags]

    field :title, :string
    field :description, :string
  end

  # …
end

This assumes that you have a join table tag_posts that looks something like:

defmodule MyApp.TagPost do

  use MyApp.Web, :model

  schema "tag_posts" do
    belongs_to :tag, MyApp.Tag
    belongs_to :post, MyApp.Post

    # Any other fields to attach, like timestamps...
  end

  # …
end

Make sure if you want to be able to see all the posts associated with a given tag, that you define the relationship the other way in the Tag model:

defmodule MyApp.Tag do

  use MyApp.Web, :model

  schema "posts" do
    has_many :tag_posts, MyApp.TagPost
    has_many :posts, through: [:tag_posts, :posts]

    # other post fields
  end

  # …
end

Then, in your controller, you want to create new tag_posts with both the ID of the post you are saving, and the id of the tags from your list.