EDIT: I’ve changed the last section of this article to reflect my findings in actually using this in production.
I’ve decided to do the backend for a new app I’m creating myself, using Elixir/Phoenix. I’ve been learning Elixir for the past few months and thought it would be a great opportunity to put what I’ve learned to good use.
There’s a lot of documentation about the new
many_to_many macro in Ecto 2 — but somehow, it still was a bit of a pain to get this right the first time. I couldn’t find a concrete example of how a self-referencing many-to-many relationship would work with Ecto, and after some digging and the help of the awesome Elixir community, I got it working. I’m writing this post just to put this out there so you can go through this quicker.
In the app, a
User can have contacts, which are themselves
Users. (Let’s omit the authentication stuff for now) The migration looks like this:
# priv/repo/migrations/create_users_table.ex defmodule MyApp.Repo.Migrations.CreateUsersTable do use Ecto.Migration def change do create table(:users) do add :username, :string end create unique_index(:users, [:username]) end end
Then, the module that defines the schema looks like this:
defmodule MyApp.User do use MyApp.Web, :model schema "users" do field :username, :string timestamps end # Omitting changesets end
In order to create a many-to-many relationship correctly we must have an associative table, that we’ll call “contacts”. This is the migration for that:
defmodule MyApp.Repo.Migrations.CreateContactsTable do use Ecto.Migration def change do create table(:contacts) do add :user_id, references(:users, on_delete: :nothing), primary_key: true add :contact_id, references(:users, on_delete: :nothing), primary_key: true add :status, :int timestamps() end end end
To expand a bit, the “Contacts” table has… :
- a column
user_id, which is the
idfor the user that initiated the “friend request”.
- a column
contact_id, which is the
idfor the user that must accept or reject the “friend request”
- a column
status, which represents the friendship status (0=pending, 1=accepted, -1=rejected).
Now for the interesting part, we have to create a “Contacts” schema that Ecto can work with in order for the association to work correctly. On to it:
defmodule MyApp.Contact do use MyApp.Web, :model alias MyApp.User schema "contacts" do belongs_to :user, User belongs_to :contact, User end end
We define the
Contact module, that belongs to both a
user and a
contact, both being
User types. We now have to update the
User model to reflect the many-to-many relation:
defmodule MyApp.User do use MyApp.Web, :model schema "users" do field :username, :string # Add the many-to-many association many_to_many :contacts, MyApp.User, join_through: "contacts", on_replace: :delete timestamps end # Omitting changesets end
many_to_many macro we only have to specify the field on the
User model, tell Ecto what table use as association table, and that’s it!
You can now do this in your code:
user = Repo.get(User, 1) |> Repo.preload(:contacts) contact = hd(user.contacts)
A member of the Elixir community told me on Slack that this looks more like a one-to-many relationship, and that I should try to represent it that way in my app. It would look like this:
defmodule MyApp.User do use MyApp.Web, :model alias MyApp.Contact schema "users" do field :username, :string # Add the many-to-many association has_many :_contacts, MyApp.Contact has_many :contacts, through: [:_contacts, :contact] timestamps end # Omitting changesets end
It works, however I don’t like the fact that now we basically have cluttered our model, having
contacts in it. Then there’s the fact that Ecto models are just backed by a simple struct, so there’s no way to “hide” details about it (as-in: make
_contacts private). I decided to stick with the
many_to_many version of this code because it hides the association table quite nicely.
After playing with this for a bit, I realised that this is the correct approach, although it clutters the
user instance space.
During actual testing in my Phoenix app, I noticed that the query that Ecto was generating with the former association type was the wrong one. Switching it from
has_many as above, fixed the issue.
Have something to add to this article, or did I miss something? Hit me up on Twitter.