has_many - limit to just one for a given scope (e.g. 1 like per user for a given comment)

Question

I hope the description in the title is explanatory enough, but to elaborate a bit more, having models like this (written off the top of my head, please forgive if there are any slight syntax errors, etc.):

defmodule MyApp.Like do
  use Ecto.Schema
  import Ecto.Changeset

  schema "likes" do
    belongs_to :user, MyApp.User
    belongs_to :comment, MyApp.Comment

    timestamps()
  end
end

defmodule MyApp.Comment do
  use Ecto.Schema
  import Ecto.Changeset

  schema "comments" do
    belongs_to :user, MyApp.User
    has_many :likes, MyApp.Likes

    timestamps()
  end
end

defmodule MyApp.User do
  use Ecto.Schema
  import Ecto.Changeset

  schema "users" do
    has_many :comments, MyApp.Comment
    has_many :likes, MyApp.Like

    timestamps()
  end
end

Now: I have set everything up in a way that it's possible to create likes, no problem. But I don't quite have an idea of how to go about limiting a user, so that he/she could only have a maximum of one like per comment.

If this might help anyone in explaining it to me, I'm coming from Rails background and have only started exploring functional paradigms of Elixir/Phoenix.

Answer 1

Define a unique key (user_id, comment_id) in likes table on the database level.

BTW, in Rails and in any other programming language / framework existing in the world, this problem is being solved in the same way.