Add ability to watch a thread

lib/firestorm_web/forums/forums.ex

@@ -6,7 +6,7 @@ defmodule FirestormWeb.Forums do
   import Ecto.{Query, Changeset}, warn: false
   alias FirestormWeb.{Repo, Notifications}
   alias Ecto.Multi
-  alias FirestormWeb.Forums.{User, Category, Thread, Post}
+  alias FirestormWeb.Forums.{User, Category, Thread, Post, Watch}
 
   @doc """
   Returns the list of users.
@@ -412,4 +412,59 @@ defmodule FirestormWeb.Forums do
     |> preload([p], [thread: [:category]])
     |> Repo.paginate(page: page)
   end
+
+  @doc """
+  Have a user watch a thread:
+
+      iex> %User{} |> watch(%Thread{})
+      {:ok, %Watch{}}
+
+  """
+  def watch(%User{} = user, %Thread{} = thread) do
+    thread
+    |> Ecto.build_assoc(:watches, %{user_id: user.id})
+    |> watch_changeset(%{})
+    |> Repo.insert()
+  end
+
+  @doc """
+  Determine if a user is watching a given watchable (Thread, etc):
+
+      iex> %Thread{} |> watched_by?(%User{})
+      false
+
+  """
+  def watched_by?(watchable, user = %User{}) do
+    watch_count(watchable, user) > 0
+  end
+
+  def watcher_ids(watchable) do
+    watchable
+    |> watches()
+    |> select([f], f.user_id)
+    |> Repo.all
+  end
+
+  def watch_count(watchable) do
+    watchable
+    |> watches()
+    |> Repo.aggregate(:count, :id)
+  end
+  defp watch_count(watchable, user = %User{}) do
+    watchable
+    |> watches()
+    |> where([f], f.user_id == ^user.id)
+    |> Repo.aggregate(:count, :id)
+  end
+
+  defp watches(watchable) do
+    watchable
+    |> Ecto.assoc(:watches)
+  end
+
+  defp watch_changeset(%Watch{} = watch, attrs) do
+    watch
+    |> cast(attrs, [:assoc_id, :user_id])
+    |> validate_required([:assoc_id, :user_id])
+  end
 end

lib/firestorm_web/forums/thread.ex

@@ -1,14 +1,16 @@
 defmodule FirestormWeb.Forums.Thread do
   use Ecto.Schema
 
-  alias FirestormWeb.Forums.{Category, Post}
+  alias FirestormWeb.Forums.{Category, Post, Watch, User}
   alias FirestormWeb.Forums.Slugs.ThreadTitleSlug
 
   schema "forums_threads" do
     field :title, :string
     field :slug, ThreadTitleSlug.Type
     belongs_to :category, Category
     has_many :posts, Post
+    has_many :watches, {"forums_threads_watches", Watch}, foreign_key: :assoc_id
+    many_to_many :watchers, User, join_through: "forums_threads_watches", join_keys: [assoc_id: :id, user_id: :id]
 
     timestamps()
   end

lib/firestorm_web/forums/watch.ex

@@ -0,0 +1,16 @@
+defmodule FirestormWeb.Forums.Watch do
+  @moduledoc """
+  A `Watch` is a polymorphic representation that a user is watching a thing in our system.
+  """
+
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  schema "abstract table: watches" do
+    # This will be used by associations on each "concrete" table
+    field :assoc_id, :integer
+    field :user_id, :integer
+
+    timestamps()
+  end
+end

priv/repo/migrations/20170619194021_add_forums_threads_watches.exs

@@ -0,0 +1,11 @@
+defmodule FirestormWeb.Repo.Migrations.AddForumsThreadsWatches do
+  use Ecto.Migration
+
+  def change do
+    create table(:forums_threads_watches) do
+      add :assoc_id, references(:forums_threads)
+      add :user_id, references(:forums_users)
+      timestamps()
+    end
+  end
+end

test/forums_test.exs

@@ -230,6 +230,15 @@ defmodule FirestormWeb.ForumsTest do
     end
   end
 
+  describe "watching a thread" do
+    setup [:create_user, :create_category, :create_thread]
+
+    test "watching a thread", %{thread: thread, user: user} do
+      {:ok, _watch} = user |> Forums.watch(thread)
+      assert thread |> Forums.watched_by?(user)
+    end
+  end
+
   def create_category(_) do
     category = fixture(:category, @create_category_attrs)
     {:ok, category: category}