From 28cd37966a12747114c0c9395b78c8a2972c4330 Mon Sep 17 00:00:00 2001
From: Shadowfacts <me@shadowfacts.net>
Date: Sun, 2 Aug 2020 16:12:38 -0400
Subject: [PATCH] Add backlinks

---
 lib/wiki/content.ex                           | 26 ++++++++++++++++
 lib/wiki/content/page.ex                      | 31 ++++++++++++++++++-
 lib/wiki/content/page_link.ex                 | 16 ++++++++++
 lib/wiki/content/renderer.ex                  | 16 ++++++----
 lib/wiki_web/controllers/page_controller.ex   |  2 +-
 lib/wiki_web/templates/page/show.html.eex     | 11 +++++++
 .../20200801233918_create_page_links.exs      | 17 ++++++++++
 7 files changed, 111 insertions(+), 8 deletions(-)
 create mode 100644 lib/wiki/content/page_link.ex
 create mode 100644 priv/repo/migrations/20200801233918_create_page_links.exs

diff --git a/lib/wiki/content.ex b/lib/wiki/content.ex
index 4c6ea94..c4189bc 100644
--- a/lib/wiki/content.ex
+++ b/lib/wiki/content.ex
@@ -56,6 +56,19 @@ defmodule Wiki.Content do
     %Page{}
     |> Page.changeset(attrs)
     |> Repo.insert()
+    |> case do
+      {:ok, page} ->
+        content = Map.get(attrs, "content") || Map.get(attrs, :content)
+
+        unless is_nil(content) do
+          Page.update_page_links(%Page{page | content: content})
+        end
+
+        {:ok, page}
+
+      {:error, reason} ->
+        {:error, reason}
+    end
   end
 
   @doc """
@@ -74,6 +87,19 @@ defmodule Wiki.Content do
     page
     |> Page.changeset(attrs)
     |> Repo.update()
+    |> case do
+      {:ok, page} ->
+        content = Map.get(attrs, "content") || Map.get(attrs, :content)
+
+        unless is_nil(content) do
+          Page.update_page_links(%Page{page | content: content})
+        end
+
+        {:ok, page}
+
+      {:error, reason} ->
+        {:error, reason}
+    end
   end
 
   @doc """
diff --git a/lib/wiki/content/page.ex b/lib/wiki/content/page.ex
index df45b2e..06c29bd 100644
--- a/lib/wiki/content/page.ex
+++ b/lib/wiki/content/page.ex
@@ -1,6 +1,9 @@
 defmodule Wiki.Content.Page do
   use Ecto.Schema
   import Ecto.Changeset
+  import Ecto.Query
+  alias Wiki.Repo
+  alias Wiki.Content.PageLink
 
   @type t() :: %__MODULE__{
           encrypted_content: binary(),
@@ -32,6 +35,14 @@ defmodule Wiki.Content.Page do
     belongs_to :user, Wiki.Accounts.User
     has_many :uploads, Wiki.Content.Upload
 
+    # Pages that this page links to
+    has_many :_pages_linked_to, PageLink, foreign_key: :from_id
+    has_many :pages_linked_to, through: [:_pages_linked_to, :to]
+
+    # Pages that link to this page
+    has_many :_pages_linked_from, PageLink, foreign_key: :to_id
+    has_many :pages_linked_from, through: [:_pages_linked_from, :from]
+
     timestamps()
   end
 
@@ -65,7 +76,7 @@ defmodule Wiki.Content.Page do
     {encrypted_content, tag, iv} = do_encrypt(content, key)
 
     user_id = get_field(changeset, :user_id)
-    {html, _linked_pages} = Wiki.Content.Renderer.render(content, user_id)
+    html = Wiki.Content.Renderer.render(content, user_id)
     {encrypted_html, html_tag, html_iv} = do_encrypt(html, key)
 
     changeset
@@ -122,4 +133,22 @@ defmodule Wiki.Content.Page do
       false
     )
   end
+
+  @spec update_page_links(page :: __MODULE__.t()) :: :ok
+  def update_page_links(%__MODULE__{content: content, id: id} = page) when is_binary(content) do
+    linked_page_ids = Wiki.Content.Renderer.get_linked_pages(page)
+
+    if length(linked_page_ids) == 0 do
+      Repo.delete_all(from link in PageLink, where: link.from_id == ^id)
+    else
+      Repo.delete_all(
+        from link in PageLink, where: link.from_id == ^id and not (link.to_id in ^linked_page_ids)
+      )
+
+      values = Enum.map(linked_page_ids, fn to_id -> [from_id: id, to_id: to_id] end)
+      Repo.insert_all(PageLink, values, on_conflict: :nothing)
+    end
+
+    :ok
+  end
 end
diff --git a/lib/wiki/content/page_link.ex b/lib/wiki/content/page_link.ex
new file mode 100644
index 0000000..ab29c2c
--- /dev/null
+++ b/lib/wiki/content/page_link.ex
@@ -0,0 +1,16 @@
+defmodule Wiki.Content.PageLink do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  schema "page_links" do
+    belongs_to :from, Wiki.Content.Page
+    belongs_to :to, Wiki.Content.Page
+  end
+
+  @doc false
+  def changeset(page_link, attrs) do
+    page_link
+    |> cast(attrs, [:from_id, :to_id])
+    |> validate_required([:from_id, :to_id])
+  end
+end
diff --git a/lib/wiki/content/renderer.ex b/lib/wiki/content/renderer.ex
index 5c4a689..0726fdc 100644
--- a/lib/wiki/content/renderer.ex
+++ b/lib/wiki/content/renderer.ex
@@ -5,14 +5,18 @@ defmodule Wiki.Content.Renderer do
   alias WikiWeb.Endpoint
   import Ecto.Query
 
-  @spec render(String.t(), integer()) :: {String.t(), [integer()]}
+  @spec render(String.t(), integer()) :: String.t()
   def render(content, user_id) do
-    {content, linked_pages} = replace_page_links(content, user_id)
+    {content, _linked_pages} = replace_page_links(content, user_id)
 
-    {
-      render_markdown(content),
-      Enum.uniq(linked_pages)
-    }
+    content
+    |> render_markdown()
+  end
+
+  @spec get_linked_pages(page :: Page.t()) :: [integer()]
+  def get_linked_pages(%Page{content: content, user_id: user_id}) do
+    {_content, linked_pages} = replace_page_links(content, user_id)
+    Enum.uniq(linked_pages)
   end
 
   @page_link_regex ~r/\[\[.*?\]\]/
diff --git a/lib/wiki_web/controllers/page_controller.ex b/lib/wiki_web/controllers/page_controller.ex
index 1c020bf..a8d7a40 100644
--- a/lib/wiki_web/controllers/page_controller.ex
+++ b/lib/wiki_web/controllers/page_controller.ex
@@ -85,7 +85,7 @@ defmodule WikiWeb.PageController do
   end
 
   def show(conn, _params) do
-    page = Repo.preload(conn.assigns.page, :uploads)
+    page = Repo.preload(conn.assigns.page, [:uploads, :pages_linked_from])
     render(conn, "show.html", page: page)
   end
 
diff --git a/lib/wiki_web/templates/page/show.html.eex b/lib/wiki_web/templates/page/show.html.eex
index 9b381ad..b1f057e 100644
--- a/lib/wiki_web/templates/page/show.html.eex
+++ b/lib/wiki_web/templates/page/show.html.eex
@@ -20,3 +20,14 @@
     <% end %>
   </div>
 <% end %>
+
+<h2>Backlinks</h2>
+<ul>
+  <%= for page <- @page.pages_linked_from do %>
+    <li>
+      <a href="<%= Routes.page_path(@conn, :show, page.id) %>">
+        <%= page.title %>
+      </a>
+    </li>
+  <% end %>
+</ul>
diff --git a/priv/repo/migrations/20200801233918_create_page_links.exs b/priv/repo/migrations/20200801233918_create_page_links.exs
new file mode 100644
index 0000000..f8257c5
--- /dev/null
+++ b/priv/repo/migrations/20200801233918_create_page_links.exs
@@ -0,0 +1,17 @@
+defmodule Wiki.Repo.Migrations.CreatePageLinks do
+  use Ecto.Migration
+
+  def change do
+    create table(:page_links) do
+      add :from_id, references(:pages, on_delete: :nothing), primary_key: true
+      add :to_id, references(:pages, on_delete: :nothing), primary_key: true
+    end
+
+    create index(:page_links, [:from_id])
+    create index(:page_links, [:to_id])
+
+    create unique_index(:page_links, [:from_id, :to_id],
+             name: :page_link_from_id_to_id_unique_index
+           )
+  end
+end