Simplify included release groups logic

This makes the code slightly less efficient as it constructs
intermediate structures BEFORE filtering, but it makes it for simpler
code to maintain down the line.

Performance is not visible affected as the largest lists involved count
around 200 items.

lib/music_library/records/musicbrainz_data.ex

@@ -1,10 +1,7 @@
 defmodule MusicLibrary.Records.MusicbrainzData do
-  def included_release_groups(musicbrainz_data, included_release_group_ids) do
+  def included_release_groups(musicbrainz_data) do
     musicbrainz_data
-    |> Map.get("relations", [])
-    |> Enum.filter(fn relation ->
-      relation["release_group"]["id"] in included_release_group_ids
-    end)
+    |> get_release_groups()
     |> Enum.map(fn relation ->
       MusicBrainz.ReleaseGroup.from_api_response(relation["release_group"])
     end)
@@ -17,6 +14,12 @@ defmodule MusicLibrary.Records.MusicbrainzData do
   end
 
   def included_release_group_ids(musicbrainz_data) do
+    musicbrainz_data
+    |> included_release_groups()
+    |> Enum.map(fn rg -> rg.id end)
+  end
+
+  defp get_release_groups(musicbrainz_data) do
     musicbrainz_data
     |> Map.get("relations", [])
     |> Enum.filter(fn relation ->
@@ -24,6 +27,5 @@ defmodule MusicLibrary.Records.MusicbrainzData do
         relation["type"] == "included in" and
         relation["direction"] == "backward"
     end)
-    |> Enum.map(fn relation -> relation["release_group"]["id"] end)
   end
 end

lib/music_library/records/record.ex

@@ -33,10 +33,9 @@ defmodule MusicLibrary.Records.Record do
   def types, do: @types
 
   def included_release_groups(record) do
-    MusicbrainzData.included_release_groups(
-      record.musicbrainz_data,
-      record.included_release_group_ids
-    )
+    record.musicbrainz_data
+    |> MusicbrainzData.included_release_groups()
+    |> Enum.filter(fn rg -> rg.id in record.included_release_group_ids end)
   end
 
   def included_release_groups_count(record) do