Add GroupScore, Group and StageSerializer

app/serializers/group_score_serializer.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class GroupScoreSerializer < ApplicationSerializer
+  attributes :group_points, :received_points, :scored_points
+
+  belongs_to :team
+end

app/serializers/group_serializer.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class GroupSerializer < ApplicationSerializer
+  attributes :number
+
+  has_many :matches
+  has_many :group_scores
+end

app/serializers/stage_serializer.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class StageSerializer < ApplicationSerializer
+  attributes :level
+
+  has_many :matches
+  has_many :groups
+end