Add collection serializer for bets

app/serializers/bets_serializer.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class BetsSerializer < ActiveModel::Serializer::CollectionSerializer
+  def serializable_hash(_adapter_options, _options, _adapter_instance)
+    @object.group_by(&:team).map do |team, bets|
+      {
+        team: ActiveModelSerializers::SerializableResource.new(team).as_json,
+        bets: bets.size
+      }
+    end
+  end
+end