add Axis, Category and Subject

spec/factories/categories.rb

@@ -0,0 +1,18 @@
+# == Schema Information
+#
+# Table name: categories
+#
+#  id         :bigint           not null, primary key
+#  name       :string
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+# Indexes
+#
+#  index_categories_on_name  (name) UNIQUE
+#
+FactoryBot.define do
+  factory :category do
+    name { Faker::Superhero.name }
+  end
+end