add Axis, Category and Subject

spec/models/axis_spec.rb

@@ -0,0 +1,27 @@
+# == Schema Information
+#
+# Table name: axes
+#
+#  id         :bigint           not null, primary key
+#  name       :string
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+# Indexes
+#
+#  index_axes_on_name  (name) UNIQUE
+#
+require 'rails_helper'
+
+RSpec.describe(Axis, type: :model) do
+  describe "associations" do
+    it { is_expected.to(have_many(:subjects)) }
+  end
+
+  describe "validations" do
+    subject { create :axis }
+
+    it { is_expected.to(validate_presence_of(:name)) }
+    it { is_expected.to(validate_uniqueness_of(:name)) }
+  end
+end