add Axis, Category and Subject

app/models/category.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
+#
+class Category < ApplicationRecord
+  has_many :subjects
+
+  validates :name, presence: true, uniqueness: true
+end