add Axis, Category and Subject

app/models/subject.rb

@@ -0,0 +1,28 @@
+# == Schema Information
+#
+# Table name: subjects
+#
+#  id          :bigint           not null, primary key
+#  name        :string
+#  created_at  :datetime         not null
+#  updated_at  :datetime         not null
+#  axis_id     :bigint           not null
+#  category_id :bigint           not null
+#
+# Indexes
+#
+#  index_subjects_on_axis_id      (axis_id)
+#  index_subjects_on_category_id  (category_id)
+#  index_subjects_on_name         (name) UNIQUE
+#
+# Foreign Keys
+#
+#  fk_rails_...  (axis_id => axes.id)
+#  fk_rails_...  (category_id => categories.id)
+#
+class Subject < ApplicationRecord
+  belongs_to :category
+  belongs_to :axis
+
+  validates :name, presence: true, uniqueness: true
+end