add Axis, Category and Subject

db/migrate/20220721123327_create_subjects.rb

@@ -0,0 +1,13 @@
+class CreateSubjects < ActiveRecord::Migration[7.0]
+  def change
+    create_table :subjects do |t|
+      t.string :name
+      t.references :category, null: false, foreign_key: true
+      t.references :axis, null: false, foreign_key: true
+
+      t.timestamps
+    end
+
+    add_index(:subjects, :name, unique: true)
+  end
+end