Add subject column to semantic links

db/migrate/20241030023153_add_subject_to_semantic_links.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+# rails g migration AddSubjectToSemanticLinks subject:references{polymorphic}
+#
+# We want to add links to Exchanges as well as Variants.
+# The word subject comes from the triple structure of the Semantic Web:
+#
+#   Subject predicate object (variant has linke URL)
+class AddSubjectToSemanticLinks < ActiveRecord::Migration[7.0]
+  def change
+    # We allow `null` until we filled the new columns with existing data.
+    add_reference :semantic_links, :subject, polymorphic: true, null: true
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2024_10_23_054951) do
+ActiveRecord::Schema[7.0].define(version: 2024_10_30_023153) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_stat_statements"
   enable_extension "plpgsql"
@@ -405,6 +405,9 @@ ActiveRecord::Schema[7.0].define(version: 2024_10_23_054951) do
     t.string "semantic_id", null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.string "subject_type"
+    t.bigint "subject_id"
+    t.index ["subject_type", "subject_id"], name: "index_semantic_links_on_subject"
     t.index ["variant_id"], name: "index_semantic_links_on_variant_id"
   end