Add SDG::Relation model

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_11_12_155047) do
+ActiveRecord::Schema.define(version: 2020_11_17_200945) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -1304,6 +1304,18 @@ ActiveRecord::Schema.define(version: 2020_11_12_155047) do
     t.index ["code"], name: "index_sdg_goals_on_code", unique: true
   end
 
+  create_table "sdg_relations", force: :cascade do |t|
+    t.string "related_sdg_type"
+    t.bigint "related_sdg_id"
+    t.string "relatable_type"
+    t.bigint "relatable_id"
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["relatable_type", "relatable_id"], name: "index_sdg_relations_on_relatable_type_and_relatable_id"
+    t.index ["related_sdg_id", "related_sdg_type", "relatable_id", "relatable_type"], name: "sdg_relations_unique", unique: true
+    t.index ["related_sdg_type", "related_sdg_id"], name: "index_sdg_relations_on_related_sdg_type_and_related_sdg_id"
+  end
+
   create_table "sdg_targets", force: :cascade do |t|
     t.bigint "goal_id"
     t.string "code", null: false