Add SDG::Relation model

app/models/concerns/sdg/relatable.rb

@@ -0,0 +1,7 @@
+module SDG::Relatable
+  extend ActiveSupport::Concern
+
+  included do
+    has_many :sdg_relations, as: :relatable, dependent: :destroy, class_name: "SDG::Relation"
+  end
+end

app/models/concerns/sdg/related.rb

@@ -0,0 +1,7 @@
+module SDG::Related
+  extend ActiveSupport::Concern
+
+  included do
+    has_many :relations, as: :related_sdg, dependent: :destroy
+  end
+end

app/models/sdg/goal.rb

@@ -1,4 +1,6 @@
 class SDG::Goal < ApplicationRecord
+  include SDG::Related
+
   validates :code, presence: true, uniqueness: true, inclusion: { in: 1..17 }
 
   has_many :targets, dependent: :destroy

app/models/sdg/relation.rb

@@ -0,0 +1,6 @@
+class SDG::Relation < ApplicationRecord
+  validates :related_sdg_id, uniqueness: { scope: [:related_sdg_type, :relatable_id, :relatable_type] }
+
+  belongs_to :relatable, polymorphic: true, optional: false
+  belongs_to :related_sdg, polymorphic: true, optional: false
+end

app/models/sdg/target.rb

@@ -1,5 +1,6 @@
 class SDG::Target < ApplicationRecord
   include Comparable
+  include SDG::Related
 
   validates :code, presence: true, uniqueness: true
   validates :goal, presence: true

db/migrate/20201117200945_create_sdg_relations.rb

@@ -0,0 +1,12 @@
+class CreateSDGRelations < ActiveRecord::Migration[5.2]
+  def change
+    create_table :sdg_relations do |t|
+      t.references :related_sdg, polymorphic: true
+      t.references :relatable, polymorphic: true
+
+      t.index [:related_sdg_id, :related_sdg_type, :relatable_id, :relatable_type], name: "sdg_relations_unique", unique: true
+
+      t.timestamps
+    end
+  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.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

spec/models/sdg/relation_spec.rb

@@ -0,0 +1,33 @@
+require "rails_helper"
+
+describe SDG::Relation do
+  describe "Validations" do
+    it "is valid with a related SDG and a relatable model" do
+      relation = SDG::Relation.new(related_sdg: SDG::Goal[1], relatable: create(:proposal))
+
+      expect(relation).to be_valid
+    end
+
+    it "is not valid without a related SDG" do
+      relation = SDG::Relation.new(relatable: create(:proposal))
+
+      expect(relation).not_to be_valid
+    end
+
+    it "is not valid without a relatable model" do
+      relation = SDG::Relation.new(related_sdg: SDG::Goal[1])
+
+      expect(relation).not_to be_valid
+    end
+
+    it "is not valid when a relation already exists" do
+      proposal = create(:proposal)
+      goal = SDG::Goal[1]
+
+      SDG::Relation.create!(related_sdg: goal, relatable: proposal)
+      relation = SDG::Relation.new(related_sdg: goal, relatable: proposal)
+
+      expect(relation).not_to be_valid
+    end
+  end
+end