Add SDG::Review model

and its relation with relatables

Note about sdg_review factory: Cannot use the constantize method on
the relatable_type as long as the relatable classes will be loaded and
this will throw an exception because the database is not available at
factiry definition time.

spec/models/sdg/relatable_spec.rb

@@ -117,6 +117,18 @@ describe SDG::Relatable do
     end
   end
 
+  describe "#sdg_review" do
+    it "returns nil when relatable is not reviewed" do
+      expect(relatable.sdg_review).to be_blank
+    end
+
+    it "returns the review when relatable is reviewed" do
+      review = create(:sdg_review, relatable: relatable)
+
+      expect(relatable.sdg_review).to eq(review)
+    end
+  end
+
   describe ".by_goal" do
     it "returns everything if no code is provided" do
       expect(relatable.class.by_goal("")).to eq [relatable]

spec/models/sdg/review_spec.rb

@@ -0,0 +1,23 @@
+require "rails_helper"
+
+describe SDG::Review do
+  describe "Validations" do
+    it "is valid for any given relatable" do
+      review = build(:sdg_review, :debate_review)
+
+      expect(review).to be_valid
+    end
+
+    it "is not valid without a relatable" do
+      review = build(:sdg_review, relatable: nil)
+
+      expect(review).not_to be_valid
+    end
+
+    it "is not valid when a review for given relatable already exists" do
+      relatable = create(:sdg_review, :proposal_review).relatable
+
+      expect(build(:sdg_review, relatable: relatable)).not_to be_valid
+    end
+  end
+end