Add basic testing for subscribe links

app/models/subscribe_link.rb

@@ -18,6 +18,7 @@ class SubscribeLink < ApplicationRecord
   scope :with_apple_id, -> { where(platform: APPLE_PLATFORMS) }
   scope :with_platform_id, -> { where(platform: UNIQUE_PLATFORMS) }
 
+  validates :platform, presence: true, inclusion: {in: PLATFORMS}
   validates :external_id, presence: true
 
   def uses_apple_id?

test/models/podcast_test.rb

@@ -105,4 +105,16 @@
       end
     end
   end
+
+  describe "subscribe links" do
+    it "can only have one of each link platform" do
+      podcast.subscribe_links.create(platform: "apple", podcast: podcast, external_id: "12345")
+      podcast.subscribe_links.create(platform: "spotify", podcast: podcast, external_id: "12345")
+      podcast.save!
+      assert podcast.valid?
+
+      podcast.subscribe_links.create(platform: "apple", podcast: podcast, external_id: "54321")
+      refute podcast.valid?
+    end
+  end
 end

test/models/subscribe_link_test.rb

@@ -0,0 +1,22 @@
+require "test_helper"
+
+describe SubscribeLink do
+  let(:podcast) { create(:podcast) }
+  let(:apple_link) { SubscribeLink.create(platform: "apple", podcast: podcast, external_id: "12345") }
+
+  describe "#valid?" do
+    it "requires a currently supported platform" do
+      assert apple_link.valid?
+      apple_link.platform = "wnyc"
+      refute apple_link.valid?
+      apple_link.platform = nil
+      refute apple_link.valid?
+    end
+
+    it "requires an external id" do
+      assert apple_link.valid?
+      apple_link.external_id = nil
+      refute apple_link.valid?
+    end
+  end
+end