Add external_id to User Serializer

spec/serializers/user_serializer_spec.rb

@@ -196,4 +196,28 @@ describe UserSerializer do
       expect(json[:custom_fields]['secret_field']).to eq(nil)
     end
   end
+
+  context "when SSO is enabled" do
+    it "sets the external_id field" do
+      SiteSetting.sso_url = "http://example.com/discourse_sso"
+      SiteSetting.sso_secret = "abcdefghijklmnop"
+      SiteSetting.enable_sso = true
+      sso = DiscourseSingleSignOn.new
+      sso.username = "test"
+      sso.email = "test@example.com"
+      sso.external_id = "1"
+      user = sso.lookup_or_create_user
+      json = UserSerializer.new(user, scope: Guardian.new, root: false).as_json
+      expect(json[:external_id]).to eq("1")
+    end
+  end
+
+  context "when SSO is not enabled" do
+    let(:user) { Fabricate(:user) }
+    let(:json) { UserSerializer.new(user, scope: Guardian.new, root: false).as_json }
+    it "doesn't include the external_id field" do
+      SiteSetting.enable_sso = false
+      expect(json).not_to have_key(:external_id)
+    end
+  end
 end