DEV: Allow theme CLI to specify which theme to synchronize (#6963)

Currently the theme is matched by name, which can be fragile when there are many themes with the same name. This functionality will be used by the next version of theme CLI.

spec/requests/admin/themes_controller_spec.rb

@@ -113,7 +113,8 @@ describe Admin::ThemesController do
       expect(UserHistory.where(action: UserHistory.actions[:change_theme]).count).to eq(1)
     end
 
-    it 'updates an existing theme from an archive' do
+    it 'updates an existing theme from an archive by name' do
+      # Old theme CLI method, remove Jan 2020
       existing_theme = Fabricate(:theme, name: "Header Icons")
 
       expect do
@@ -126,6 +127,39 @@ describe Admin::ThemesController do
       expect(json["theme"]["theme_fields"].length).to eq(5)
       expect(UserHistory.where(action: UserHistory.actions[:change_theme]).count).to eq(1)
     end
+
+    it 'updates an existing theme from an archive by id' do
+      # Used by theme CLI
+      existing_theme = Fabricate(:theme, name: "Header Icons")
+      other_existing_theme = Fabricate(:theme, name: "Some other name")
+
+      expect do
+        post "/admin/themes/import.json", params: { bundle: theme_archive, theme_id: other_existing_theme.id }
+      end.to change { Theme.count }.by (0)
+      expect(response.status).to eq(201)
+      json = ::JSON.parse(response.body)
+
+      expect(json["theme"]["name"]).to eq("Some other name")
+      expect(json["theme"]["id"]).to eq(other_existing_theme.id)
+      expect(json["theme"]["theme_fields"].length).to eq(5)
+      expect(UserHistory.where(action: UserHistory.actions[:change_theme]).count).to eq(1)
+    end
+
+    it 'creates a new theme when id specified as nil' do
+      # Used by theme CLI
+      existing_theme = Fabricate(:theme, name: "Header Icons")
+
+      expect do
+        post "/admin/themes/import.json", params: { bundle: theme_archive, theme_id: nil }
+      end.to change { Theme.count }.by (1)
+      expect(response.status).to eq(201)
+      json = ::JSON.parse(response.body)
+
+      expect(json["theme"]["name"]).to eq("Header Icons")
+      expect(json["theme"]["id"]).not_to eq(existing_theme.id)
+      expect(json["theme"]["theme_fields"].length).to eq(5)
+      expect(UserHistory.where(action: UserHistory.actions[:change_theme]).count).to eq(1)
+    end
   end
 
   describe '#index' do