Connect to writing database during thumbnail generation

In a multi db setup where you have a reading and a writing database
we need to make sure that we are using the writing database.

This is necessary, because we are writing (caching) picture
thumbnails in a GET request. Usually Rails would handle this for us
on POST, PATCH/PUT and DELETE requests.

ActiveStorage does the same thing to support multi database
setups.

Closes #2428

app/models/alchemy/picture/url.rb

@@ -35,7 +35,9 @@ def uid
           thumb.uid
         else
           uid = PictureThumb::Uid.call(signature, variant)
-          PictureThumb.generator_class.call(variant, signature, uid)
+          ActiveRecord::Base.connected_to(role: ActiveRecord::Base.writing_role) do
+            PictureThumb.generator_class.call(variant, signature, uid)
+          end
           uid
         end
       end

spec/models/alchemy/picture/url_spec.rb

@@ -34,5 +34,11 @@
     it "returns the url to the thumbnail" do
       is_expected.to match(/\/pictures\/\d+\/.+\/image\.png/)
     end
+
+    it "connects to writing database" do
+      writing_role = ActiveRecord::Base.writing_role
+      expect(ActiveRecord::Base).to receive(:connected_to).with(role: writing_role)
+      subject
+    end
   end
 end