Specify what is missing if tag assignment failed

app/models/classification.rb

@@ -99,10 +99,14 @@ def ns
 
   def self.classify(obj, category_name, entry_name, is_request = true)
     cat = Classification.lookup_by_name(category_name, obj.region_id)
-    unless cat.nil?
-      ent = cat.find_entry_by_name(entry_name, obj.region_id)
-      ent.assign_entry_to(obj, is_request) unless ent.nil? || obj.is_tagged_with?(ent.to_tag, :ns => "none")
-    end
+    raise "Tag category '#{category_name}' not found in region #{obj.region_id}" if cat.nil?
+
+    ent = cat.find_entry_by_name(entry_name, obj.region_id)
+    raise "Tag name '#{entry_name}' not found  in region #{obj.region_id}" if ent.nil?
+
+    raise "Object already tagged with ':ns' set to 'none'" if obj.is_tagged_with?(ent.to_tag, :ns => "none")
+
+    ent.assign_entry_to(obj, is_request)
   end
 
   def self.unclassify(obj, category_name, entry_name, is_request = true)

spec/models/classification_spec.rb

@@ -61,6 +61,39 @@
       FactoryBot.create(:classification_tag,      :name => "multi_entry_2", :parent => parent)
     end
 
+    describe ".classify" do
+      let(:entry) { "test_entry" }
+      let(:category) { "test_category" }
+      before { @vm = FactoryBot.create(:vm) }
+
+      it "raise error if tag category not found" do
+        category = "Hello, World"
+        expect { Classification.classify(@vm, category, entry) }.to raise_error(
+          RuntimeError, "Tag category '#{category}' not found in region #{@vm.region_id}"
+        )
+      end
+
+      it "raise error if tag entry not found" do
+        entry = "Hello, World"
+        expect { Classification.classify(@vm, category, entry) }.to raise_error(
+          RuntimeError, "Tag name '#{entry}' not found  in region #{@vm.region_id}"
+        )
+      end
+
+      it "raise error if object already tagged with tag namespace set to 'none'" do
+        allow(@vm).to receive(:is_tagged_with?)
+        allow(@vm).to receive(:is_tagged_with?).with("/managed/test_category/test_entry", :ns => "none").and_return(true)
+        expect { Classification.classify(@vm, category, entry) }.to raise_error(
+          RuntimeError, "Object already tagged with ':ns' set to 'none'"
+        )
+      end
+
+      it "assign tag entry to object if tag category and tag name exist" do
+        Classification.classify(@vm, category, entry)
+        expect(@vm.tags.first.name).to eq("/managed/test_category/test_entry")
+      end
+    end
+
     context "#destroy" do
       it "a category deletes all entries" do
         cat = Classification.lookup_by_name("test_category")