BURs: don't allow implying tags from different categories.

Don't allow requests for implications between tags of different
categories. For example, don't allow character tags to imply copyright
tags.

app/models/artist.rb

@@ -192,6 +192,7 @@ def ban!(banner: CurrentUser.user)
 
         # potential race condition but unlikely
         unless TagImplication.where(:antecedent_name => name, :consequent_name => "banned_artist").exists?
+          Tag.find_or_create_by_name("artist:banned_artist") # ensure the banned_artist exists and is an artist tag.
           TagImplication.approve!(antecedent_name: name, consequent_name: "banned_artist", approver: banner)
         end
 

app/models/tag_implication.rb

@@ -7,6 +7,7 @@ class TagImplication < TagRelationship
   validate :absence_of_transitive_relation
   validate :antecedent_is_not_aliased
   validate :consequent_is_not_aliased
+  validate :tag_categories_are_compatible
   validate :has_wiki_page, on: :request
 
   concerning :HierarchyMethods do
@@ -93,6 +94,12 @@ def consequent_is_not_aliased
       end
     end
 
+    def tag_categories_are_compatible
+      if antecedent_tag.category != consequent_tag.category
+        errors[:base] << "Can't imply a #{antecedent_tag.category_name.downcase} tag to a #{consequent_tag.category_name.downcase} tag"
+      end
+    end
+
     def has_wiki_page
       if !antecedent_tag.empty? && antecedent_wiki.blank?
         errors[:base] << "'#{antecedent_name}' must have a wiki page"

test/unit/bulk_update_request_test.rb

@@ -140,6 +140,18 @@ def create_bur!(script, approver)
           assert_equal(false, @bur.valid?)
           assert_equal(["Can't create implication a -> c (a already implies c through another implication)"], @bur.errors.full_messages)
         end
+
+        should "fail for an implication between tags of different categories" do
+          create(:tag, name: "hatsune_miku", category: Tag.categories.character)
+          create(:tag, name: "vocaloid", category: Tag.categories.copyright)
+          create(:wiki_page, title: "hatsune_miku")
+          create(:wiki_page, title: "vocaloid")
+
+          @bur = build(:bulk_update_request, script: "imply hatsune_miku -> vocaloid")
+
+          assert_equal(false, @bur.valid?)
+          assert_equal(["Can't create implication hatsune_miku -> vocaloid (Can't imply a character tag to a copyright tag)"], @bur.errors.full_messages)
+        end
       end
 
       context "the remove alias command" do