Reduce Namespace queries in UserReferenceFilterbanzai-user-filter-queries

This changes UserReferenceFilter so it operates using the following
steps:

1. Grab all username references from the input document.
2. Query the corresponding Namespace objects using a single query.
3. Iterate over all nodes to build links while re-using the objects
   queried in step 2.

The impact of these changes is that a comment mentioning 5 different
usernames no longer runs 5 different queries (1 for every username),
instead it only runs a single query.

1 files changed, 27 insertions, 2 deletions

diff --git a/lib/banzai/filter/user_reference_filter.rb b/lib/banzai/filter/user_reference_filter.rb
index 331d8007257..5b0a6d8541b 100644
--- a/lib/banzai/filter/user_reference_filter.rb
+++ b/lib/banzai/filter/user_reference_filter.rb
@@ -29,7 +29,7 @@ module Banzai
         ref_pattern = User.reference_pattern
         ref_pattern_start = /\A#{ref_pattern}\z/
 
-        each_node do |node|
+        nodes.each do |node|
           if text_node?(node)
             replace_text_when_pattern_matches(node, ref_pattern) do |content|
               user_link_filter(content)
@@ -59,7 +59,7 @@ module Banzai
         self.class.references_in(text) do |match, username|
           if username == 'all'
             link_to_all(link_text: link_text)
-          elsif namespace = Namespace.find_by(path: username)
+          elsif namespace = namespaces[username]
             link_to_namespace(namespace, link_text: link_text) || match
           else
             match
@@ -67,6 +67,31 @@ module Banzai
         end
       end
 
+      # Returns a Hash containing all Namespace objects for the username
+      # references in the current document.
+      #
+      # The keys of this Hash are the namespace paths, the values the
+      # corresponding Namespace objects.
+      def namespaces
+        @namespaces ||=
+          Namespace.where(path: usernames).each_with_object({}) do |row, hash|
+            hash[row.path] = row
+          end
+      end
+
+      # Returns all usernames referenced in the current document.
+      def usernames
+        refs = Set.new
+
+        nodes.each do |node|
+          node.to_html.scan(User.reference_pattern) do
+            refs << $~[:user]
+          end
+        end
+
+        refs.to_a
+      end
+
       private
 
       def urls