Efficiently load multiple references of one type.

lib/gitlab/markdown/issue_reference_filter.rb

@@ -28,10 +28,7 @@ module Gitlab
       end
 
       def self.referenced_by(node)
-        issue = Issue.find(node.attr("data-issue")) rescue nil
-        return unless issue
-
-        { issue: issue }
+        { issue: LazyReference.new(Issue, node.attr("data-issue")) }
       end
 
       def call

lib/gitlab/markdown/label_reference_filter.rb

@@ -23,10 +23,7 @@ module Gitlab
       end
 
       def self.referenced_by(node)
-        label = Label.find(node.attr("data-label")) rescue nil
-        return unless label
-
-        { label: label }
+        { label: LazyReference.new(Label, node.attr("data-label")) }
       end
 
       def call

lib/gitlab/markdown/merge_request_reference_filter.rb

@@ -28,10 +28,7 @@ module Gitlab
       end
 
       def self.referenced_by(node)
-        merge_request = MergeRequest.find(node.attr("data-merge-request")) rescue nil
-        return unless merge_request
-
-        { merge_request: merge_request }
+        { merge_request: LazyReference.new(MergeRequest, node.attr("data-merge-request")) }
       end
 
       def call

lib/gitlab/markdown/reference_filter.rb

@@ -12,6 +12,8 @@ module Gitlab
     #   :project (required) - Current project, ignored if reference is cross-project.
     #   :only_path          - Generate path-only links.
     class ReferenceFilter < HTML::Pipeline::Filter
+      LazyReference = Struct.new(:klass, :ids)
+
       def self.user_can_reference?(user, node, context)
         if node.has_attribute?('data-project')
           project_id = node.attr('data-project').to_i

lib/gitlab/markdown/reference_gatherer_filter.rb

@@ -12,7 +12,8 @@ module Gitlab
       def initialize(*)
         super
 
-        result[:references] ||= Hash.new { |hash, type| hash[type] = [] }
+        result[:lazy_references]  ||= Hash.new { |hash, type| hash[type] = [] }
+        result[:references]       ||= Hash.new { |hash, type| hash[type] = [] }
       end
 
       def call
@@ -20,6 +21,8 @@ module Gitlab
           gather_references(node)
         end
 
+        load_lazy_references
+
         doc
       end
 
@@ -37,7 +40,27 @@ module Gitlab
         return unless references
 
         references.each do |type, values|
-          result[:references][type].push(*values)
+          Array.wrap(values).each do |value|
+            refs = 
+              if value.is_a?(ReferenceFilter::LazyReference)
+                result[:lazy_references]
+              else
+                result[:references]
+              end
+
+            refs[type] << value
+          end
+        end
+      end
+
+      # Will load all references of one type using one query.
+      def load_lazy_references
+        result[:lazy_references].each do |type, refs|
+          refs.group_by(&:klass).each do |klass, refs|
+            ids = refs.map(&:ids).flatten
+            values = klass.find(ids)
+            result[:references][type].push(*values)
+          end
         end
       end
 

lib/gitlab/markdown/snippet_reference_filter.rb

@@ -28,10 +28,7 @@ module Gitlab
       end
 
       def self.referenced_by(node)
-        snippet = Snippet.find(node.attr("data-snippet")) rescue nil
-        return unless snippet
-
-        { snippet: snippet }
+        { snippet: LazyReference.new(Snippet, node.attr("data-snippet")) }
       end
 
       def call

lib/gitlab/markdown/user_reference_filter.rb

@@ -30,10 +30,7 @@ module Gitlab
 
           { user: group.users }
         elsif node.has_attribute?('data-user')
-          user = User.find(node.attr('data-user')) rescue nil
-          return unless user
-
-          { user: user }
+          { user: LazyReference.new(User, node.attr('data-user')) }
         elsif node.has_attribute?('data-project')
           project = Project.find(node.attr('data-project')) rescue nil
           return unless project