Index: test/functional/issues_controller_test.rb
===================================================================
--- test/functional/issues_controller_test.rb	(revision 3709)
+++ test/functional/issues_controller_test.rb	(working copy)
@@ -1249,7 +1249,21 @@
     assert_not_nil assigns(:issues)
     assert assigns(:issues).include?(Issue.find(13))
   end
+
+  def test_auto_complete_with_scope_all_and_cross_project_relations
+    Setting.cross_project_issue_relations = '1'
+    get :aut_complete, :project_id => 'ecookbook', :q => '13', :scope => 'all'
+    assert_response :success
+    assert_not_nil assigns(:issues)
+    assert assigns(:issues).include?(Issue.find(13))
+  end
   
+  def test_auto_complete_with_scope_all_without_cross_project_relations
+    get :aut_complete, :project_id => 'ecookbook', :q => '13', :scope => 'all'
+    assert_response :success
+    assert_nil assigns(:issues)
+  end
+  
   def test_destroy_issue_with_no_time_entries
     assert_nil TimeEntry.find_by_issue_id(2)
     @request.session[:user_id] = 2
Index: app/controllers/issues_controller.rb
===================================================================
--- app/controllers/issues_controller.rb	(revision 3709)
+++ app/controllers/issues_controller.rb	(working copy)
@@ -405,11 +405,12 @@
   def auto_complete
     @issues = []
     q = params[:q].to_s
+    query = (params[:scope] == "all" && Setting.cross_project_issue_relations?) ? Issue : @project.issues
     if q.match(/^\d+$/)
-      @issues << @project.issues.visible.find_by_id(q.to_i)
+      @issues << query.visible.find_by_id(q.to_i)
     end
     unless q.blank?
-      @issues += @project.issues.visible.find(:all, :conditions => ["LOWER(#{Issue.table_name}.subject) LIKE ?", "%#{q.downcase}%"], :limit => 10)
+      @issues += query.visible.find(:all, :conditions => ["LOWER(#{Issue.table_name}.subject) LIKE ?", "%#{q.downcase}%"], :limit => 10)
     end
     render :layout => false
   end
Index: public/javascripts/application.js
===================================================================
--- public/javascripts/application.js	(revision 3709)
+++ public/javascripts/application.js	(working copy)
@@ -206,6 +206,20 @@
                            }});
 }
 
+function observeRelatedIssueField(url) {
+  new Ajax.Autocompleter('relation_issue_to_id',
+                         'related_issue_candidates',
+                         url,
+                         { minChars: 3,
+                           frequency: 0.5,
+                           paramName: 'q',
+                           updateElement: function(value) {
+                             document.getElementById('relation_issue_to_id').value = value.id;
+                           },
+                           parameters: 'scope=all'
+                           });
+}
+
 /* shows and hides ajax indicator */
 Ajax.Responders.register({
     onCreate: function(){
Index: public/stylesheets/application.css
===================================================================
--- public/stylesheets/application.css	(revision 3709)
+++ public/stylesheets/application.css	(working copy)
@@ -655,6 +655,7 @@
 }
 
 #parent_issue_candidates ul li {width: 500px;}
+#related_issue_candidates ul li {width: 500px;}
 
 /***** Diff *****/
 .diff_out { background: #fcc; }
Index: app/views/issue_relations/_form.rhtml
===================================================================
--- app/views/issue_relations/_form.rhtml	(revision 3709)
+++ app/views/issue_relations/_form.rhtml	(working copy)
@@ -2,6 +2,11 @@
 
 <p><%= f.select :relation_type, collection_for_relation_type_select, {}, :onchange => "setPredecessorFieldsVisibility();" %>
 <%= l(:label_issue) %> #<%= f.text_field :issue_to_id, :size => 6 %>
+<div id="related_issue_candidates" class="autocomplete"></div>
+<%= javascript_tag "observeRelatedIssueField('#{url_for(:controller => :issues,
+                                                       :action => :auto_complete,
+                                                       :id => @issue,
+                                                       :project_id => @project) }')" %>
 <span id="predecessor_fields" style="display:none;">
 <%= l(:field_delay) %>: <%= f.text_field :delay, :size => 3 %> <%= l(:label_day_plural) %>
 </span>