Index: app/models/member_role.rb
===================================================================
--- app/models/member_role.rb	(revision 15418)
+++ app/models/member_role.rb	(working copy)
@@ -72,9 +72,8 @@
   end
 
   def remove_inherited_roles
-    MemberRole.where(:inherited_from => id).group_by(&:member).
-        each do |member, member_roles|
-      member_roles.each(&:destroy)
+    MemberRole.where(:inherited_from => id).order(:member_id).each do |member_role|
+      member_role.destroy
     end
   end
 end
Index: app/models/project.rb
===================================================================
--- app/models/project.rb	(revision 15418)
+++ app/models/project.rb	(working copy)
@@ -804,7 +804,7 @@
   end
 
   def remove_inherited_member_roles
-    member_roles = memberships.map(&:member_roles).flatten
+    member_roles = MemberRole.where(:member_id => membership_ids).to_a
     member_role_ids = member_roles.map(&:id)
     member_roles.each do |member_role|
       if member_role.inherited_from && !member_role_ids.include?(member_role.inherited_from)
Index: db/migrate/20160519091249_add_index_on_member_roles_inherited_from.rb
===================================================================
--- db/migrate/20160519091249_add_index_on_member_roles_inherited_from.rb	(nonexistent)
+++ db/migrate/20160519091249_add_index_on_member_roles_inherited_from.rb	(working copy)
@@ -0,0 +1,5 @@
+class AddIndexOnMemberRolesInheritedFrom < ActiveRecord::Migration
+  def change
+    add_index :member_roles, :inherited_from
+  end
+end