mongoid-read-preference.diff

diff --git a/lib/mongoid/persistence_context.rb b/lib/mongoid/persistence_context.rb
index e7c77ed626..9efa479589 100644
--- a/lib/mongoid/persistence_context.rb
+++ b/lib/mongoid/persistence_context.rb
@@ -150,9 +150,15 @@ def __evaluate__(name)
     end
 
     def client_options
-      @client_options ||= options.select do |k, v|
-                            Mongo::Client::VALID_OPTIONS.include?(k.to_sym)
-                          end
+      @client_options ||= begin
+        opts = options.select do |k, v|
+                              Mongo::Client::VALID_OPTIONS.include?(k.to_sym)
+                            end
+        if opts[:read].is_a?(Symbol)
+          opts[:read] = {mode: opts[:read]}
+        end
+        opts
+      end
     end
 
     def database_name_option