mongos expects read preferences to be strings, not symbols.

This should actually fix RUBY-542. Without this, using read
preferences with a mongos cluster that's backed by replica sets will
error with

  wrong type for field (mode) 14 != 2

This is raised by _extraReadPref in dbclient_rs.cpp, when it does:

  const string mode = prefDoc["mode"].String();

BSON type 14 (0xE) is the (deprecated) Symbol type, and type 2 is
String, so this indicates that mongos is getting a symbol when it
expects a string.

The sharding tests passed either way because they don't actually
construct replica sets for the shards, and _extractReadPref is only
used by DBClientReplicaSet. Thus, the $readPreference field passed to
mongos is completely ignored by the test mongoS, and not even parsed.

Author: nelhage

lib/mongo/util/read_preference.rb

@@ -9,11 +9,11 @@ module ReadPreference
     ]
 
     MONGOS_MODES = {
-      :primary              => :primary,
-      :primary_preferred    => :primaryPreferred,
-      :secondary            => :secondary,
-      :secondary_preferred  => :secondaryPreferred,
-      :nearest              => :nearest
+      :primary              => 'primary',
+      :primary_preferred    => 'primaryPreferred',
+      :secondary            => 'secondary',
+      :secondary_preferred  => 'secondaryPreferred',
+      :nearest              => 'nearest'
     }
 
     def self.mongos(mode, tag_sets)

test/sharded_cluster/basic_test.rb

@@ -36,7 +36,7 @@ def test_read_from_client
     @client = MongoClient.new(host, port, {:read => :secondary, :tag_sets => tags})
     assert @client.connected?
     cursor = Cursor.new(@client[MONGO_TEST_DB]['whatever'], {})
-    assert_equal cursor.construct_query_spec['$readPreference'], {:mode => :secondary, :tags => tags}
+    assert_equal cursor.construct_query_spec['$readPreference'], {:mode => 'secondary', :tags => tags}
   end
 
   def test_find_one_with_read_secondary
@@ -68,7 +68,7 @@ def test_read_from_sharded_client
     @client = MongoShardedClient.new(@seeds, {:read => :secondary, :tag_sets => tags})
     assert @client.connected?
     cursor = Cursor.new(@client[MONGO_TEST_DB]['whatever'], {})
-    assert_equal cursor.construct_query_spec['$readPreference'], {:mode => :secondary, :tags => tags}
+    assert_equal cursor.construct_query_spec['$readPreference'], {:mode => 'secondary', :tags => tags}
   end
 
   def test_hard_refresh

test/unit/cursor_test.rb

@@ -124,47 +124,47 @@ class Mongo::Cursor
 
         spec = cursor.construct_query_spec
         assert spec.has_key?('$readPreference')
-        assert_equal :secondary, spec['$readPreference'][:mode]
+        assert_equal 'secondary', spec['$readPreference'][:mode]
         assert !spec['$readPreference'].has_key?(:tags)
 
         # secondary preferred with tags
         cursor = Cursor.new(@collection, { :read => :secondary_preferred, :tag_sets => @tag_sets })
 
         spec = cursor.construct_query_spec
         assert spec.has_key?('$readPreference')
-        assert_equal :secondaryPreferred, spec['$readPreference'][:mode]
+        assert_equal 'secondaryPreferred', spec['$readPreference'][:mode]
         assert_equal @tag_sets, spec['$readPreference'][:tags]
 
         # primary preferred
         cursor = Cursor.new(@collection, { :read => :primary_preferred })
 
         spec = cursor.construct_query_spec
         assert spec.has_key?('$readPreference')
-        assert_equal :primaryPreferred, spec['$readPreference'][:mode]
+        assert_equal 'primaryPreferred', spec['$readPreference'][:mode]
         assert !spec['$readPreference'].has_key?(:tags)
 
         # primary preferred with tags
         cursor = Cursor.new(@collection, { :read => :primary_preferred, :tag_sets => @tag_sets })
 
         spec = cursor.construct_query_spec
         assert spec.has_key?('$readPreference')
-        assert_equal :primaryPreferred, spec['$readPreference'][:mode]
+        assert_equal 'primaryPreferred', spec['$readPreference'][:mode]
         assert_equal @tag_sets, spec['$readPreference'][:tags]
 
         # nearest
         cursor = Cursor.new(@collection, { :read => :nearest })
 
         spec = cursor.construct_query_spec
         assert spec.has_key?('$readPreference')
-        assert_equal :nearest, spec['$readPreference'][:mode]
+        assert_equal 'nearest', spec['$readPreference'][:mode]
         assert !spec['$readPreference'].has_key?(:tags)
 
         # nearest with tags
         cursor = Cursor.new(@collection, { :read => :nearest, :tag_sets => @tag_sets })
 
         spec = cursor.construct_query_spec
         assert spec.has_key?('$readPreference')
-        assert_equal :nearest, spec['$readPreference'][:mode]
+        assert_equal 'nearest', spec['$readPreference'][:mode]
         assert_equal @tag_sets, spec['$readPreference'][:tags]
       end