disable automatic explain if there is no logger [closes rails#4671]

Author: fxn

activerecord/lib/active_record/explain.rb

@@ -19,6 +19,8 @@ def self.extended(base)
     # currently collected. A false value indicates collecting is turned
     # off. Otherwise it is an array of queries.
     def logging_query_plan # :nodoc:
+      return yield unless logger
+
       threshold = auto_explain_threshold_in_seconds
       current   = Thread.current
       if threshold && current[:available_queries_for_explain].nil?

activerecord/test/cases/explain_test.rb

@@ -14,7 +14,7 @@ def connection
       base.connection
     end
 
-    def test_logging_query_plan
+    def test_logging_query_plan_with_logger
       base.logger.expects(:warn).with do |message|
         message.starts_with?('EXPLAIN for:')
       end
@@ -24,6 +24,20 @@ def test_logging_query_plan
       end
     end
 
+    def test_logging_query_plan_without_logger
+      original = base.logger
+      base.logger = nil
+
+      base.logger.expects(:warn).never
+
+      with_threshold(0) do
+        car = Car.where(:name => 'honda').first
+        assert_equal 'honda', car.name
+      end
+    ensure
+      base.logger = original
+    end
+
     def test_collect_queries_for_explain
       base.auto_explain_threshold_in_seconds = nil
       queries = Thread.current[:available_queries_for_explain] = []

railties/guides/source/active_record_querying.textile

@@ -1400,6 +1400,9 @@ A threshold of +nil+ disables automatic EXPLAINs.
 The default threshold in development mode is 0.5 seconds, and +nil+ in test and
 production modes.
 
+INFO. Automatic EXPLAIN gets disabled if Active Record has no logger, regardless
+of the value of the threshold.
+
 h5. Disabling Automatic EXPLAIN
 
 Automatic EXPLAIN can be selectively silenced with +ActiveRecord::Base.silence_auto_explain+: