Add high_precision_current_timestamp to adapters

CURRENT_TIMESTAMP provides differing precision on different databases.

This method can be used in queries instead to provide a high precision
current time regardless of the database being connected to.

CURRENT_TIMESTAMP continues to be used as the default, unless overriden.

activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb

@@ -441,6 +441,19 @@ module ActiveRecord
         end
       end
 
+      # This is a safe default, even if not high precision on all databases
+      HIGH_PRECISION_CURRENT_TIMESTAMP = Arel.sql("CURRENT_TIMESTAMP").freeze # :nodoc:
+      private_constant :HIGH_PRECISION_CURRENT_TIMESTAMP
+
+      # Returns an Arel SQL literal for the CURRENT_TIMESTAMP for usage with
+      # arbitrary precision date/time columns.
+      #
+      # Adapters supporting datetime with precision should override this to
+      # provide as much precision as is available.
+      def high_precision_current_timestamp
+        HIGH_PRECISION_CURRENT_TIMESTAMP
+      end
+
       private
         def execute_batch(statements, name = nil)
           statements.each do |statement|

activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb

@@ -79,6 +79,15 @@ module ActiveRecord
         end
         alias :exec_update :exec_delete
 
+        # https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_current-timestamp
+        # https://dev.mysql.com/doc/refman/5.7/en/date-and-time-type-syntax.html
+        HIGH_PRECISION_CURRENT_TIMESTAMP = Arel.sql("CURRENT_TIMESTAMP(6)").freeze # :nodoc:
+        private_constant :HIGH_PRECISION_CURRENT_TIMESTAMP
+
+        def high_precision_current_timestamp
+          HIGH_PRECISION_CURRENT_TIMESTAMP
+        end
+
         private
           def execute_batch(statements, name = nil)
             combine_multi_statements(statements).each do |statement|

activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb

@@ -122,6 +122,14 @@ module ActiveRecord
           execute("ROLLBACK", "TRANSACTION")
         end
 
+        # From https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT
+        HIGH_PRECISION_CURRENT_TIMESTAMP = Arel.sql("CURRENT_TIMESTAMP").freeze # :nodoc:
+        private_constant :HIGH_PRECISION_CURRENT_TIMESTAMP
+
+        def high_precision_current_timestamp
+          HIGH_PRECISION_CURRENT_TIMESTAMP
+        end
+
         private
           def execute_batch(statements, name = nil)
             execute(combine_multi_statements(statements))

activerecord/lib/active_record/connection_adapters/sqlite3/database_statements.rb

@@ -95,6 +95,15 @@ module ActiveRecord
           reset_read_uncommitted
         end
 
+        # https://stackoverflow.com/questions/17574784
+        # https://www.sqlite.org/lang_datefunc.html
+        HIGH_PRECISION_CURRENT_TIMESTAMP = Arel.sql("STRFTIME('%Y-%m-%d %H:%M:%f', 'NOW')").freeze # :nodoc:
+        private_constant :HIGH_PRECISION_CURRENT_TIMESTAMP
+
+        def high_precision_current_timestamp
+          HIGH_PRECISION_CURRENT_TIMESTAMP
+        end
+
         private
           def reset_read_uncommitted
             read_uncommitted = Thread.current.thread_variable_get("read_uncommitted")