Configurable read concern with start_transaction

Author: ajdavis

pymongo/client_session.py

@@ -130,12 +130,12 @@ def __init__(self, read_concern=None, write_concern=None):
  raise ConfigurationError(
  "transactions must use an acknowledged write concern, "
  "not: %r" % (write_concern,))
- 
+
  @property
  def read_concern(self):
  """This transaction's :class:`~read_concern.ReadConcern`."""
  return self._read_concern
- 
+
  @property
  def write_concern(self):
  """This transaction's :class:`~write_concern.WriteConcern`."""
@@ -387,11 +387,18 @@ def _apply_to(self, command, is_retryable, read_preference):
  # First statement begins a new transaction.
  self._server_session._transaction_id += 1
  command['startTransaction'] = True
- read_concern = command.setdefault('readConcern', {})
- read_concern['level'] = 'snapshot'
+
+ if self._transaction.opts.read_concern:
+ rc = self._transaction.opts.read_concern.document
+ else:
+ rc = {}
+
  if (self.options.causal_consistency
  and self.operation_time is not None):
- read_concern['afterClusterTime'] = self.operation_time
+ rc['afterClusterTime'] = self.operation_time
+
+ if rc:
+ command['readConcern'] = rc
 
  command['txnNumber'] = self._server_session.transaction_id
  command['stmtId'] = self._server_session.statement_id