Set all attributes of transaction before connecting to the database

The connection to the database may fail and thus stop and rollback may be
called. All the attributes must be set to avoid to raise another AttributeError
exception.

Closes #15022

trytond/trytond/transaction.py

@@ -187,13 +187,7 @@ class Transaction(object):
             # it is strictly before all transactions started after
             # but it may be also before transactions started before
             self.started_at = self.monotonic_time()
-            if not database_name:
-                database = backend.Database().connect()
-            else:
-                database = backend.Database(database_name).connect()
-            Flavor.set(backend.Database.flavor)
             self.user = user
-            self.database = database
             self.readonly = readonly
             self.context = ImmutableDict(context or {})
             self.create_records = defaultdict(list)
@@ -206,6 +200,12 @@ class Transaction(object):
             self.counter = 0
             self._datamanagers = []
 
+            if not database_name:
+                database = backend.Database().connect()
+            else:
+                database = backend.Database(database_name).connect()
+            Flavor.set(backend.Database.flavor)
+            self.database = database
             self.connection = database.get_connection(readonly=readonly,
                 autocommit=autocommit, statement_timeout=timeout)
             count = 0