- Added new feature :paramref:`.EnvironmentContext.configure.transaction_per_migration`,

which when True causes the BEGIN/COMMIT pair to incur for each migration
individually, rather than for the whole series of migrations.  This is
to assist with some database directives that need to be within individual
transactions, without the need to disable transactional DDL entirely.
fixes #201

1 files changed, 13 insertions, 19 deletions

diff --git a/alembic/environment.py b/alembic/environment.py
index cba3beb..f8875a2 100644
--- a/alembic/environment.py
+++ b/alembic/environment.py
@@ -1,5 +1,3 @@
-from contextlib import contextmanager
-
 from .operations import Operations
 from .migration import MigrationContext
 from . import util
@@ -60,7 +58,6 @@ class EnvironmentContext(object):
     """
 
     _migration_context = None
-    _default_opts = None
 
     config = None
     """An instance of :class:`.Config` representing the
@@ -87,8 +84,6 @@ class EnvironmentContext(object):
         self.config = config
         self.script = script
         self.context_opts = kw
-        if self._default_opts:
-            self.context_opts.update(self._default_opts)
 
     def __enter__(self):
         """Establish a context which provides a
@@ -261,6 +256,7 @@ class EnvironmentContext(object):
             url=None,
             dialect_name=None,
             transactional_ddl=None,
+            transaction_per_migration=False,
             output_buffer=None,
             starting_rev=None,
             tag=None,
@@ -325,6 +321,12 @@ class EnvironmentContext(object):
          DDL on or off;
          this otherwise defaults to whether or not the dialect in
          use supports it.
+        :param transaction_per_migration: if True, nest each migration script
+         in a transaction rather than the full series of migrations to
+         run.
+
+         .. versionadded:: 0.6.5
+
         :param output_buffer: a file-like object that will be used
          for textual output
          when the ``--sql`` option is used to generate SQL scripts.
@@ -635,6 +637,7 @@ class EnvironmentContext(object):
             opts['tag'] = tag
         if template_args and 'template_args' in opts:
             opts['template_args'].update(template_args)
+        opts["transaction_per_migration"] = transaction_per_migration
         opts['target_metadata'] = target_metadata
         opts['include_symbol'] = include_symbol
         opts['include_object'] = include_object
@@ -651,6 +654,7 @@ class EnvironmentContext(object):
         if compare_server_default is not None:
             opts['compare_server_default'] = compare_server_default
         opts['script'] = self.script
+
         opts.update(kw)
 
         self._migration_context = MigrationContext.configure(
@@ -709,6 +713,7 @@ class EnvironmentContext(object):
         """
         self.get_context().impl.static_output(text)
 
+
     def begin_transaction(self):
         """Return a context manager that will
         enclose an operation within a "transaction",
@@ -752,20 +757,9 @@ class EnvironmentContext(object):
         mode.
 
         """
-        if not self.is_transactional_ddl():
-            @contextmanager
-            def do_nothing():
-                yield
-            return do_nothing()
-        elif self.is_offline_mode():
-            @contextmanager
-            def begin_commit():
-                self.get_context().impl.emit_begin()
-                yield
-                self.get_context().impl.emit_commit()
-            return begin_commit()
-        else:
-            return self.get_bind().begin()
+
+        return self.get_context().begin_transaction()
+
 
     def get_context(self):
         """Return the current :class:`.MigrationContext` object.