diff options
Diffstat (limited to 'lib/sqlalchemy/sql/ddl.py')
| -rw-r--r-- | lib/sqlalchemy/sql/ddl.py | 15 |
1 files changed, 10 insertions, 5 deletions
diff --git a/lib/sqlalchemy/sql/ddl.py b/lib/sqlalchemy/sql/ddl.py index a0163ad7a..a17c8ee53 100644 --- a/lib/sqlalchemy/sql/ddl.py +++ b/lib/sqlalchemy/sql/ddl.py @@ -551,13 +551,18 @@ class CreateColumn(_DDLCompiles): The :class:`.CreateColumn` construct can also be used to skip certain columns when producing a ``CREATE TABLE``. This is accomplished by creating a compilation rule that conditionally returns ``None``. - For example, to produce a - :class:`.Table` which includes the Postgresql system column ``xmin``, - but omits this column from the ``CREATE TABLE``:: + This is essentially how to produce the same effect as using the + ``system=True`` argument on :class:`.Column`, which marks a column + as an implicitly-present "system" column. + + For example, suppose we wish to produce a :class:`.Table` which skips + rendering of the Postgresql ``xmin`` column against the Postgresql backend, + but on other backends does render it, in anticipation of a triggered rule. + A conditional compilation rule could skip this name only on Postgresql:: from sqlalchemy.schema import CreateColumn - @compiles(CreateColumn) + @compiles(CreateColumn, "postgresql") def skip_xmin(element, compiler, **kw): if element.element.name == 'xmin': return None @@ -572,7 +577,7 @@ class CreateColumn(_DDLCompiles): Above, a :class:`.CreateTable` construct will generate a ``CREATE TABLE`` which only includes the ``id`` column in the string; the ``xmin`` column - will be omitted. + will be omitted, but only against the Postgresql backend. .. versionadded:: 0.8.3 The :class:`.CreateColumn` construct supports skipping of columns by returning ``None`` from a custom compilation rule. |