On 09/14/2011 12:25 PM, Wichert Akkerman wrote:
Constraints marked as deferrable result in a syntax error when using
SQLite. Is this deliberate, or a bug in the sqlite dialect?
As a workaround I figured I could use events to only add deferrable
constraint variants on PostgreSQL and use the non-deferrable version on
other database. That resulted in this code:
_generic_constraints = [UniqueConstraint('article_id', 'colour, 'size')]
_ deferrable_variant_constraints = [
UniqueConstraint('article_id', 'uuid', deferrable=True,
initially='DEFERRED'),
]
class Article(BaseObject):
__table_args = (_generic_constraints[0], _deferrable_constraints[0], {})
def deferrable_supported(ddl, target, bind, **kw):
"""Check if deferrable constraints are supported.
This function can be used as a callable for
:ref:`execute_if<sqlalchemy:sqlalchemy.schema.DDLElement.execute_if>` to
only run DDL statements on databases that support deferrable constraints.
"""
return bind.dialect == 'postgresql'
def deferrable_not_supported(ddl, target, bind, **kw):
"""Check if deferrable constraints are not supported.
This function can be used as a callable for
:ref:`execute_if<sqlalchemy:sqlalchemy.schema.DDLElement.execute_if>` to
only run DDL statements on databases that do not support deferrable
constraints.
"""
return not deferrable_supported(ddl, target, bind, **kw)
for constraint in _generic_variant_constraints:
listen(Article.__table__, 'after_create',
AddConstraint(constraint)
.execute_if(callable_=deferrable_not_supported))
for constraint in _deferrable_variant_constraints:
listen(Article.__table__, 'after_create',
AddConstraint(constraint)
.execute_if(callable_=deferrable_supported))
But this fails as well since SQLite does not support ALTER TABLE .. ADD
CONSTRAINT. Is there another way to create deferrable constraints only
on databases that support it?
Wichert.
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