4.8 KiB
The Operations Object
Within migration scripts, actual database migration operations are
handled via an instance of .Operations
. The .Operations
class lists out available migration
operations that are linked to a .MigrationContext
, which communicates instructions
originated by the .Operations
object into SQL that is sent to a
database or SQL output stream.
Most methods on the .Operations
class are generated dynamically using a
"plugin" system, described in the next section operation_plugins
.
Additionally, when Alembic migration scripts actually run, the methods
on the current .Operations
object are proxied out to the
alembic.op
module, so that they are available using
module-style access.
For an overview of how to use an .Operations
object directly in programs, as well as
for reference to the standard operation methods as well as "batch"
methods, see ops
.
Operation Plugins
The Operations object is extensible using a plugin system. This
system allows one to add new op.<some_operation>
methods at runtime. The steps to use this system are to first create a
subclass of .MigrateOperation
, register it using the .Operations.register_operation
class decorator, then
build a default "implementation" function which is established using the
.Operations.implementation_for
decorator.
0.8.0 - the .Operations
class is now an open namespace that is
extensible via the creation of new .MigrateOperation
subclasses.
Below we illustrate a very simple operation
CreateSequenceOp
which will implement a new method
op.create_sequence()
for use in migration scripts:
from alembic.operations import Operations, MigrateOperation
@Operations.register_operation("create_sequence")
class CreateSequenceOp(MigrateOperation):
"""Create a SEQUENCE."""
def __init__(self, sequence_name, **kw):
self.sequence_name = sequence_name
self.kw = kw
@classmethod
def create_sequence(cls, operations, sequence_name, **kw):
"""Issue a "CREATE SEQUENCE" instruction."""
op = CreateSequenceOp(sequence_name, **kw)
return operations.invoke(op)
Above, the CreateSequenceOp
class represents a new
operation that will be available as op.create_sequence()
.
The reason the operation is represented as a stateful class is so that
an operation and a specific set of arguments can be represented
generically; the state can then correspond to different kinds of
operations, such as invoking the instruction against a database, or
autogenerating Python code for the operation into a script.
In order to establish the migrate-script behavior of the new
operation, we use the .Operations.implementation_for
decorator:
@Operations.implementation_for(CreateSequenceOp)
def create_sequence(operations, operation):
operations.execute("CREATE SEQUENCE %s" % operation.sequence_name)
Above, we use the simplest possible technique of invoking our DDL,
which is just to call .Operations.execute
with literal SQL. If this is all
a custom operation needs, then this is fine. However, options for more
comprehensive support include building out a custom SQL construct, as
documented at sqlalchemy.ext.compiler_toplevel
.
With the above two steps, a migration script can now use a new method
op.create_sequence()
that will proxy to our object as a
classmethod:
def upgrade():
op.create_sequence("my_sequence")
The registration of new operations only needs to occur in time for
the env.py
script to invoke .MigrationContext.run_migrations
; within the module
level of the env.py
script is sufficient.
0.8 - the migration operations available via the .Operations
class as well
as the alembic.op
namespace is now extensible using a
plugin system.
Built-in Operation Objects
The migration operations present on .Operations
are themselves delivered via operation
objects that represent an operation and its arguments. All operations
descend from the .MigrateOperation
class, and are registered with the
.Operations
class
using the .Operations.register_operation
class decorator. The
.MigrateOperation
objects also serve as the basis for how the autogenerate system renders
new migration scripts.
operation_plugins
customizing_revision
The built-in operation objects are listed below.
alembic.operations.ops