Edit File: ddl.py
# sql/ddl.py # Copyright (C) 2009-2021 the SQLAlchemy authors and contributors # <see AUTHORS file> # # This module is part of SQLAlchemy and is released under # the MIT License: http://www.opensource.org/licenses/mit-license.php """ Provides the hierarchy of DDL-defining schema items as well as routines to invoke them for a create/drop call. """ from .base import _bind_or_error from .base import _generative from .base import Executable from .base import SchemaVisitor from .elements import ClauseElement from .. import event from .. import exc from .. import util from ..util import topological class _DDLCompiles(ClauseElement): def _compiler(self, dialect, **kw): """Return a compiler appropriate for this ClauseElement, given a Dialect.""" return dialect.ddl_compiler(dialect, self, **kw) class DDLElement(Executable, _DDLCompiles): """Base class for DDL expression constructs. This class is the base for the general purpose :class:`.DDL` class, as well as the various create/drop clause constructs such as :class:`.CreateTable`, :class:`.DropTable`, :class:`.AddConstraint`, etc. :class:`.DDLElement` integrates closely with SQLAlchemy events, introduced in :ref:`event_toplevel`. An instance of one is itself an event receiving callable:: event.listen( users, 'after_create', AddConstraint(constraint).execute_if(dialect='postgresql') ) .. seealso:: :class:`.DDL` :class:`.DDLEvents` :ref:`event_toplevel` :ref:`schema_ddl_sequences` """ _execution_options = Executable._execution_options.union( {"autocommit": True} ) target = None on = None dialect = None callable_ = None def _execute_on_connection(self, connection, multiparams, params): return connection._execute_ddl(self, multiparams, params) def execute(self, bind=None, target=None): """Execute this DDL immediately. Executes the DDL statement in isolation using the supplied :class:`.Connectable` or :class:`.Connectable` assigned to the ``.bind`` property, if not supplied. If the DDL has a conditional ``on`` criteria, it will be invoked with None as the event. :param bind: Optional, an ``Engine`` or ``Connection``. If not supplied, a valid :class:`.Connectable` must be present in the ``.bind`` property. :param target: Optional, defaults to None. The target :class:`_schema.SchemaItem` for the execute call. Will be passed to the ``on`` callable if any, and may also provide string expansion data for the statement. See ``execute_at`` for more information. """ if bind is None: bind = _bind_or_error(self) if self._should_execute(target, bind): return bind.execute(self.against(target)) else: bind.engine.logger.info("DDL execution skipped, criteria not met.") @util.deprecated( "0.7", "The :meth:`.DDLElement.execute_at` method is deprecated and will " "be removed in a future release. Please use the :class:`.DDLEvents` " "listener interface in conjunction with the " ":meth:`.DDLElement.execute_if` method.", ) def execute_at(self, event_name, target): """Link execution of this DDL to the DDL lifecycle of a SchemaItem. Links this ``DDLElement`` to a ``Table`` or ``MetaData`` instance, executing it when that schema item is created or dropped. The DDL statement will be executed using the same Connection and transactional context as the Table create/drop itself. The ``.bind`` property of this statement is ignored. :param event: One of the events defined in the schema item's ``.ddl_events``; e.g. 'before-create', 'after-create', 'before-drop' or 'after-drop' :param target: The Table or MetaData instance for which this DDLElement will be associated with. A DDLElement instance can be linked to any number of schema items. ``execute_at`` builds on the ``append_ddl_listener`` interface of :class:`_schema.MetaData` and :class:`_schema.Table` objects. Caveat: Creating or dropping a Table in isolation will also trigger any DDL set to ``execute_at`` that Table's MetaData. This may change in a future release. """ def call_event(target, connection, **kw): if self._should_execute_deprecated( event_name, target, connection, **kw ): return connection.execute(self.against(target)) event.listen(target, "" + event_name.replace("-", "_"), call_event) @_generative def against(self, target): """Return a copy of this DDL against a specific schema item.""" self.target = target @_generative def execute_if(self, dialect=None, callable_=None, state=None): r"""Return a callable that will execute this DDLElement conditionally. Used to provide a wrapper for event listening:: event.listen( metadata, 'before_create', DDL("my_ddl").execute_if(dialect='postgresql') ) :param dialect: May be a string, tuple or a callable predicate. If a string, it will be compared to the name of the executing database dialect:: DDL('something').execute_if(dialect='postgresql') If a tuple, specifies multiple dialect names:: DDL('something').execute_if(dialect=('postgresql', 'mysql')) :param callable\_: A callable, which will be invoked with four positional arguments as well as optional keyword arguments: :ddl: This DDL element. :target: The :class:`_schema.Table` or :class:`_schema.MetaData` object which is the target of this event. May be None if the DDL is executed explicitly. :bind: The :class:`_engine.Connection` being used for DDL execution :tables: Optional keyword argument - a list of Table objects which are to be created/ dropped within a MetaData.create_all() or drop_all() method call. :state: Optional keyword argument - will be the ``state`` argument passed to this function. :checkfirst: Keyword argument, will be True if the 'checkfirst' flag was set during the call to ``create()``, ``create_all()``, ``drop()``, ``drop_all()``. If the callable returns a True value, the DDL statement will be executed. :param state: any value which will be passed to the callable\_ as the ``state`` keyword argument. .. seealso:: :class:`.DDLEvents` :ref:`event_toplevel` """ self.dialect = dialect self.callable_ = callable_ self.state = state def _should_execute(self, target, bind, **kw): if self.on is not None and not self._should_execute_deprecated( None, target, bind, **kw ): return False if isinstance(self.dialect, util.string_types): if self.dialect != bind.engine.name: return False elif isinstance(self.dialect, (tuple, list, set)): if bind.engine.name not in self.dialect: return False if self.callable_ is not None and not self.callable_( self, target, bind, state=self.state, **kw ): return False return True def _should_execute_deprecated(self, event, target, bind, **kw): if self.on is None: return True elif isinstance(self.on, util.string_types): return self.on == bind.engine.name elif isinstance(self.on, (tuple, list, set)): return bind.engine.name in self.on else: return self.on(self, event, target, bind, **kw) def __call__(self, target, bind, **kw): """Execute the DDL as a ddl_listener.""" if self._should_execute(target, bind, **kw): return bind.execute(self.against(target)) def _check_ddl_on(self, on): if on is not None and ( not isinstance(on, util.string_types + (tuple, list, set)) and not util.callable(on) ): raise exc.ArgumentError( "Expected the name of a database dialect, a tuple " "of names, or a callable for " "'on' criteria, got type '%s'." % type(on).__name__ ) def bind(self): if self._bind: return self._bind def _set_bind(self, bind): self._bind = bind bind = property(bind, _set_bind) def _generate(self): s = self.__class__.__new__(self.__class__) s.__dict__ = self.__dict__.copy() return s class DDL(DDLElement): """A literal DDL statement. Specifies literal SQL DDL to be executed by the database. DDL objects function as DDL event listeners, and can be subscribed to those events listed in :class:`.DDLEvents`, using either :class:`_schema.Table` or :class:`_schema.MetaData` objects as targets. Basic templating support allows a single DDL instance to handle repetitive tasks for multiple tables. Examples:: from sqlalchemy import event, DDL tbl = Table('users', metadata, Column('uid', Integer)) event.listen(tbl, 'before_create', DDL('DROP TRIGGER users_trigger')) spow = DDL('ALTER TABLE %(table)s SET secretpowers TRUE') event.listen(tbl, 'after_create', spow.execute_if(dialect='somedb')) drop_spow = DDL('ALTER TABLE users SET secretpowers FALSE') connection.execute(drop_spow) When operating on Table events, the following ``statement`` string substitutions are available:: %(table)s - the Table name, with any required quoting applied %(schema)s - the schema name, with any required quoting applied %(fullname)s - the Table name including schema, quoted if needed The DDL's "context", if any, will be combined with the standard substitutions noted above. Keys present in the context will override the standard substitutions. """ __visit_name__ = "ddl" @util.deprecated_params( on=( "0.7", "The :paramref:`.DDL.on` parameter is deprecated and will be " "removed in a future release. Please refer to " ":meth:`.DDLElement.execute_if`.", ) ) def __init__(self, statement, on=None, context=None, bind=None): """Create a DDL statement. :param statement: A string or unicode string to be executed. Statements will be processed with Python's string formatting operator. See the ``context`` argument and the ``execute_at`` method. A literal '%' in a statement must be escaped as '%%'. SQL bind parameters are not available in DDL statements. :param on: Optional filtering criteria. May be a string, tuple or a callable predicate. If a string, it will be compared to the name of the executing database dialect:: DDL('something', on='postgresql') If a tuple, specifies multiple dialect names:: DDL('something', on=('postgresql', 'mysql')) If a callable, it will be invoked with four positional arguments as well as optional keyword arguments: :ddl: This DDL element. :event: The name of the event that has triggered this DDL, such as 'after-create' Will be None if the DDL is executed explicitly. :target: The ``Table`` or ``MetaData`` object which is the target of this event. May be None if the DDL is executed explicitly. :connection: The ``Connection`` being used for DDL execution :tables: Optional keyword argument - a list of Table objects which are to be created/ dropped within a MetaData.create_all() or drop_all() method call. If the callable returns a true value, the DDL statement will be executed. :param context: Optional dictionary, defaults to None. These values will be available for use in string substitutions on the DDL statement. :param bind: Optional. A :class:`.Connectable`, used by default when ``execute()`` is invoked without a bind argument. .. seealso:: :class:`.DDLEvents` :ref:`event_toplevel` """ if not isinstance(statement, util.string_types): raise exc.ArgumentError( "Expected a string or unicode SQL statement, got '%r'" % statement ) self.statement = statement self.context = context or {} self._check_ddl_on(on) self.on = on self._bind = bind def __repr__(self): return "<%s@%s; %s>" % ( type(self).__name__, id(self), ", ".join( [repr(self.statement)] + [ "%s=%r" % (key, getattr(self, key)) for key in ("on", "context") if getattr(self, key) ] ), ) class _CreateDropBase(DDLElement): """Base class for DDL constructs that represent CREATE and DROP or equivalents. The common theme of _CreateDropBase is a single ``element`` attribute which refers to the element to be created or dropped. """ def __init__(self, element, on=None, bind=None): self.element = element self._check_ddl_on(on) self.on = on self.bind = bind def _create_rule_disable(self, compiler): """Allow disable of _create_rule using a callable. Pass to _create_rule using util.portable_instancemethod(self._create_rule_disable) to retain serializability. """ return False class CreateSchema(_CreateDropBase): """Represent a CREATE SCHEMA statement. The argument here is the string name of the schema. """ __visit_name__ = "create_schema" def __init__(self, name, quote=None, **kw): """Create a new :class:`.CreateSchema` construct.""" self.quote = quote super(CreateSchema, self).__init__(name, **kw) class DropSchema(_CreateDropBase): """Represent a DROP SCHEMA statement. The argument here is the string name of the schema. """ __visit_name__ = "drop_schema" def __init__(self, name, quote=None, cascade=False, **kw): """Create a new :class:`.DropSchema` construct.""" self.quote = quote self.cascade = cascade super(DropSchema, self).__init__(name, **kw) class CreateTable(_CreateDropBase): """Represent a CREATE TABLE statement.""" __visit_name__ = "create_table" def __init__( self, element, on=None, bind=None, include_foreign_key_constraints=None ): """Create a :class:`.CreateTable` construct. :param element: a :class:`_schema.Table` that's the subject of the CREATE :param on: See the description for 'on' in :class:`.DDL`. :param bind: See the description for 'bind' in :class:`.DDL`. :param include_foreign_key_constraints: optional sequence of :class:`_schema.ForeignKeyConstraint` objects that will be included inline within the CREATE construct; if omitted, all foreign key constraints that do not specify use_alter=True are included. .. versionadded:: 1.0.0 """ super(CreateTable, self).__init__(element, on=on, bind=bind) self.columns = [CreateColumn(column) for column in element.columns] self.include_foreign_key_constraints = include_foreign_key_constraints class _DropView(_CreateDropBase): """Semi-public 'DROP VIEW' construct. Used by the test suite for dialect-agnostic drops of views. This object will eventually be part of a public "view" API. """ __visit_name__ = "drop_view" class CreateColumn(_DDLCompiles): """Represent a :class:`_schema.Column` as rendered in a CREATE TABLE statement, via the :class:`.CreateTable` construct. This is provided to support custom column DDL within the generation of CREATE TABLE statements, by using the compiler extension documented in :ref:`sqlalchemy.ext.compiler_toplevel` to extend :class:`.CreateColumn`. Typical integration is to examine the incoming :class:`_schema.Column` object, and to redirect compilation if a particular flag or condition is found:: from sqlalchemy import schema from sqlalchemy.ext.compiler import compiles @compiles(schema.CreateColumn) def compile(element, compiler, **kw): column = element.element if "special" not in column.info: return compiler.visit_create_column(element, **kw) text = "%s SPECIAL DIRECTIVE %s" % ( column.name, compiler.type_compiler.process(column.type) ) default = compiler.get_column_default_string(column) if default is not None: text += " DEFAULT " + default if not column.nullable: text += " NOT NULL" if column.constraints: text += " ".join( compiler.process(const) for const in column.constraints) return text The above construct can be applied to a :class:`_schema.Table` as follows:: from sqlalchemy import Table, Metadata, Column, Integer, String from sqlalchemy import schema metadata = MetaData() table = Table('mytable', MetaData(), Column('x', Integer, info={"special":True}, primary_key=True), Column('y', String(50)), Column('z', String(20), info={"special":True}) ) metadata.create_all(conn) Above, the directives we've added to the :attr:`_schema.Column.info` collection will be detected by our custom compilation scheme:: CREATE TABLE mytable ( x SPECIAL DIRECTIVE INTEGER NOT NULL, y VARCHAR(50), z SPECIAL DIRECTIVE VARCHAR(20), PRIMARY KEY (x) ) 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``. This is essentially how to produce the same effect as using the ``system=True`` argument on :class:`_schema.Column`, which marks a column as an implicitly-present "system" column. For example, suppose we wish to produce a :class:`_schema.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, "postgresql") def skip_xmin(element, compiler, **kw): if element.element.name == 'xmin': return None else: return compiler.visit_create_column(element, **kw) my_table = Table('mytable', metadata, Column('id', Integer, primary_key=True), Column('xmin', Integer) ) 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, but only against the PostgreSQL backend. """ __visit_name__ = "create_column" def __init__(self, element): self.element = element class DropTable(_CreateDropBase): """Represent a DROP TABLE statement.""" __visit_name__ = "drop_table" class CreateSequence(_CreateDropBase): """Represent a CREATE SEQUENCE statement.""" __visit_name__ = "create_sequence" class DropSequence(_CreateDropBase): """Represent a DROP SEQUENCE statement.""" __visit_name__ = "drop_sequence" class CreateIndex(_CreateDropBase): """Represent a CREATE INDEX statement.""" __visit_name__ = "create_index" class DropIndex(_CreateDropBase): """Represent a DROP INDEX statement.""" __visit_name__ = "drop_index" class AddConstraint(_CreateDropBase): """Represent an ALTER TABLE ADD CONSTRAINT statement.""" __visit_name__ = "add_constraint" def __init__(self, element, *args, **kw): super(AddConstraint, self).__init__(element, *args, **kw) element._create_rule = util.portable_instancemethod( self._create_rule_disable ) class DropConstraint(_CreateDropBase): """Represent an ALTER TABLE DROP CONSTRAINT statement.""" __visit_name__ = "drop_constraint" def __init__(self, element, cascade=False, **kw): self.cascade = cascade super(DropConstraint, self).__init__(element, **kw) element._create_rule = util.portable_instancemethod( self._create_rule_disable ) class SetTableComment(_CreateDropBase): """Represent a COMMENT ON TABLE IS statement.""" __visit_name__ = "set_table_comment" class DropTableComment(_CreateDropBase): """Represent a COMMENT ON TABLE '' statement. Note this varies a lot across database backends. """ __visit_name__ = "drop_table_comment" class SetColumnComment(_CreateDropBase): """Represent a COMMENT ON COLUMN IS statement.""" __visit_name__ = "set_column_comment" class DropColumnComment(_CreateDropBase): """Represent a COMMENT ON COLUMN IS NULL statement.""" __visit_name__ = "drop_column_comment" class DDLBase(SchemaVisitor): def __init__(self, connection): self.connection = connection class SchemaGenerator(DDLBase): def __init__( self, dialect, connection, checkfirst=False, tables=None, **kwargs ): super(SchemaGenerator, self).__init__(connection, **kwargs) self.checkfirst = checkfirst self.tables = tables self.preparer = dialect.identifier_preparer self.dialect = dialect self.memo = {} def _can_create_table(self, table): self.dialect.validate_identifier(table.name) effective_schema = self.connection.schema_for_object(table) if effective_schema: self.dialect.validate_identifier(effective_schema) return not self.checkfirst or not self.dialect.has_table( self.connection, table.name, schema=effective_schema ) def _can_create_sequence(self, sequence): effective_schema = self.connection.schema_for_object(sequence) return self.dialect.supports_sequences and ( (not self.dialect.sequences_optional or not sequence.optional) and ( not self.checkfirst or not self.dialect.has_sequence( self.connection, sequence.name, schema=effective_schema ) ) ) def visit_metadata(self, metadata): if self.tables is not None: tables = self.tables else: tables = list(metadata.tables.values()) collection = sort_tables_and_constraints( [t for t in tables if self._can_create_table(t)] ) seq_coll = [ s for s in metadata._sequences.values() if s.column is None and self._can_create_sequence(s) ] event_collection = [t for (t, fks) in collection if t is not None] metadata.dispatch.before_create( metadata, self.connection, tables=event_collection, checkfirst=self.checkfirst, _ddl_runner=self, ) for seq in seq_coll: self.traverse_single(seq, create_ok=True) for table, fkcs in collection: if table is not None: self.traverse_single( table, create_ok=True, include_foreign_key_constraints=fkcs, _is_metadata_operation=True, ) else: for fkc in fkcs: self.traverse_single(fkc) metadata.dispatch.after_create( metadata, self.connection, tables=event_collection, checkfirst=self.checkfirst, _ddl_runner=self, ) def visit_table( self, table, create_ok=False, include_foreign_key_constraints=None, _is_metadata_operation=False, ): if not create_ok and not self._can_create_table(table): return table.dispatch.before_create( table, self.connection, checkfirst=self.checkfirst, _ddl_runner=self, _is_metadata_operation=_is_metadata_operation, ) for column in table.columns: if column.default is not None: self.traverse_single(column.default) if not self.dialect.supports_alter: # e.g., don't omit any foreign key constraints include_foreign_key_constraints = None self.connection.execute( # fmt: off CreateTable( table, include_foreign_key_constraints= # noqa include_foreign_key_constraints, # noqa ) # fmt: on ) if hasattr(table, "indexes"): for index in table.indexes: self.traverse_single(index) if self.dialect.supports_comments and not self.dialect.inline_comments: if table.comment is not None: self.connection.execute(SetTableComment(table)) for column in table.columns: if column.comment is not None: self.connection.execute(SetColumnComment(column)) table.dispatch.after_create( table, self.connection, checkfirst=self.checkfirst, _ddl_runner=self, _is_metadata_operation=_is_metadata_operation, ) def visit_foreign_key_constraint(self, constraint): if not self.dialect.supports_alter: return self.connection.execute(AddConstraint(constraint)) def visit_sequence(self, sequence, create_ok=False): if not create_ok and not self._can_create_sequence(sequence): return self.connection.execute(CreateSequence(sequence)) def visit_index(self, index): self.connection.execute(CreateIndex(index)) class SchemaDropper(DDLBase): def __init__( self, dialect, connection, checkfirst=False, tables=None, **kwargs ): super(SchemaDropper, self).__init__(connection, **kwargs) self.checkfirst = checkfirst self.tables = tables self.preparer = dialect.identifier_preparer self.dialect = dialect self.memo = {} def visit_metadata(self, metadata): if self.tables is not None: tables = self.tables else: tables = list(metadata.tables.values()) try: unsorted_tables = [t for t in tables if self._can_drop_table(t)] collection = list( reversed( sort_tables_and_constraints( unsorted_tables, filter_fn=lambda constraint: False if not self.dialect.supports_alter or constraint.name is None else None, ) ) ) except exc.CircularDependencyError as err2: if not self.dialect.supports_alter: util.warn( "Can't sort tables for DROP; an " "unresolvable foreign key " "dependency exists between tables: %s; and backend does " "not support ALTER. To restore at least a partial sort, " "apply use_alter=True to ForeignKey and " "ForeignKeyConstraint " "objects involved in the cycle to mark these as known " "cycles that will be ignored." % (", ".join(sorted([t.fullname for t in err2.cycles]))) ) collection = [(t, ()) for t in unsorted_tables] else: util.raise_( exc.CircularDependencyError( err2.args[0], err2.cycles, err2.edges, msg="Can't sort tables for DROP; an " "unresolvable foreign key " "dependency exists between tables: %s. Please ensure " "that the ForeignKey and ForeignKeyConstraint objects " "involved in the cycle have " "names so that they can be dropped using " "DROP CONSTRAINT." % ( ", ".join( sorted([t.fullname for t in err2.cycles]) ) ), ), from_=err2, ) seq_coll = [ s for s in metadata._sequences.values() if self._can_drop_sequence(s) ] event_collection = [t for (t, fks) in collection if t is not None] metadata.dispatch.before_drop( metadata, self.connection, tables=event_collection, checkfirst=self.checkfirst, _ddl_runner=self, ) for table, fkcs in collection: if table is not None: self.traverse_single( table, drop_ok=True, _is_metadata_operation=True, _ignore_sequences=seq_coll, ) else: for fkc in fkcs: self.traverse_single(fkc) for seq in seq_coll: self.traverse_single(seq, drop_ok=seq.column is None) metadata.dispatch.after_drop( metadata, self.connection, tables=event_collection, checkfirst=self.checkfirst, _ddl_runner=self, ) def _can_drop_table(self, table): self.dialect.validate_identifier(table.name) effective_schema = self.connection.schema_for_object(table) if effective_schema: self.dialect.validate_identifier(effective_schema) return not self.checkfirst or self.dialect.has_table( self.connection, table.name, schema=effective_schema ) def _can_drop_sequence(self, sequence): effective_schema = self.connection.schema_for_object(sequence) return self.dialect.supports_sequences and ( (not self.dialect.sequences_optional or not sequence.optional) and ( not self.checkfirst or self.dialect.has_sequence( self.connection, sequence.name, schema=effective_schema ) ) ) def visit_index(self, index): self.connection.execute(DropIndex(index)) def visit_table( self, table, drop_ok=False, _is_metadata_operation=False, _ignore_sequences=[], ): if not drop_ok and not self._can_drop_table(table): return table.dispatch.before_drop( table, self.connection, checkfirst=self.checkfirst, _ddl_runner=self, _is_metadata_operation=_is_metadata_operation, ) self.connection.execute(DropTable(table)) # traverse client side defaults which may refer to server-side # sequences. noting that some of these client side defaults may also be # set up as server side defaults (see http://docs.sqlalchemy.org/en/ # latest/core/defaults.html#associating-a-sequence-as-the-server-side- # default), so have to be dropped after the table is dropped. for column in table.columns: if ( column.default is not None and column.default not in _ignore_sequences ): self.traverse_single(column.default) table.dispatch.after_drop( table, self.connection, checkfirst=self.checkfirst, _ddl_runner=self, _is_metadata_operation=_is_metadata_operation, ) def visit_foreign_key_constraint(self, constraint): if not self.dialect.supports_alter: return self.connection.execute(DropConstraint(constraint)) def visit_sequence(self, sequence, drop_ok=False): if not drop_ok and not self._can_drop_sequence(sequence): return self.connection.execute(DropSequence(sequence)) def sort_tables( tables, skip_fn=None, extra_dependencies=None, ): """Sort a collection of :class:`_schema.Table` objects based on dependency. This is a dependency-ordered sort which will emit :class:`_schema.Table` objects such that they will follow their dependent :class:`_schema.Table` objects. Tables are dependent on another based on the presence of :class:`_schema.ForeignKeyConstraint` objects as well as explicit dependencies added by :meth:`_schema.Table.add_is_dependent_on`. .. warning:: The :func:`._schema.sort_tables` function cannot by itself accommodate automatic resolution of dependency cycles between tables, which are usually caused by mutually dependent foreign key constraints. When these cycles are detected, the foreign keys of these tables are omitted from consideration in the sort. A warning is emitted when this condition occurs, which will be an exception raise in a future release. Tables which are not part of the cycle will still be returned in dependency order. To resolve these cycles, the :paramref:`_schema.ForeignKeyConstraint.use_alter` parameter may be applied to those constraints which create a cycle. Alternatively, the :func:`_schema.sort_tables_and_constraints` function will automatically return foreign key constraints in a separate collection when cycles are detected so that they may be applied to a schema separately. .. versionchanged:: 1.3.17 - a warning is emitted when :func:`_schema.sort_tables` cannot perform a proper sort due to cyclical dependencies. This will be an exception in a future release. Additionally, the sort will continue to return other tables not involved in the cycle in dependency order which was not the case previously. :param tables: a sequence of :class:`_schema.Table` objects. :param skip_fn: optional callable which will be passed a :class:`_schema.ForeignKey` object; if it returns True, this constraint will not be considered as a dependency. Note this is **different** from the same parameter in :func:`.sort_tables_and_constraints`, which is instead passed the owning :class:`_schema.ForeignKeyConstraint` object. :param extra_dependencies: a sequence of 2-tuples of tables which will also be considered as dependent on each other. .. seealso:: :func:`.sort_tables_and_constraints` :attr:`_schema.MetaData.sorted_tables` - uses this function to sort """ if skip_fn is not None: def _skip_fn(fkc): for fk in fkc.elements: if skip_fn(fk): return True else: return None else: _skip_fn = None return [ t for (t, fkcs) in sort_tables_and_constraints( tables, filter_fn=_skip_fn, extra_dependencies=extra_dependencies, _warn_for_cycles=True, ) if t is not None ] def sort_tables_and_constraints( tables, filter_fn=None, extra_dependencies=None, _warn_for_cycles=False ): """Sort a collection of :class:`_schema.Table` / :class:`_schema.ForeignKeyConstraint` objects. This is a dependency-ordered sort which will emit tuples of ``(Table, [ForeignKeyConstraint, ...])`` such that each :class:`_schema.Table` follows its dependent :class:`_schema.Table` objects. Remaining :class:`_schema.ForeignKeyConstraint` objects that are separate due to dependency rules not satisfied by the sort are emitted afterwards as ``(None, [ForeignKeyConstraint ...])``. Tables are dependent on another based on the presence of :class:`_schema.ForeignKeyConstraint` objects, explicit dependencies added by :meth:`_schema.Table.add_is_dependent_on`, as well as dependencies stated here using the :paramref:`~.sort_tables_and_constraints.skip_fn` and/or :paramref:`~.sort_tables_and_constraints.extra_dependencies` parameters. :param tables: a sequence of :class:`_schema.Table` objects. :param filter_fn: optional callable which will be passed a :class:`_schema.ForeignKeyConstraint` object, and returns a value based on whether this constraint should definitely be included or excluded as an inline constraint, or neither. If it returns False, the constraint will definitely be included as a dependency that cannot be subject to ALTER; if True, it will **only** be included as an ALTER result at the end. Returning None means the constraint is included in the table-based result unless it is detected as part of a dependency cycle. :param extra_dependencies: a sequence of 2-tuples of tables which will also be considered as dependent on each other. .. versionadded:: 1.0.0 .. seealso:: :func:`.sort_tables` """ fixed_dependencies = set() mutable_dependencies = set() if extra_dependencies is not None: fixed_dependencies.update(extra_dependencies) remaining_fkcs = set() for table in tables: for fkc in table.foreign_key_constraints: if fkc.use_alter is True: remaining_fkcs.add(fkc) continue if filter_fn: filtered = filter_fn(fkc) if filtered is True: remaining_fkcs.add(fkc) continue dependent_on = fkc.referred_table if dependent_on is not table: mutable_dependencies.add((dependent_on, table)) fixed_dependencies.update( (parent, table) for parent in table._extra_dependencies ) try: candidate_sort = list( topological.sort( fixed_dependencies.union(mutable_dependencies), tables, deterministic_order=True, ) ) except exc.CircularDependencyError as err: if _warn_for_cycles: util.warn( "Cannot correctly sort tables; there are unresolvable cycles " 'between tables "%s", which is usually caused by mutually ' "dependent foreign key constraints. Foreign key constraints " "involving these tables will not be considered; this warning " "may raise an error in a future release." % (", ".join(sorted(t.fullname for t in err.cycles)),) ) for edge in err.edges: if edge in mutable_dependencies: table = edge[1] if table not in err.cycles: continue can_remove = [ fkc for fkc in table.foreign_key_constraints if filter_fn is None or filter_fn(fkc) is not False ] remaining_fkcs.update(can_remove) for fkc in can_remove: dependent_on = fkc.referred_table if dependent_on is not table: mutable_dependencies.discard((dependent_on, table)) candidate_sort = list( topological.sort( fixed_dependencies.union(mutable_dependencies), tables, deterministic_order=True, ) ) return [ (table, table.foreign_key_constraints.difference(remaining_fkcs)) for table in candidate_sort ] + [(None, list(remaining_fkcs))]