The schema module provides the building blocks for database metadata.
Each element within this module describes a database entity which can be created and dropped, or is otherwise part of such an entity. Examples include tables, columns, sequences, and indexes.
All entities are subclasses of SchemaItem, and as defined in this module they are intended to be agnostic of any vendor-specific constructs.
A collection of entities are grouped into a unit called MetaData. MetaData serves as a logical grouping of schema elements, and can also be associated with an actual database connection such that operations involving the contained elements can contact the database as needed.
Two of the elements here also build upon their "syntactic" counterparts, which are defined in module sqlalchemy.sql.expression, specifically Table and Column. Since these objects are part of the SQL expression language, they are usable as components in SQL expressions.
A table- or column-level CHECK constraint.
Can be included in the definition of a Table or Column.
Construct a CHECK constraint.
Represent a column in a database table.
This is a subclass of expression.ColumnClause and represents an actual existing table in the database, in a similar fashion as TableClause/Table.
Construct a new Column object.
Arguments are:
Keyword arguments include:
Defaults to the column name: a Python-only alias name for this column.
The column will then be identified everywhere in an application, including the column list on its Table, by this key, and not the given name. Generated SQL, however, will still reference the column by its actual name.
A plain default value on a column.
This could correspond to a constant, a callable function, or a SQL clause.
A table-level SQL constraint, such as a KEY.
Implements a hybrid of dict/setlike behavior with regards to the list of underying columns.
Create a SQL constraint.
A literal DDL statement.
Specifies literal SQL DDL to be executed by the database. DDL objects can be attached to Tables or MetaData instances, conditionally executing SQL as part of the DDL lifecycle of those schema items. Basic templating support allows a single DDL instance to handle repetitive tasks for multiple tables.
Examples:
tbl = Table('users', metadata, Column('uid', Integer)) # ... DDL('DROP TRIGGER users_trigger').execute_at('before-create', tbl) spow = DDL('ALTER TABLE %(table)s SET secretpowers TRUE', on='somedb') spow.execute_at('after-create', tbl) drop_spow = DDL('ALTER TABLE users SET secretpowers FALSE') connection.execute(drop_spow)
Create a DDL 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.
Optional filtering criteria. May be a string or a callable predicate. If a string, it will be compared to the name of the executing database dialect:
DDL('something', on='postgres')
If a callable, it will be invoked with three positional arguments:
- event
- The name of the event that has triggered this DDL, such as 'after-create' Will be None if the DDL is executed explicitly.
- schema_item
- A SchemaItem instance, such as Table or MetaData. May be None if the DDL is executed explicitly.
- connection
- The Connection being used for DDL execution
If the callable returns a true value, the DDL statement will be executed.
An Engine or Connection to which this DDL is bound.
This property may be assigned an Engine or Connection, or assigned a string or URL to automatically create a basic Engine for this bind with create_engine().
Execute this DDL immediately.
Executes the DDL statement in isolation using the supplied Connectable or 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.
Link execution of this DDL to the DDL lifecycle of a SchemaItem.
Links this DDL 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.
When operating on Table events, the following additional statement string substitions 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 substutions noted above. Keys present in the context will override the standard substitutions.
A DDL instance can be linked to any number of schema items. The statement subsitution support allows for DDL instances to be used in a template fashion.
execute_at builds on the append_ddl_listener interface of MetaDta and 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.
Defines a column-level FOREIGN KEY constraint between two columns.
ForeignKey is specified as an argument to a Column object.
For a composite (multiple column) FOREIGN KEY, use a ForeignKeyConstraint within the Table definition.
Construct a column-level FOREIGN KEY.
Return the column in the given table referenced by this ForeignKey.
Returns None if this ForeignKey does not reference the given table.
A table-level FOREIGN KEY constraint.
Defines a single column or composite FOREIGN KEY ... REFERENCES constraint. For a no-frills, single column foreign key, adding a ForeignKey to the definition of a Column is a shorthand equivalent for an unnamed, single column ForeignKeyConstraint.
Construct a composite-capable FOREIGN KEY.
A table-level INDEX.
Defines a composite (one or more column) INDEX. For a no-frills, single column index, adding index=True to the Column definition is a shorthand equivalent for an unnamed, single column Index.
Construct an index object.
Arguments are:
Keyword arguments include:
A collection of Tables and their associated schema constructs.
Holds a collection of Tables and an optional binding to an Engine or Connection. If bound, the Table objects in the collection and their columns may participate in implicit SQL execution.
The bind property may be assigned to dynamically. A common pattern is to start unbound and then bind later when an engine is available:
metadata = MetaData() # define tables Table('mytable', metadata, ...) # connect to an engine later, perhaps after loading a URL from a # configuration file metadata.bind = an_engine
MetaData is a thread-safe object after tables have been explicitly defined or loaded via reflection.
Create a new MetaData object.
Append a DDL event listener to this MetaData.
The listener callable will be triggered when this MetaData is involved in DDL creates or drops, and will be invoked either before all Table-related actions or after.
Arguments are:
A callable, invoked with three positional arguments:
Listeners are added to the MetaData's ddl_listeners attribute.
Note: MetaData listeners are invoked even when Tables are created in isolation. This may change in a future release. I.e.:
# triggers all MetaData and Table listeners: metadata.create_all() # triggers MetaData listeners too: some.table.create()
An Engine or Connection to which this MetaData is bound.
This property may be assigned an Engine or Connection, or assigned a string or URL to automatically create a basic Engine for this bind with create_engine().
Deprecated. Bind this MetaData to an Engine.
Use metadata.bind = <engine> or metadata.bind = <url>.
- bind
- A string, URL, Engine or Connection instance. If a string or URL, will be passed to create_engine() along with \**kwargs to produce the engine which to connect to. Otherwise connects directly to the given Engine.
Create all tables stored in this metadata.
Conditional by default, will not attempt to recreate tables already present in the target database.
Drop all tables stored in this metadata.
Conditional by default, will not attempt to drop tables not present in the target database.
Load all available table definitions from the database.
Automatically creates Table entries in this MetaData for any table available in the database but not yet present in the MetaData. May be called multiple times to pick up tables recently added to the database, however no special action is taken if a table in this MetaData no longer exists in the database.
Optional. Load only a sub-set of available named tables. May be specified as a sequence of names or a callable.
If a sequence of names is provided, only those tables will be reflected. An error is raised if a table is requested but not available. Named tables already present in this MetaData are ignored.
If a callable is provided, it will be used as a boolean predicate to filter the list of potential table names. The callable is called with a table name and this MetaData instance as positional arguments and should return a true value for any table to reflect.
A default that takes effect on the database side.
A table-level PRIMARY KEY constraint.
Defines a single column or composite PRIMARY KEY constraint. For a no-frills primary key, adding primary_key=True to one or more Column definitions is a shorthand equivalent for an unnamed single- or multiple-column PrimaryKeyConstraint.
Construct a composite-capable PRIMARY KEY.
Represents a named database sequence.
Construct a new Sequence.
Represent a relational database table.
Construct a Table.
Table objects can be constructed directly. Arguments are:
The name of this table, exactly as it appears, or will appear, in the database.
This property, along with the schema, indicates the singleton identity of this table.
Further tables constructed with the same name/schema combination will return the same Table instance.
kwargs include:
Append a DDL event listener to this Table.
The listener callable will be triggered when this Table is created or dropped, either directly before or after the DDL is issued to the database. The listener may modify the Table, but may not abort the event itself.
Arguments are:
A callable, invoked with three positional arguments:
Listeners are added to the Table's ddl_listeners attribute.
Issue a CREATE statement for this table.
See also metadata.create_all().
Issue a DROP statement for this table.
See also metadata.drop_all().
Return a copy of this Table associated with a different MetaData.
A MetaData variant that presents a different bind in every thread.
Makes the bind property of the MetaData a thread-local value, allowing this collection of tables to be bound to different Engine implementations or connections in each thread.
The ThreadLocalMetaData starts off bound to None in each thread. Binds must be made explicitly by assigning to the bind property or using connect(). You can also re-bind dynamically multiple times per thread, just like a regular MetaData.
Use this type of MetaData when your tables are present in more than one database and you need to address them simultanesouly.
The bound Engine or Connection for this thread.
This property may be assigned an Engine or Connection, or assigned a string or URL to automatically create a basic Engine for this bind with create_engine().
Deprecated. Bind to an Engine in the caller's thread.
Use metadata.bind=<engine> or metadata.bind=<url>.
- bind
- A string, URL, Engine or Connection instance. If a string or URL, will be passed to create_engine() along with \**kwargs to produce the engine which to connect to. Otherwise connects directly to the given Engine.
A table-level UNIQUE constraint.
Defines a single column or composite UNIQUE constraint. For a no-frills, single column constraint, adding unique=True to the Column definition is a shorthand equivalent for an unnamed, single column UniqueConstraint.
Construct a UNIQUE constraint.