Reflecting Database Objects¶

A Table object can be instructed to load information about itself from the corresponding database schema object already existing within the database. This process is called reflection. In the most simple case you need only specify the table name, a MetaData object, and the autoload=True flag. If the MetaData is not persistently bound, also add the autoload_with argument:

>>> messages = Table('messages', meta, autoload=True, autoload_with=engine)
>>> [c.name for c in messages.columns]
['message_id', 'message_name', 'date']

The above operation will use the given engine to query the database for information about the messages table, and will then generate Column, ForeignKey, and other objects corresponding to this information as though the Table object were hand-constructed in Python.

When tables are reflected, if a given table references another one via foreign key, a second Table object is created within the MetaData object representing the connection. Below, assume the table shopping_cart_items references a table named shopping_carts. Reflecting the shopping_cart_items table has the effect such that the shopping_carts table will also be loaded:

>>> shopping_cart_items = Table('shopping_cart_items', meta, autoload=True, autoload_with=engine)
>>> 'shopping_carts' in meta.tables:
True

The MetaData has an interesting “singleton-like” behavior such that if you requested both tables individually, MetaData will ensure that exactly one Table object is created for each distinct table name. The Table constructor actually returns to you the already-existing Table object if one already exists with the given name. Such as below, we can access the already generated shopping_carts table just by naming it:

shopping_carts = Table('shopping_carts', meta)

Of course, it’s a good idea to use autoload=True with the above table regardless. This is so that the table’s attributes will be loaded if they have not been already. The autoload operation only occurs for the table if it hasn’t already been loaded; once loaded, new calls to Table with the same name will not re-issue any reflection queries.

Overriding Reflected Columns¶

Individual columns can be overridden with explicit values when reflecting tables; this is handy for specifying custom datatypes, constraints such as primary keys that may not be configured within the database, etc.:

>>> mytable = Table('mytable', meta,
... Column('id', Integer, primary_key=True),   # override reflected 'id' to have primary key
... Column('mydata', Unicode(50)),    # override reflected 'mydata' to be Unicode
... # additional Column objects which require no change are reflected normally
... autoload_with=some_engine)

Reflecting Views¶

The reflection system can also reflect views. Basic usage is the same as that of a table:

my_view = Table("some_view", metadata, autoload=True)

Above, my_view is a Table object with Column objects representing the names and types of each column within the view “some_view”.

Usually, it’s desired to have at least a primary key constraint when reflecting a view, if not foreign keys as well. View reflection doesn’t extrapolate these constraints.

Use the “override” technique for this, specifying explicitly those columns which are part of the primary key or have foreign key constraints:

my_view = Table("some_view", metadata,
                Column("view_id", Integer, primary_key=True),
                Column("related_thing", Integer, ForeignKey("othertable.thing_id")),
                autoload=True
)

Reflecting All Tables at Once¶

The MetaData object can also get a listing of tables and reflect the full set. This is achieved by using the reflect() method. After calling it, all located tables are present within the MetaData object’s dictionary of tables:

meta = MetaData()
meta.reflect(bind=someengine)
users_table = meta.tables['users']
addresses_table = meta.tables['addresses']

metadata.reflect() also provides a handy way to clear or delete all the rows in a database:

meta = MetaData()
meta.reflect(bind=someengine)
for table in reversed(meta.sorted_tables):
    someengine.execute(table.delete())

Fine Grained Reflection with Inspector¶

A low level interface which provides a backend-agnostic system of loading lists of schema, table, column, and constraint descriptions from a given database is also available. This is known as the “Inspector”:

from sqlalchemy import create_engine
from sqlalchemy.engine import reflection
engine = create_engine('...')
insp = reflection.Inspector.from_engine(engine)
print(insp.get_table_names())

class sqlalchemy.engine.reflection.Inspector(bind)¶

Performs database schema inspection.

The Inspector acts as a proxy to the reflection methods of the Dialect, providing a consistent interface as well as caching support for previously fetched metadata.

A Inspector object is usually created via the inspect() function:

from sqlalchemy import inspect, create_engine
engine = create_engine('...')
insp = inspect(engine)

The inspection method above is equivalent to using the Inspector.from_engine() method, i.e.:

engine = create_engine('...')
insp = Inspector.from_engine(engine)

Where above, the Dialect may opt to return an Inspector subclass that provides additional methods specific to the dialect’s target database.

__init__(bind)¶

Initialize a new Inspector.

Parameters:	bind¶ – a `Connectable`, which is typically an instance of `Engine` or `Connection`.

For a dialect-specific instance of Inspector, see Inspector.from_engine()

default_schema_name¶

Return the default schema name presented by the dialect for the current engine’s database user.

E.g. this is typically public for PostgreSQL and dbo for SQL Server.

classmethod from_engine(bind)¶

Construct a new dialect-specific Inspector object from the given engine or connection.

Parameters:	bind¶ – a `Connectable`, which is typically an instance of `Engine` or `Connection`.

This method differs from direct a direct constructor call of Inspector in that the Dialect is given a chance to provide a dialect-specific Inspector instance, which may provide additional methods.

See the example at Inspector.

get_check_constraints(table_name, schema=None, **kw)¶

Return information about check constraints in table_name.

Given a string table_name and an optional string schema, return check constraint information as a list of dicts with these keys:

name: the check constraint’s name
sqltext: the check constraint’s SQL expression
dialect_options: may or may not be present; a dictionary with additional dialect-specific options for this CHECK constraint

New in version 1.3.8.

Parameters:	table_name¶ – string name of the table. For special quoting, use `quoted_name`. schema¶ – string schema name; if omitted, uses the default schema of the database connection. For special quoting, use `quoted_name`.

New in version 1.1.0.

get_columns(table_name, schema=None, **kw)¶

Return information about columns in table_name.

Given a string table_name and an optional string schema, return column information as a list of dicts with these keys:

name - the column’s name
type - the type of this column; an instance of TypeEngine
nullable - boolean flag if the column is NULL or NOT NULL
default - the column’s server default value - this is returned as a string SQL expression.
attrs - dict containing optional column attributes

Parameters:	table_name¶ – string name of the table. For special quoting, use `quoted_name`. schema¶ – string schema name; if omitted, uses the default schema of the database connection. For special quoting, use `quoted_name`.
Returns:	list of dictionaries, each representing the definition of a database column.

get_foreign_keys(table_name, schema=None, **kw)¶

Return information about foreign_keys in table_name.

Given a string table_name, and an optional string schema, return foreign key information as a list of dicts with these keys:

constrained_columns: a list of column names that make up the foreign key
referred_schema: the name of the referred schema
referred_table: the name of the referred table
referred_columns: a list of column names in the referred table that correspond to constrained_columns
name: optional name of the foreign key constraint.

Parameters:	table_name¶ – string name of the table. For special quoting, use `quoted_name`. schema¶ – string schema name; if omitted, uses the default schema of the database connection. For special quoting, use `quoted_name`.

get_indexes(table_name, schema=None, **kw)¶

Return information about indexes in table_name.

Given a string table_name and an optional string schema, return index information as a list of dicts with these keys:

name: the index’s name
column_names: list of column names in order
unique: boolean
column_sorting: optional dict mapping column names to tuple of sort keywords, which may include asc, desc, nullsfirst, nullslast.

New in version 1.3.5.
dialect_options: dict of dialect-specific index options. May not be present for all dialects.

New in version 1.0.0.

Parameters:	table_name¶ – string name of the table. For special quoting, use `quoted_name`. schema¶ – string schema name; if omitted, uses the default schema of the database connection. For special quoting, use `quoted_name`.

get_pk_constraint(table_name, schema=None, **kw)¶

Return information about primary key constraint on table_name.

Given a string table_name, and an optional string schema, return primary key information as a dictionary with these keys:

constrained_columns: a list of column names that make up the primary key
name: optional name of the primary key constraint.

Parameters:	table_name¶ – string name of the table. For special quoting, use `quoted_name`. schema¶ – string schema name; if omitted, uses the default schema of the database connection. For special quoting, use `quoted_name`.

get_primary_keys(table_name, schema=None, **kw)¶

Return information about primary keys in table_name.

Deprecated since version 0.7: The Inspector.get_primary_keys() method is deprecated and will be removed in a future release. Please refer to the Inspector.get_pk_constraint() method.

Given a string table_name, and an optional string schema, return primary key information as a list of column names.

get_schema_names()¶: Return all schema names.

get_sorted_table_and_fkc_names(schema=None)¶

Return dependency-sorted table and foreign key constraint names in referred to within a particular schema.

This will yield 2-tuples of (tablename, [(tname, fkname), (tname, fkname), ...]) consisting of table names in CREATE order grouped with the foreign key constraint names that are not detected as belonging to a cycle. The final element will be (None, [(tname, fkname), (tname, fkname), ..]) which will consist of remaining foreign key constraint names that would require a separate CREATE step after-the-fact, based on dependencies between tables.

New in version 1.0.-.

Limitations of Reflection¶

It’s important to note that the reflection process recreates Table metadata using only information which is represented in the relational database. This process by definition cannot restore aspects of a schema that aren’t actually stored in the database. State which is not available from reflection includes but is not limited to:

Client side defaults, either Python functions or SQL expressions defined using the default keyword of Column (note this is separate from server_default, which specifically is what’s available via reflection).
Column information, e.g. data that might have been placed into the Column.info dictionary
The value of the .quote setting for Column or Table
The association of a particular Sequence with a given Column

The relational database also in many cases reports on table metadata in a different format than what was specified in SQLAlchemy. The Table objects returned from reflection cannot be always relied upon to produce the identical DDL as the original Python-defined Table objects. Areas where this occurs includes server defaults, column-associated sequences and various idiosyncrasies regarding constraints and datatypes. Server side defaults may be returned with cast directives (typically PostgreSQL will include a ::<type> cast) or different quoting patterns than originally specified.

Another category of limitation includes schema structures for which reflection is only partially or not yet defined. Recent improvements to reflection allow things like views, indexes and foreign key options to be reflected. As of this writing, structures like CHECK constraints, table comments, and triggers are not reflected.

SQLAlchemy 1.3 Documentation