SQLAlchemy 1.1 Documentation
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
... autoload=True)
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 theinspect()
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 anInspector
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 ofEngine
orConnection
.For a dialect-specific instance of
Inspector
, seeInspector.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 anddbo
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 ofEngine
orConnection
.This method differs from direct a direct constructor call of
Inspector
in that theDialect
is given a chance to provide a dialect-specificInspector
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
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
TypeEngine
- nullable
- boolean
- default
- the column’s default value
- 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
.
-
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
- 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: Call to deprecated method get_primary_keys. Use get_pk_constraint instead.
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.-.
See also
sort_tables_and_constraints()
- similar method which works- with an already-given
MetaData
.
-
get_table_names
(schema=None, order_by=None)¶ Return all table names in referred to within a particular schema.
The names are expected to be real tables only, not views. Views are instead returned using the
Inspector.get_view_names()
method.Parameters: - schema¶ – Schema name. If
schema
is left atNone
, the database’s default schema is used, else the named schema is searched. If the database does not support named schemas, behavior is undefined ifschema
is not passed asNone
. For special quoting, usequoted_name
. - order_by¶ –
Optional, may be the string “foreign_key” to sort the result on foreign key dependencies. Does not automatically resolve cycles, and will raise
CircularDependencyError
if cycles exist.Deprecated since version 1.0.0: - see
Inspector.get_sorted_table_and_fkc_names()
for a version of this which resolves foreign key cycles between tables automatically.Changed in version 0.8: the “foreign_key” sorting sorts tables in order of dependee to dependent; that is, in creation order, rather than in drop order. This is to maintain consistency with similar features such as
MetaData.sorted_tables
andutil.sort_tables()
.
- schema¶ – Schema name. If
-
get_table_options
(table_name, schema=None, **kw)¶ Return a dictionary of options specified when the table of the given name was created.
This currently includes some options that apply to MySQL tables.
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
.
- table_name¶ – string name of the table. For special quoting, use
-
get_temp_table_names
()¶ return a list of temporary table names for the current bind.
This method is unsupported by most dialects; currently only SQLite implements it.
New in version 1.0.0.
-
get_temp_view_names
()¶ return a list of temporary view names for the current bind.
This method is unsupported by most dialects; currently only SQLite implements it.
New in version 1.0.0.
-
get_unique_constraints
(table_name, schema=None, **kw)¶ Return information about unique constraints in table_name.
Given a string table_name and an optional string schema, return unique constraint information as a list of dicts with these keys:
- name
- the unique constraint’s name
- column_names
- list of column names in order
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 0.8.4.
-
get_view_definition
(view_name, schema=None)¶ Return definition for view_name.
Parameters: schema¶ – Optional, retrieve names from a non-default schema. For special quoting, use quoted_name
.
-
get_view_names
(schema=None)¶ Return all view names in schema.
Parameters: schema¶ – Optional, retrieve names from a non-default schema. For special quoting, use quoted_name
.
-
reflecttable
(table, include_columns, exclude_columns=())¶ Given a Table object, load its internal constructs based on introspection.
This is the underlying method used by most dialects to produce table reflection. Direct usage is like:
from sqlalchemy import create_engine, MetaData, Table from sqlalchemy.engine import reflection engine = create_engine('...') meta = MetaData() user_table = Table('user', meta) insp = Inspector.from_engine(engine) insp.reflecttable(user_table, None)
Parameters:
-
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 ofColumn
(note this is separate fromserver_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 forColumn
orTable
- The association of a particular
Sequence
with a givenColumn
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 idosyncrasies 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.