SQLAlchemy 2.0 Documentation
SQLAlchemy Core
- SQL Statements and Expressions API
- Schema Definition Language
- Describing Databases with MetaData
- Reflecting Database Objects
- Column INSERT/UPDATE Defaults¶
- Scalar Defaults
- Python-Executed Functions
- Client-Invoked SQL Expressions
- Server-invoked DDL-Explicit Default Expressions
- Marking Implicitly Generated Values, timestamps, and Triggered Columns
- Defining Sequences
- Computed Columns (GENERATED ALWAYS AS)
- Identity Columns (GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY)
- Default Objects API
- Defining Constraints and Indexes
- Customizing DDL
- SQL Datatype Objects
- Engine and Connection Use
- Core API Basics
Project Versions
- Previous: Reflecting Database Objects
- Next: Defining Constraints and Indexes
- Up: Home
- On this page:
- Column INSERT/UPDATE Defaults
- Scalar Defaults
- Python-Executed Functions
- Client-Invoked SQL Expressions
- Server-invoked DDL-Explicit Default Expressions
- Marking Implicitly Generated Values, timestamps, and Triggered Columns
- Defining Sequences
- Computed Columns (GENERATED ALWAYS AS)
- Identity Columns (GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY)
- Default Objects API
Column INSERT/UPDATE Defaults¶
Column INSERT and UPDATE defaults refer to functions that create a default value for a particular column in a row as an INSERT or UPDATE statement is proceeding against that row, in the case where no value was provided to the INSERT or UPDATE statement for that column. That is, if a table has a column called “timestamp”, and an INSERT statement proceeds which does not include a value for this column, an INSERT default would create a new value, such as the current time, that is used as the value to be INSERTed into the “timestamp” column. If the statement does include a value for this column, then the default does not take place.
Column defaults can be server-side functions or constant values which are defined in the database along with the schema in DDL, or as SQL expressions which are rendered directly within an INSERT or UPDATE statement emitted by SQLAlchemy; they may also be client-side Python functions or constant values which are invoked by SQLAlchemy before data is passed to the database.
Note
A column default handler should not be confused with a construct that intercepts and modifies incoming values for INSERT and UPDATE statements which are provided to the statement as it is invoked. This is known as data marshalling, where a column value is modified in some way by the application before being sent to the database. SQLAlchemy provides a few means of achieving this which include using custom datatypes, SQL execution events and in the ORM custom validators as well as attribute events. Column defaults are only invoked when there is no value present for a column in a SQL DML statement.
SQLAlchemy provides an array of features regarding default generation functions which take place for non-present values during INSERT and UPDATE statements. Options include:
Scalar values used as defaults during INSERT and UPDATE operations
Python functions which execute upon INSERT and UPDATE operations
SQL expressions which are embedded in INSERT statements (or in some cases execute beforehand)
SQL expressions which are embedded in UPDATE statements
Server side default values used during INSERT
Markers for server-side triggers used during UPDATE
The general rule for all insert/update defaults is that they only take effect
if no value for a particular column is passed as an execute()
parameter;
otherwise, the given value is used.
Scalar Defaults¶
The simplest kind of default is a scalar value used as the default value of a column:
Table("mytable", metadata_obj, Column("somecolumn", Integer, default=12))
Above, the value “12” will be bound as the column value during an INSERT if no other value is supplied.
A scalar value may also be associated with an UPDATE statement, though this is not very common (as UPDATE statements are usually looking for dynamic defaults):
Table("mytable", metadata_obj, Column("somecolumn", Integer, onupdate=25))
Python-Executed Functions¶
The Column.default
and Column.onupdate
keyword arguments also accept Python
functions. These functions are invoked at the time of insert or update if no
other value for that column is supplied, and the value returned is used for
the column’s value. Below illustrates a crude “sequence” that assigns an
incrementing counter to a primary key column:
# a function which counts upwards
i = 0
def mydefault():
global i
i += 1
return i
t = Table(
"mytable",
metadata_obj,
Column("id", Integer, primary_key=True, default=mydefault),
)
It should be noted that for real “incrementing sequence” behavior, the
built-in capabilities of the database should normally be used, which may
include sequence objects or other autoincrementing capabilities. For primary
key columns, SQLAlchemy will in most cases use these capabilities
automatically. See the API documentation for
Column
including the Column.autoincrement
flag, as
well as the section on Sequence
later in this
chapter for background on standard primary key generation techniques.
To illustrate onupdate, we assign the Python datetime
function now
to
the Column.onupdate
attribute:
import datetime
t = Table(
"mytable",
metadata_obj,
Column("id", Integer, primary_key=True),
# define 'last_updated' to be populated with datetime.now()
Column("last_updated", DateTime, onupdate=datetime.datetime.now),
)
When an update statement executes and no value is passed for last_updated
,
the datetime.datetime.now()
Python function is executed and its return
value used as the value for last_updated
. Notice that we provide now
as the function itself without calling it (i.e. there are no parenthesis
following) - SQLAlchemy will execute the function at the time the statement
executes.
Context-Sensitive Default Functions¶
The Python functions used by Column.default
and
Column.onupdate
may also make use of the current statement’s
context in order to determine a value. The context of a statement is an
internal SQLAlchemy object which contains all information about the statement
being executed, including its source expression, the parameters associated with
it and the cursor. The typical use case for this context with regards to
default generation is to have access to the other values being inserted or
updated on the row. To access the context, provide a function that accepts a
single context
argument:
def mydefault(context):
return context.get_current_parameters()["counter"] + 12
t = Table(
"mytable",
metadata_obj,
Column("counter", Integer),
Column("counter_plus_twelve", Integer, default=mydefault, onupdate=mydefault),
)
The above default generation function is applied so that it will execute for
all INSERT and UPDATE statements where a value for counter_plus_twelve
was
otherwise not provided, and the value will be that of whatever value is present
in the execution for the counter
column, plus the number 12.
For a single statement that is being executed using “executemany” style, e.g.
with multiple parameter sets passed to Connection.execute()
, the
user-defined function is called once for each set of parameters. For the use case of
a multi-valued Insert
construct (e.g. with more than one VALUES
clause set up via the Insert.values()
method), the user-defined function
is also called once for each set of parameters.
When the function is invoked, the special method
DefaultExecutionContext.get_current_parameters()
is available from
the context object (an subclass of DefaultExecutionContext
). This
method returns a dictionary of column-key to values that represents the
full set of values for the INSERT or UPDATE statement. In the case of a
multi-valued INSERT construct, the subset of parameters that corresponds to
the individual VALUES clause is isolated from the full parameter dictionary
and returned alone.
New in version 1.2: Added DefaultExecutionContext.get_current_parameters()
method,
which improves upon the still-present
DefaultExecutionContext.current_parameters
attribute
by offering the service of organizing multiple VALUES clauses
into individual parameter dictionaries.
Client-Invoked SQL Expressions¶
The Column.default
and Column.onupdate
keywords may
also be passed SQL expressions, which are in most cases rendered inline within the
INSERT or UPDATE statement:
t = Table(
"mytable",
metadata_obj,
Column("id", Integer, primary_key=True),
# define 'create_date' to default to now()
Column("create_date", DateTime, default=func.now()),
# define 'key' to pull its default from the 'keyvalues' table
Column(
"key",
String(20),
default=select(keyvalues.c.key).where(keyvalues.c.type="type1"),
),
# define 'last_modified' to use the current_timestamp SQL function on update
Column("last_modified", DateTime, onupdate=func.utc_timestamp()),
)
Above, the create_date
column will be populated with the result of the
now()
SQL function (which, depending on backend, compiles into NOW()
or CURRENT_TIMESTAMP
in most cases) during an INSERT statement, and the
key
column with the result of a SELECT subquery from another table. The
last_modified
column will be populated with the value of
the SQL UTC_TIMESTAMP()
MySQL function when an UPDATE statement is
emitted for this table.
Note
When using SQL functions with the func
construct, we “call” the
named function, e.g. with parenthesis as in func.now()
. This differs
from when we specify a Python callable as a default such as
datetime.datetime
, where we pass the function itself, but we don’t
invoke it ourselves. In the case of a SQL function, invoking
func.now()
returns the SQL expression object that will render the
“NOW” function into the SQL being emitted.
Default and update SQL expressions specified by Column.default
and
Column.onupdate
are invoked explicitly by SQLAlchemy when an
INSERT or UPDATE statement occurs, typically rendered inline within the DML
statement except in certain cases listed below. This is different than a
“server side” default, which is part of the table’s DDL definition, e.g. as
part of the “CREATE TABLE” statement, which are likely more common. For
server side defaults, see the next section Server-invoked DDL-Explicit Default Expressions.
When a SQL expression indicated by Column.default
is used with
primary key columns, there are some cases where SQLAlchemy must “pre-execute”
the default generation SQL function, meaning it is invoked in a separate SELECT
statement, and the resulting value is passed as a parameter to the INSERT.
This only occurs for primary key columns for an INSERT statement that is being
asked to return this primary key value, where RETURNING or cursor.lastrowid
may not be used. An Insert
construct that specifies the
insert.inline
flag will always render default expressions
inline.
When the statement is executed with a single set of parameters (that is, it is
not an “executemany” style execution), the returned
CursorResult
will contain a collection accessible
via CursorResult.postfetch_cols()
which contains a list of all
Column
objects which had an inline-executed
default. Similarly, all parameters which were bound to the statement, including
all Python and SQL expressions which were pre-executed, are present in the
CursorResult.last_inserted_params()
or
CursorResult.last_updated_params()
collections on
CursorResult
. The
CursorResult.inserted_primary_key
collection contains a list of primary
key values for the row inserted (a list so that single-column and
composite-column primary keys are represented in the same format).
Server-invoked DDL-Explicit Default Expressions¶
A variant on the SQL expression default is the Column.server_default
, which gets
placed in the CREATE TABLE statement during a Table.create()
operation:
t = Table(
"test",
metadata_obj,
Column("abc", String(20), server_default="abc"),
Column("created_at", DateTime, server_default=func.sysdate()),
Column("index_value", Integer, server_default=text("0")),
)
A create call for the above table will produce:
CREATE TABLE test (
abc varchar(20) default 'abc',
created_at datetime default sysdate,
index_value integer default 0
)
The above example illustrates the two typical use cases for Column.server_default
,
that of the SQL function (SYSDATE in the above example) as well as a server-side constant
value (the integer “0” in the above example). It is advisable to use the
text()
construct for any literal SQL values as opposed to passing the
raw value, as SQLAlchemy does not typically perform any quoting or escaping on
these values.
Like client-generated expressions, Column.server_default
can accommodate
SQL expressions in general, however it is expected that these will usually be simple
functions and expressions, and not the more complex cases like an embedded SELECT.
Marking Implicitly Generated Values, timestamps, and Triggered Columns¶
Columns which generate a new value on INSERT or UPDATE based on other
server-side database mechanisms, such as database-specific auto-generating
behaviors such as seen with TIMESTAMP columns on some platforms, as well as
custom triggers that invoke upon INSERT or UPDATE to generate a new value,
may be called out using FetchedValue
as a marker:
from sqlalchemy.schema import FetchedValue
t = Table(
"test",
metadata_obj,
Column("id", Integer, primary_key=True),
Column("abc", TIMESTAMP, server_default=FetchedValue()),
Column("def", String(20), server_onupdate=FetchedValue()),
)
The FetchedValue
indicator does not affect the rendered DDL for the
CREATE TABLE. Instead, it marks the column as one that will have a new value
populated by the database during the process of an INSERT or UPDATE statement,
and for supporting databases may be used to indicate that the column should be
part of a RETURNING or OUTPUT clause for the statement. Tools such as the
SQLAlchemy ORM then make use of this marker in order to know how to get at the
value of the column after such an operation. In particular, the
ValuesBase.return_defaults()
method can be used with an Insert
or Update
construct to indicate that these values should be
returned.
For details on using FetchedValue
with the ORM, see
Fetching Server-Generated Defaults.
Warning
The Column.server_onupdate
directive
does not currently produce MySQL’s
“ON UPDATE CURRENT_TIMESTAMP()” clause. See
Rendering ON UPDATE CURRENT TIMESTAMP for MySQL / MariaDB’s explicit_defaults_for_timestamp for background on how to produce
this clause.
See also
Defining Sequences¶
SQLAlchemy represents database sequences using the
Sequence
object, which is considered to be a
special case of “column default”. It only has an effect on databases which have
explicit support for sequences, which among SQLAlchemy’s included dialects
includes PostgreSQL, Oracle, MS SQL Server, and MariaDB. The
Sequence
object is otherwise ignored.
Tip
In newer database engines, the Identity
construct should likely
be preferred vs. Sequence
for generation of integer primary key
values. See the section Identity Columns (GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY) for background on this
construct.
The Sequence
may be placed on any column as a
“default” generator to be used during INSERT operations, and can also be
configured to fire off during UPDATE operations if desired. It is most
commonly used in conjunction with a single integer primary key column:
table = Table(
"cartitems",
metadata_obj,
Column(
"cart_id",
Integer,
Sequence("cart_id_seq", start=1),
primary_key=True,
),
Column("description", String(40)),
Column("createdate", DateTime()),
)
Where above, the table cartitems
is associated with a sequence named
cart_id_seq
. Emitting MetaData.create_all()
for the above
table will include:
CREATE SEQUENCE cart_id_seq START WITH 1
CREATE TABLE cartitems (
cart_id INTEGER NOT NULL,
description VARCHAR(40),
createdate TIMESTAMP WITHOUT TIME ZONE,
PRIMARY KEY (cart_id)
)
Tip
When using tables with explicit schema names (detailed at
Specifying the Schema Name), the configured schema of the Table
is not automatically shared by an embedded Sequence
, instead,
specify Sequence.schema
:
Sequence("cart_id_seq", start=1, schema="some_schema")
The Sequence
may also be made to automatically make use of the
MetaData.schema
setting on the MetaData
in use;
see Associating a Sequence with the MetaData for background.
When Insert
DML constructs are invoked against the cartitems
table, without an explicit value passed for the cart_id
column, the
cart_id_seq
sequence will be used to generate a value on participating
backends. Typically, the sequence function is embedded in the INSERT statement,
which is combined with RETURNING so that the newly generated value can be
returned to the Python process:
INSERT INTO cartitems (cart_id, description, createdate)
VALUES (next_val(cart_id_seq), 'some description', '2015-10-15 12:00:15')
RETURNING cart_id
When using Connection.execute()
to invoke an Insert
construct,
newly generated primary key identifiers, including but not limited to those
generated using Sequence
, are available from the CursorResult
construct using the CursorResult.inserted_primary_key
attribute.
When the Sequence
is associated with a
Column
as its Python-side default generator, the
Sequence
will also be subject to “CREATE SEQUENCE” and “DROP
SEQUENCE” DDL when similar DDL is emitted for the owning Table
,
such as when using MetaData.create_all()
to generate DDL for a series
of tables.
The Sequence
may also be associated with a
MetaData
construct directly. This allows the Sequence
to be used in more than one Table
at a time and also allows the
MetaData.schema
parameter to be inherited. See the section
Associating a Sequence with the MetaData for background.
Associating a Sequence on a SERIAL column¶
PostgreSQL’s SERIAL datatype is an auto-incrementing type that implies
the implicit creation of a PostgreSQL sequence when CREATE TABLE is emitted.
The Sequence
construct, when indicated for a Column
,
may indicate that it should not be used in this specific case by specifying
a value of True
for the Sequence.optional
parameter.
This allows the given Sequence
to be used for backends that have no
alternative primary key generation system but to ignore it for backends
such as PostgreSQL which will automatically generate a sequence for a particular
column:
table = Table(
"cartitems",
metadata_obj,
Column(
"cart_id",
Integer,
# use an explicit Sequence where available, but not on
# PostgreSQL where SERIAL will be used
Sequence("cart_id_seq", start=1, optional=True),
primary_key=True,
),
Column("description", String(40)),
Column("createdate", DateTime()),
)
In the above example, CREATE TABLE
for PostgreSQL will make use of the
SERIAL
datatype for the cart_id
column, and the cart_id_seq
sequence will be ignored. However on Oracle, the cart_id_seq
sequence
will be created explicitly.
Tip
This particular interaction of SERIAL and SEQUENCE is fairly legacy, and
as in other cases, using Identity
instead will simplify the
operation to simply use IDENTITY
on all supported backends.
Executing a Sequence Standalone¶
A SEQUENCE is a first class schema object in SQL and can be used to generate
values independently in the database. If you have a Sequence
object, it can be invoked with its “next value” instruction by
passing it directly to a SQL execution method:
with my_engine.connect() as conn:
seq = Sequence("some_sequence", start=1)
nextid = conn.execute(seq)
In order to embed the “next value” function of a Sequence
inside of a SQL statement like a SELECT or INSERT, use the Sequence.next_value()
method, which will render at statement compilation time a SQL function that is
appropriate for the target backend:
>>> my_seq = Sequence("some_sequence", start=1)
>>> stmt = select(my_seq.next_value())
>>> print(stmt.compile(dialect=postgresql.dialect()))
SELECT nextval('some_sequence') AS next_value_1
Associating a Sequence with the MetaData¶
For a Sequence
that is to be associated with arbitrary
Table
objects, the Sequence
may be associated with
a particular MetaData
, using the
Sequence.metadata
parameter:
seq = Sequence("my_general_seq", metadata=metadata_obj, start=1)
Such a sequence can then be associated with columns in the usual way:
table = Table(
"cartitems",
metadata_obj,
seq,
Column("description", String(40)),
Column("createdate", DateTime()),
)
In the above example, the Sequence
object is treated as an
independent schema construct that can exist on its own or be shared among
tables.
Explicitly associating the Sequence
with MetaData
allows for the following behaviors:
The
Sequence
will inherit theMetaData.schema
parameter specified to the targetMetaData
, which affects the production of CREATE / DROP DDL as well as how theSequence.next_value()
function is rendered in SQL statements.The
MetaData.create_all()
andMetaData.drop_all()
methods will emit CREATE / DROP for thisSequence
, even if theSequence
is not associated with anyTable
/Column
that’s a member of thisMetaData
.
Associating a Sequence as the Server Side Default¶
Note
The following technique is known to work only with the PostgreSQL database. It does not work with Oracle.
The preceding sections illustrate how to associate a Sequence
with a
Column
as the Python side default generator:
Column(
"cart_id",
Integer,
Sequence("cart_id_seq", metadata=metadata_obj, start=1),
primary_key=True,
)
In the above case, the Sequence
will automatically be subject
to CREATE SEQUENCE / DROP SEQUENCE DDL when the related Table
is subject to CREATE / DROP. However, the sequence will not be present
as the server-side default for the column when CREATE TABLE is emitted.
If we want the sequence to be used as a server-side default,
meaning it takes place even if we emit INSERT commands to the table from
the SQL command line, we can use the Column.server_default
parameter in conjunction with the value-generation function of the
sequence, available from the Sequence.next_value()
method. Below
we illustrate the same Sequence
being associated with the
Column
both as the Python-side default generator as well as
the server-side default generator:
cart_id_seq = Sequence("cart_id_seq", metadata=metadata_obj, start=1)
table = Table(
"cartitems",
metadata_obj,
Column(
"cart_id",
Integer,
cart_id_seq,
server_default=cart_id_seq.next_value(),
primary_key=True,
),
Column("description", String(40)),
Column("createdate", DateTime()),
)
or with the ORM:
class CartItem(Base):
__tablename__ = "cartitems"
cart_id_seq = Sequence("cart_id_seq", metadata=Base.metadata, start=1)
cart_id = Column(
Integer, cart_id_seq, server_default=cart_id_seq.next_value(), primary_key=True
)
description = Column(String(40))
createdate = Column(DateTime)
When the “CREATE TABLE” statement is emitted, on PostgreSQL it would be emitted as:
CREATE TABLE cartitems (
cart_id INTEGER DEFAULT nextval('cart_id_seq') NOT NULL,
description VARCHAR(40),
createdate TIMESTAMP WITHOUT TIME ZONE,
PRIMARY KEY (cart_id)
)
Placement of the Sequence
in both the Python-side and server-side
default generation contexts ensures that the “primary key fetch” logic
works in all cases. Typically, sequence-enabled databases also support
RETURNING for INSERT statements, which is used automatically by SQLAlchemy
when emitting this statement. However if RETURNING is not used for a particular
insert, then SQLAlchemy would prefer to “pre-execute” the sequence outside
of the INSERT statement itself, which only works if the sequence is
included as the Python-side default generator function.
The example also associates the Sequence
with the enclosing
MetaData
directly, which again ensures that the Sequence
is fully associated with the parameters of the MetaData
collection
including the default schema, if any.
See also
Sequences/SERIAL/IDENTITY - in the PostgreSQL dialect documentation
RETURNING Support - in the Oracle dialect documentation
Computed Columns (GENERATED ALWAYS AS)¶
New in version 1.3.11.
The Computed
construct allows a Column
to be declared in
DDL as a “GENERATED ALWAYS AS” column, that is, one which has a value that is
computed by the database server. The construct accepts a SQL expression
typically declared textually using a string or the text()
construct, in
a similar manner as that of CheckConstraint
. The SQL expression is
then interpreted by the database server in order to determine the value for the
column within a row.
Example:
from sqlalchemy import Table, Column, MetaData, Integer, Computed
metadata_obj = MetaData()
square = Table(
"square",
metadata_obj,
Column("id", Integer, primary_key=True),
Column("side", Integer),
Column("area", Integer, Computed("side * side")),
Column("perimeter", Integer, Computed("4 * side")),
)
The DDL for the square
table when run on a PostgreSQL 12 backend will look
like:
CREATE TABLE square (
id SERIAL NOT NULL,
side INTEGER,
area INTEGER GENERATED ALWAYS AS (side * side) STORED,
perimeter INTEGER GENERATED ALWAYS AS (4 * side) STORED,
PRIMARY KEY (id)
)
Whether the value is persisted upon INSERT and UPDATE, or if it is calculated
on fetch, is an implementation detail of the database; the former is known as
“stored” and the latter is known as “virtual”. Some database implementations
support both, but some only support one or the other. The optional
Computed.persisted
flag may be specified as True
or False
to indicate if the “STORED” or “VIRTUAL” keyword should be rendered in DDL,
however this will raise an error if the keyword is not supported by the target
backend; leaving it unset will use a working default for the target backend.
The Computed
construct is a subclass of the FetchedValue
object, and will set itself up as both the “server default” and “server
onupdate” generator for the target Column
, meaning it will be treated
as a default generating column when INSERT and UPDATE statements are generated,
as well as that it will be fetched as a generating column when using the ORM.
This includes that it will be part of the RETURNING clause of the database
for databases which support RETURNING and the generated values are to be
eagerly fetched.
Note
A Column
that is defined with the Computed
construct may not store any value outside of that which the server applies
to it; SQLAlchemy’s behavior when a value is passed for such a column
to be written in INSERT or UPDATE is currently that the value will be
ignored.
“GENERATED ALWAYS AS” is currently known to be supported by:
MySQL version 5.7 and onwards
MariaDB 10.x series and onwards
PostgreSQL as of version 12
Oracle - with the caveat that RETURNING does not work correctly with UPDATE (a warning will be emitted to this effect when the UPDATE..RETURNING that includes a computed column is rendered)
Microsoft SQL Server
SQLite as of version 3.31
When Computed
is used with an unsupported backend, if the target
dialect does not support it, a CompileError
is raised when attempting
to render the construct. Otherwise, if the dialect supports it but the
particular database server version in use does not, then a subclass of
DBAPIError
, usually OperationalError
, is raised when the
DDL is emitted to the database.
See also
Identity Columns (GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY)¶
New in version 1.4.
The Identity
construct allows a Column
to be declared
as an identity column and rendered in DDL as “GENERATED { ALWAYS | BY DEFAULT }
AS IDENTITY”. An identity column has its value automatically generated by the
database server using an incrementing (or decrementing) sequence. The construct
shares most of its option to control the database behaviour with
Sequence
.
Example:
from sqlalchemy import Table, Column, MetaData, Integer, Identity, String
metadata_obj = MetaData()
data = Table(
"data",
metadata_obj,
Column("id", Integer, Identity(start=42, cycle=True), primary_key=True),
Column("data", String),
)
The DDL for the data
table when run on a PostgreSQL 12 backend will look
like:
CREATE TABLE data (
id INTEGER GENERATED BY DEFAULT AS IDENTITY (START WITH 42 CYCLE) NOT NULL,
data VARCHAR,
PRIMARY KEY (id)
)
The database will generate a value for the id
column upon insert,
starting from 42
, if the statement did not already contain a value for
the id
column.
An identity column can also require that the database generates the value
of the column, ignoring the value passed with the statement or raising an
error, depending on the backend. To activate this mode, set the parameter
Identity.always
to True
in the
Identity
construct. Updating the previous
example to include this parameter will generate the following DDL:
CREATE TABLE data (
id INTEGER GENERATED ALWAYS AS IDENTITY (START WITH 42 CYCLE) NOT NULL,
data VARCHAR,
PRIMARY KEY (id)
)
The Identity
construct is a subclass of the FetchedValue
object, and will set itself up as the “server default” generator for the
target Column
, meaning it will be treated
as a default generating column when INSERT statements are generated,
as well as that it will be fetched as a generating column when using the ORM.
This includes that it will be part of the RETURNING clause of the database
for databases which support RETURNING and the generated values are to be
eagerly fetched.
The Identity
construct is currently known to be supported by:
PostgreSQL as of version 10.
Oracle as of version 12. It also supports passing
always=None
to enable the default generated mode and the parameteron_null=True
to specify “ON NULL” in conjunction with a “BY DEFAULT” identity column.Microsoft SQL Server. MSSQL uses a custom syntax that only supports the
start
andincrement
parameters, and ignores all other.
When Identity
is used with an unsupported backend, it is ignored,
and the default SQLAlchemy logic for autoincrementing columns is used.
An error is raised when a Column
specifies both an
Identity
and also sets Column.autoincrement
to False
.
See also
Default Objects API¶
Object Name | Description |
---|---|
A plain default value on a column. |
|
Defines a generated column, i.e. “GENERATED ALWAYS AS” syntax. |
|
A DDL-specified DEFAULT column value. |
|
Base class for column default values. |
|
A marker for a transparent database-side default. |
|
Defines an identity column, i.e. “GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY” syntax. |
|
Represents a named database sequence. |
- class sqlalchemy.schema.Computed¶
Defines a generated column, i.e. “GENERATED ALWAYS AS” syntax.
The
Computed
construct is an inline construct added to the argument list of aColumn
object:from sqlalchemy import Computed Table('square', metadata_obj, Column('side', Float, nullable=False), Column('area', Float, Computed('side * side')) )
See the linked documentation below for complete details.
New in version 1.3.11.
Members
Class signature
class
sqlalchemy.schema.Computed
(sqlalchemy.schema.FetchedValue
,sqlalchemy.schema.SchemaItem
)-
method
sqlalchemy.schema.Computed.
__init__(sqltext: _DDLColumnArgument, persisted: bool | None = None) → None¶ Construct a GENERATED ALWAYS AS DDL construct to accompany a
Column
.- Parameters:
sqltext¶ –
A string containing the column generation expression, which will be used verbatim, or a SQL expression construct, such as a
text()
object. If given as a string, the object is converted to atext()
object.Warning
The
Computed.sqltext
argument toComputed
can be passed as a Python string argument, which will be treated as trusted SQL text and rendered as given. DO NOT PASS UNTRUSTED INPUT TO THIS PARAMETER.persisted¶ –
Optional, controls how this column should be persisted by the database. Possible values are:
None
, the default, it will use the default persistence defined by the database.True
, will renderGENERATED ALWAYS AS ... STORED
, or the equivalent for the target database if supported.False
, will renderGENERATED ALWAYS AS ... VIRTUAL
, or the equivalent for the target database if supported.
Specifying
True
orFalse
may raise an error when the DDL is emitted to the target database if the database does not support that persistence option. Leaving this parameter at its default ofNone
is guaranteed to succeed for all databases that supportGENERATED ALWAYS AS
.
-
method
sqlalchemy.schema.Computed.
copy(*, target_table: Table | None = None, **kw: Any) → Computed¶ Deprecated since version 1.4: The
Computed.copy()
method is deprecated and will be removed in a future release.
-
method
- class sqlalchemy.schema.ColumnDefault¶
A plain default value on a column.
This could correspond to a constant, a callable function, or a SQL clause.
ColumnDefault
is generated automatically whenever thedefault
,onupdate
arguments ofColumn
are used. AColumnDefault
can be passed positionally as well.For example, the following:
Column('foo', Integer, default=50)
Is equivalent to:
Column('foo', Integer, ColumnDefault(50))
Class signature
class
sqlalchemy.schema.ColumnDefault
(sqlalchemy.schema.DefaultGenerator
,abc.ABC
)
- class sqlalchemy.schema.DefaultClause¶
A DDL-specified DEFAULT column value.
DefaultClause
is aFetchedValue
that also generates a “DEFAULT” clause when “CREATE TABLE” is emitted.DefaultClause
is generated automatically whenever theserver_default
,server_onupdate
arguments ofColumn
are used. ADefaultClause
can be passed positionally as well.For example, the following:
Column('foo', Integer, server_default="50")
Is equivalent to:
Column('foo', Integer, DefaultClause("50"))
Class signature
class
sqlalchemy.schema.DefaultClause
(sqlalchemy.schema.FetchedValue
)
- class sqlalchemy.schema.DefaultGenerator¶
Base class for column default values.
This object is only present on column.default or column.onupdate. It’s not valid as a server default.
Class signature
class
sqlalchemy.schema.DefaultGenerator
(sqlalchemy.sql.expression.Executable
,sqlalchemy.schema.SchemaItem
)
- class sqlalchemy.schema.FetchedValue¶
A marker for a transparent database-side default.
Use
FetchedValue
when the database is configured to provide some automatic default for a column.E.g.:
Column('foo', Integer, FetchedValue())
Would indicate that some trigger or default generator will create a new value for the
foo
column during an INSERT.Class signature
class
sqlalchemy.schema.FetchedValue
(sqlalchemy.sql.expression.SchemaEventTarget
)
- class sqlalchemy.schema.Sequence¶
Represents a named database sequence.
The
Sequence
object represents the name and configurational parameters of a database sequence. It also represents a construct that can be “executed” by a SQLAlchemyEngine
orConnection
, rendering the appropriate “next value” function for the target database and returning a result.The
Sequence
is typically associated with a primary key column:some_table = Table( 'some_table', metadata, Column('id', Integer, Sequence('some_table_seq', start=1), primary_key=True) )
When CREATE TABLE is emitted for the above
Table
, if the target platform supports sequences, a CREATE SEQUENCE statement will be emitted as well. For platforms that don’t support sequences, theSequence
construct is ignored.Members
Class signature
class
sqlalchemy.schema.Sequence
(sqlalchemy.schema.HasSchemaAttr
,sqlalchemy.schema.IdentityOptions
,sqlalchemy.schema.DefaultGenerator
)-
method
sqlalchemy.schema.Sequence.
__init__(name: str, start: int | None = None, increment: int | None = None, minvalue: int | None = None, maxvalue: int | None = None, nominvalue: bool | None = None, nomaxvalue: bool | None = None, cycle: bool | None = None, schema: str | Literal[SchemaConst.BLANK_SCHEMA] | None = None, cache: int | None = None, order: bool | None = None, data_type: _TypeEngineArgument[int] | None = None, optional: bool = False, quote: bool | None = None, metadata: MetaData | None = None, quote_schema: bool | None = None, for_update: bool = False) → None¶ Construct a
Sequence
object.- Parameters:
name¶ – the name of the sequence.
start¶ –
the starting index of the sequence. This value is used when the CREATE SEQUENCE command is emitted to the database as the value of the “START WITH” clause. If
None
, the clause is omitted, which on most platforms indicates a starting value of 1.Changed in version 2.0: The
Sequence.start
parameter is required in order to have DDL emit “START WITH”. This is a reversal of a change made in version 1.4 which would implicitly render “START WITH 1” if theSequence.start
were not included. See The Sequence construct reverts to not having any explicit default “start” value; impacts MS SQL Server for more detail.increment¶ – the increment value of the sequence. This value is used when the CREATE SEQUENCE command is emitted to the database as the value of the “INCREMENT BY” clause. If
None
, the clause is omitted, which on most platforms indicates an increment of 1.minvalue¶ – the minimum value of the sequence. This value is used when the CREATE SEQUENCE command is emitted to the database as the value of the “MINVALUE” clause. If
None
, the clause is omitted, which on most platforms indicates a minvalue of 1 and -2^63-1 for ascending and descending sequences, respectively.maxvalue¶ – the maximum value of the sequence. This value is used when the CREATE SEQUENCE command is emitted to the database as the value of the “MAXVALUE” clause. If
None
, the clause is omitted, which on most platforms indicates a maxvalue of 2^63-1 and -1 for ascending and descending sequences, respectively.nominvalue¶ – no minimum value of the sequence. This value is used when the CREATE SEQUENCE command is emitted to the database as the value of the “NO MINVALUE” clause. If
None
, the clause is omitted, which on most platforms indicates a minvalue of 1 and -2^63-1 for ascending and descending sequences, respectively.nomaxvalue¶ – no maximum value of the sequence. This value is used when the CREATE SEQUENCE command is emitted to the database as the value of the “NO MAXVALUE” clause. If
None
, the clause is omitted, which on most platforms indicates a maxvalue of 2^63-1 and -1 for ascending and descending sequences, respectively.cycle¶ – allows the sequence to wrap around when the maxvalue or minvalue has been reached by an ascending or descending sequence respectively. This value is used when the CREATE SEQUENCE command is emitted to the database as the “CYCLE” clause. If the limit is reached, the next number generated will be the minvalue or maxvalue, respectively. If cycle=False (the default) any calls to nextval after the sequence has reached its maximum value will return an error.
schema¶ – optional schema name for the sequence, if located in a schema other than the default. The rules for selecting the schema name when a
MetaData
is also present are the same as that ofTable.schema
.cache¶ – optional integer value; number of future values in the sequence which are calculated in advance. Renders the CACHE keyword understood by Oracle and PostgreSQL.
order¶ – optional boolean value; if
True
, renders the ORDER keyword, understood by Oracle, indicating the sequence is definitively ordered. May be necessary to provide deterministic ordering using Oracle RAC.data_type¶ –
The type to be returned by the sequence, for dialects that allow us to choose between INTEGER, BIGINT, etc. (e.g., mssql).
New in version 1.4.0.
optional¶ – boolean value, when
True
, indicates that thisSequence
object only needs to be explicitly generated on backends that don’t provide another way to generate primary key identifiers. Currently, it essentially means, “don’t create this sequence on the PostgreSQL backend, where the SERIAL keyword creates a sequence for us automatically”.quote¶ – boolean value, when
True
orFalse
, explicitly forces quoting of theSequence.name
on or off. When left at its default ofNone
, normal quoting rules based on casing and reserved words take place.quote_schema¶ – Set the quoting preferences for the
schema
name.metadata¶ –
optional
MetaData
object which thisSequence
will be associated with. ASequence
that is associated with aMetaData
gains the following capabilities:The
Sequence
will inherit theMetaData.schema
parameter specified to the targetMetaData
, which affects the production of CREATE / DROP DDL, if any.The
Sequence.create()
andSequence.drop()
methods automatically use the engine bound to theMetaData
object, if any.The
MetaData.create_all()
andMetaData.drop_all()
methods will emit CREATE / DROP for thisSequence
, even if theSequence
is not associated with anyTable
/Column
that’s a member of thisMetaData
.
The above behaviors can only occur if the
Sequence
is explicitly associated with theMetaData
via this parameter.See also
Associating a Sequence with the MetaData - full discussion of the
Sequence.metadata
parameter.for_update¶ – Indicates this
Sequence
, when associated with aColumn
, should be invoked for UPDATE statements on that column’s table, rather than for INSERT statements, when no value is otherwise present for that column in the statement.
-
method
sqlalchemy.schema.Sequence.
create(bind: _CreateDropBind, checkfirst: bool = True) → None¶ Creates this sequence in the database.
-
method
sqlalchemy.schema.Sequence.
drop(bind: _CreateDropBind, checkfirst: bool = True) → None¶ Drops this sequence from the database.
-
method
sqlalchemy.schema.Sequence.
next_value() → Function[int]¶ Return a
next_value
function element which will render the appropriate increment function for thisSequence
within any SQL expression.
-
method
- class sqlalchemy.schema.Identity¶
Defines an identity column, i.e. “GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY” syntax.
The
Identity
construct is an inline construct added to the argument list of aColumn
object:from sqlalchemy import Identity Table('foo', metadata_obj, Column('id', Integer, Identity()) Column('description', Text), )
See the linked documentation below for complete details.
New in version 1.4.
Members
Class signature
class
sqlalchemy.schema.Identity
(sqlalchemy.schema.IdentityOptions
,sqlalchemy.schema.FetchedValue
,sqlalchemy.schema.SchemaItem
)-
method
sqlalchemy.schema.Identity.
__init__(always: bool = False, on_null: bool | None = None, start: int | None = None, increment: int | None = None, minvalue: int | None = None, maxvalue: int | None = None, nominvalue: bool | None = None, nomaxvalue: bool | None = None, cycle: bool | None = None, cache: int | None = None, order: bool | None = None) → None¶ Construct a GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY DDL construct to accompany a
Column
.See the
Sequence
documentation for a complete description of most parameters.Note
MSSQL supports this construct as the preferred alternative to generate an IDENTITY on a column, but it uses non standard syntax that only support
Identity.start
andIdentity.increment
. All other parameters are ignored.- Parameters:
always¶ – A boolean, that indicates the type of identity column. If
False
is specified, the default, then the user-specified value takes precedence. IfTrue
is specified, a user-specified value is not accepted ( on some backends, like PostgreSQL, OVERRIDING SYSTEM VALUE, or similar, may be specified in an INSERT to override the sequence value). Some backends also have a default value for this parameter,None
can be used to omit rendering this part in the DDL. It will be treated asFalse
if a backend does not have a default value.on_null¶ – Set to
True
to specify ON NULL in conjunction with aalways=False
identity column. This option is only supported on some backends, like Oracle.start¶ – the starting index of the sequence.
increment¶ – the increment value of the sequence.
minvalue¶ – the minimum value of the sequence.
maxvalue¶ – the maximum value of the sequence.
nominvalue¶ – no minimum value of the sequence.
nomaxvalue¶ – no maximum value of the sequence.
cycle¶ – allows the sequence to wrap around when the maxvalue or minvalue has been reached.
cache¶ – optional integer value; number of future values in the sequence which are calculated in advance.
order¶ – optional boolean value; if true, renders the ORDER keyword.
-
method
sqlalchemy.schema.Identity.
copy(**kw: Any) → Identity¶ Deprecated since version 1.4: The
Identity.copy()
method is deprecated and will be removed in a future release.
-
method
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