SQLAlchemy provides a very rich featureset regarding column level events which take place during INSERT and UPDATE statements. Options include:
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.
The simplest kind of default is a scalar value used as the default value of a column:
Table("mytable", meta,
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", meta,
Column("somecolumn", Integer, onupdate=25)
)
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", meta,
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", meta,
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.
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.current_parameters['counter'] + 12
t = Table('mytable', meta,
Column('counter', Integer),
Column('counter_plus_twelve', Integer, default=mydefault, onupdate=mydefault)
)
Above we illustrate a default function which 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.
While the context object passed to the default function has many attributes,
the current_parameters
member is a special member provided only during the
execution of a default function for the purposes of deriving defaults from its
existing values. For a single statement that is executing many sets of bind
parameters, the user-defined function is called for each set of parameters,
and current_parameters
will be provided with each individual parameter set
for each execution.
The “default” and “onupdate” keywords may also be passed SQL expressions, including select statements or direct function calls:
t = Table("mytable", meta,
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=keyvalues.select(keyvalues.c.type='type1', limit=1)),
# 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
UTC_TIMESTAMP()
, a function specific to MySQL, when an UPDATE statement is
emitted for this table.
Note that when using func
functions, unlike when using Python datetime
functions we do call the function, i.e. with parenthesis “()” - this is
because what we want in this case is the return value of the function, which
is the SQL expression construct that will be rendered into the INSERT or
UPDATE statement.
The above SQL functions are usually executed “inline” with the INSERT or UPDATE statement being executed, meaning, a single statement is executed which embeds the given expressions or subqueries within the VALUES or SET clause of the statement. Although in some cases, the function is “pre-executed” in a SELECT statement of its own beforehand. This happens when all of the following is true:
cursor.lastrowid
accessor
(or equivalent); this currently includes PostgreSQL, Oracle, and Firebird, as
well as some MySQL dialects.implicit_returning
flag is set to False
for the dialect. Dialects
which support RETURNING currently include PostgreSQL, Oracle, Firebird, and
MS-SQL.inline=True
flag is not set on the
Insert()
or
Update()
construct, and the statement has
not defined an explicit returning() clause.Whether or not the default generation clause “pre-executes” is not something that normally needs to be considered, unless it is being addressed for performance reasons.
When the statement is executed with a single set of parameters (that is, it is
not an “executemany” style execution), the returned
ResultProxy
will contain a collection
accessible via ResultProxy.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 ResultProxy.last_inserted_params()
or ResultProxy.last_updated_params()
collections on
ResultProxy
. The ResultProxy.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).
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', meta,
Column('abc', String(20), server_default='abc'),
Column('created_at', DateTime, server_default=text("sysdate"))
)
A create call for the above table will produce:
CREATE TABLE test (
abc varchar(20) default 'abc',
created_at datetime default sysdate
)
The behavior of Column.server_default
is similar to that of a regular SQL
default; if it’s placed on a primary key column for a database which doesn’t
have a way to “postfetch” the ID, and the statement is not “inlined”, the SQL
expression is pre-executed; otherwise, SQLAlchemy lets the default fire off on
the database side normally.
Columns with values set by a database trigger or other external process may be
called out using FetchedValue
as a marker:
t = Table('test', meta,
Column('abc', String(20), server_default=FetchedValue()),
Column('def', String(20), server_onupdate=FetchedValue())
)
These markers do not emit a “default” clause when the table is created,
however they do set the same internal flags as a static server_default
clause, providing hints to higher-level tools that a “post-fetch” of these
rows should be performed after an insert or update.
Note
It’s generally not appropriate to use FetchedValue
in
conjunction with a primary key column, particularly when using the
ORM or any other scenario where the ResultProxy.inserted_primary_key
attribute is required. This is becaue the “post-fetch” operation requires
that the primary key value already be available, so that the
row can be selected on its primary key.
For a server-generated primary key value, all databases provide special
accessors or other techniques in order to acquire the “last inserted
primary key” column of a table. These mechanisms aren’t affected by the presence
of FetchedValue
. For special situations where triggers are
used to generate primary key values, and the database in use does not
support the RETURNING
clause, it may be necessary to forego the usage
of the trigger and instead apply the SQL expression or function as a
“pre execute” expression:
t = Table('test', meta,
Column('abc', MyType, default=func.generate_new_value(), primary_key=True)
)
Where above, when Table.insert()
is used,
the func.generate_new_value()
expression will be pre-executed
in the context of a scalar SELECT
statement, and the new value will
be applied to the subsequent INSERT
, while at the same time being
made available to the ResultProxy.inserted_primary_key
attribute.
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 currently includes PostgreSQL,
Oracle, and Firebird. The Sequence
object is
otherwise ignored.
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", meta,
Column("cart_id", Integer, Sequence('cart_id_seq'), primary_key=True),
Column("description", String(40)),
Column("createdate", DateTime())
)
Where above, the table “cartitems” is associated with a sequence named “cart_id_seq”. When INSERT statements take place for “cartitems”, and no value is passed for the “cart_id” column, the “cart_id_seq” sequence will be used to generate a value.
When the Sequence
is associated with a table,
CREATE and DROP statements issued for that table will also issue CREATE/DROP
for the sequence object as well, thus “bundling” the sequence object with its
parent table.
The Sequence
object also implements special
functionality to accommodate PostgreSQL’s SERIAL datatype. The SERIAL type in
PG automatically generates a sequence that is used implicitly during inserts.
This means that if a Table
object defines a
Sequence
on its primary key column so that it
works with Oracle and Firebird, the Sequence
would
get in the way of the “implicit” sequence that PG would normally use. For this
use case, add the flag optional=True
to the
Sequence
object - this indicates that the
Sequence
should only be used if the database
provides no other option for generating primary key identifiers.
The Sequence
object also has the ability to be
executed standalone like a SQL expression, which has the effect of calling its
“next value” function:
seq = Sequence('some_sequence')
nextid = connection.execute(seq)
When we associate a Sequence
with a Column
as above,
this association is an in-Python only association. The CREATE TABLE
that would be generated for our Table
would not refer to this
sequence. 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 commandline, 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:
cart_id_seq = Sequence('cart_id_seq')
table = Table("cartitems", meta,
Column(
"cart_id", Integer, cart_id_seq,
server_default=cart_id_seq.next_value(), primary_key=True),
Column("description", String(40)),
Column("createdate", DateTime())
)
The above metadata will generate a CREATE TABLE statement on PostgreSQL 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)
)
We place the Sequence
also as a Python-side default above, that
is, it is mentioned twice in the Column
definition. Depending
on the backend in use, this may not be strictly necessary, for example
on the PostgreSQL backend the Core will use RETURNING
to access the
newly generated primary key value in any case. However, for the best
compatibility, Sequence
was originally intended to be a Python-side
directive first and foremost so it’s probably a good idea to specify it
in this way as well.
sqlalchemy.schema.
ColumnDefault
(arg, **kwargs)¶Bases: sqlalchemy.schema.DefaultGenerator
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 the default
, onupdate
arguments of
Column
are used. A ColumnDefault
can be passed positionally as well.
For example, the following:
Column('foo', Integer, default=50)
Is equivalent to:
Column('foo', Integer, ColumnDefault(50))
sqlalchemy.schema.
DefaultClause
(arg, for_update=False, _reflected=False)¶Bases: sqlalchemy.schema.FetchedValue
A DDL-specified DEFAULT column value.
DefaultClause
is a FetchedValue
that also generates a “DEFAULT” clause when
“CREATE TABLE” is emitted.
DefaultClause
is generated automatically
whenever the server_default
, server_onupdate
arguments of
Column
are used. A DefaultClause
can be passed positionally as well.
For example, the following:
Column('foo', Integer, server_default="50")
Is equivalent to:
Column('foo', Integer, DefaultClause("50"))
sqlalchemy.schema.
DefaultGenerator
(for_update=False)¶Bases: sqlalchemy.schema._NotAColumnExpr
, sqlalchemy.schema.SchemaItem
Base class for column default values.
sqlalchemy.schema.
FetchedValue
(for_update=False)¶Bases: sqlalchemy.schema._NotAColumnExpr
, sqlalchemy.sql.expression.SchemaEventTarget
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.
See also
sqlalchemy.schema.
PassiveDefault
(*arg, **kw)¶Bases: sqlalchemy.schema.DefaultClause
A DDL-specified DEFAULT column value.
Deprecated since version 0.6: PassiveDefault
is deprecated.
Use DefaultClause
.
sqlalchemy.schema.
Sequence
(name, start=None, increment=None, minvalue=None, maxvalue=None, nominvalue=None, nomaxvalue=None, cycle=None, schema=None, optional=False, quote=None, metadata=None, quote_schema=None, for_update=False)¶Bases: sqlalchemy.schema.DefaultGenerator
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 SQLAlchemy Engine
or Connection
, 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'),
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,
the Sequence
construct is ignored.
__init__
(name, start=None, increment=None, minvalue=None, maxvalue=None, nominvalue=None, nomaxvalue=None, cycle=None, schema=None, optional=False, quote=None, metadata=None, quote_schema=None, for_update=False)¶Construct a Sequence
object.
Parameters: |
|
---|
create
(bind=None, checkfirst=True)¶Creates this sequence in the database.
drop
(bind=None, checkfirst=True)¶Drops this sequence from the database.
next_value
()¶Return a next_value
function element
which will render the appropriate increment function
for this Sequence
within any SQL expression.