SQLAlchemy 2.0 Documentation
SQLAlchemy Core
- SQL Statements and Expressions API
- Column Elements and Expressions
- Operator Reference
- SELECT and Related Constructs
- Insert, Updates, Deletes
- SQL and Generic Functions
- Custom SQL Constructs and Compilation Extension¶
- Expression Serializer Extension
- SQL Expression Language Foundational Constructs
- Visitor and Traversal Utilities
- Schema Definition Language
- SQL Datatype Objects
- Engine and Connection Use
- Core API Basics
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- Custom SQL Constructs and Compilation Extension
Custom SQL Constructs and Compilation Extension¶
Provides an API for creation of custom ClauseElements and compilers.
Synopsis¶
Usage involves the creation of one or more
ClauseElement
subclasses and one or
more callables defining its compilation:
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.expression import ColumnClause
class MyColumn(ColumnClause):
inherit_cache = True
@compiles(MyColumn)
def compile_mycolumn(element, compiler, **kw):
return "[%s]" % element.name
Above, MyColumn
extends ColumnClause
,
the base expression element for named column objects. The compiles
decorator registers itself with the MyColumn
class so that it is invoked
when the object is compiled to a string:
from sqlalchemy import select
s = select(MyColumn('x'), MyColumn('y'))
print(str(s))
Produces:
SELECT [x], [y]
Dialect-specific compilation rules¶
Compilers can also be made dialect-specific. The appropriate compiler will be invoked for the dialect in use:
from sqlalchemy.schema import DDLElement
class AlterColumn(DDLElement):
inherit_cache = False
def __init__(self, column, cmd):
self.column = column
self.cmd = cmd
@compiles(AlterColumn)
def visit_alter_column(element, compiler, **kw):
return "ALTER COLUMN %s ..." % element.column.name
@compiles(AlterColumn, 'postgresql')
def visit_alter_column(element, compiler, **kw):
return "ALTER TABLE %s ALTER COLUMN %s ..." % (element.table.name,
element.column.name)
The second visit_alter_table
will be invoked when any postgresql
dialect is used.
Compiling sub-elements of a custom expression construct¶
The compiler
argument is the
Compiled
object in use. This object
can be inspected for any information about the in-progress compilation,
including compiler.dialect
, compiler.statement
etc. The
SQLCompiler
and
DDLCompiler
both include a process()
method which can be used for compilation of embedded attributes:
from sqlalchemy.sql.expression import Executable, ClauseElement
class InsertFromSelect(Executable, ClauseElement):
inherit_cache = False
def __init__(self, table, select):
self.table = table
self.select = select
@compiles(InsertFromSelect)
def visit_insert_from_select(element, compiler, **kw):
return "INSERT INTO %s (%s)" % (
compiler.process(element.table, asfrom=True, **kw),
compiler.process(element.select, **kw)
)
insert = InsertFromSelect(t1, select(t1).where(t1.c.x>5))
print(insert)
Produces:
"INSERT INTO mytable (SELECT mytable.x, mytable.y, mytable.z
FROM mytable WHERE mytable.x > :x_1)"
Note
The above InsertFromSelect
construct is only an example, this actual
functionality is already available using the
Insert.from_select()
method.
Cross Compiling between SQL and DDL compilers¶
SQL and DDL constructs are each compiled using different base compilers -
SQLCompiler
and DDLCompiler
. A common need is to access the
compilation rules of SQL expressions from within a DDL expression. The
DDLCompiler
includes an accessor sql_compiler
for this reason, such as
below where we generate a CHECK constraint that embeds a SQL expression:
@compiles(MyConstraint)
def compile_my_constraint(constraint, ddlcompiler, **kw):
kw['literal_binds'] = True
return "CONSTRAINT %s CHECK (%s)" % (
constraint.name,
ddlcompiler.sql_compiler.process(
constraint.expression, **kw)
)
Above, we add an additional flag to the process step as called by
SQLCompiler.process()
, which is the literal_binds
flag. This
indicates that any SQL expression which refers to a BindParameter
object or other “literal” object such as those which refer to strings or
integers should be rendered in-place, rather than being referred to as
a bound parameter; when emitting DDL, bound parameters are typically not
supported.
Changing the default compilation of existing constructs¶
The compiler extension applies just as well to the existing constructs. When
overriding the compilation of a built in SQL construct, the @compiles
decorator is invoked upon the appropriate class (be sure to use the class,
i.e. Insert
or Select
, instead of the creation function such
as insert()
or select()
).
Within the new compilation function, to get at the “original” compilation routine, use the appropriate visit_XXX method - this because compiler.process() will call upon the overriding routine and cause an endless loop. Such as, to add “prefix” to all insert statements:
from sqlalchemy.sql.expression import Insert
@compiles(Insert)
def prefix_inserts(insert, compiler, **kw):
return compiler.visit_insert(insert.prefix_with("some prefix"), **kw)
The above compiler will prefix all INSERT statements with “some prefix” when compiled.
Changing Compilation of Types¶
compiler
works for types, too, such as below where we implement the
MS-SQL specific ‘max’ keyword for String
/VARCHAR
:
@compiles(String, 'mssql')
@compiles(VARCHAR, 'mssql')
def compile_varchar(element, compiler, **kw):
if element.length == 'max':
return "VARCHAR('max')"
else:
return compiler.visit_VARCHAR(element, **kw)
foo = Table('foo', metadata,
Column('data', VARCHAR('max'))
)
Subclassing Guidelines¶
A big part of using the compiler extension is subclassing SQLAlchemy expression constructs. To make this easier, the expression and schema packages feature a set of “bases” intended for common tasks. A synopsis is as follows:
ClauseElement
- This is the root expression class. Any SQL expression can be derived from this base, and is probably the best choice for longer constructs such as specialized INSERT statements.ColumnElement
- The root of all “column-like” elements. Anything that you’d place in the “columns” clause of a SELECT statement (as well as order by and group by) can derive from this - the object will automatically have Python “comparison” behavior.ColumnElement
classes want to have atype
member which is expression’s return type. This can be established at the instance level in the constructor, or at the class level if its generally constant:class timestamp(ColumnElement): type = TIMESTAMP() inherit_cache = True
FunctionElement
- This is a hybrid of aColumnElement
and a “from clause” like object, and represents a SQL function or stored procedure type of call. Since most databases support statements along the line of “SELECT FROM <some function>”FunctionElement
adds in the ability to be used in the FROM clause of aselect()
construct:from sqlalchemy.sql.expression import FunctionElement class coalesce(FunctionElement): name = 'coalesce' inherit_cache = True @compiles(coalesce) def compile(element, compiler, **kw): return "coalesce(%s)" % compiler.process(element.clauses, **kw) @compiles(coalesce, 'oracle') def compile(element, compiler, **kw): if len(element.clauses) > 2: raise TypeError("coalesce only supports two arguments on Oracle") return "nvl(%s)" % compiler.process(element.clauses, **kw)
ExecutableDDLElement
- The root of all DDL expressions, like CREATE TABLE, ALTER TABLE, etc. Compilation ofExecutableDDLElement
subclasses is issued by aDDLCompiler
instead of aSQLCompiler
.ExecutableDDLElement
can also be used as an event hook in conjunction with event hooks likeDDLEvents.before_create()
andDDLEvents.after_create()
, allowing the construct to be invoked automatically during CREATE TABLE and DROP TABLE sequences.See also
Customizing DDL - contains examples of associating
DDL
objects (which are themselvesExecutableDDLElement
instances) withDDLEvents
event hooks.Executable
- This is a mixin which should be used with any expression class that represents a “standalone” SQL statement that can be passed directly to anexecute()
method. It is already implicit withinDDLElement
andFunctionElement
.
Most of the above constructs also respond to SQL statement caching. A
subclassed construct will want to define the caching behavior for the object,
which usually means setting the flag inherit_cache
to the value of
False
or True
. See the next section Enabling Caching Support for Custom Constructs
for background.
Enabling Caching Support for Custom Constructs¶
SQLAlchemy as of version 1.4 includes a SQL compilation caching facility which will allow equivalent SQL constructs to cache their stringified form, along with other structural information used to fetch results from the statement.
For reasons discussed at Object will not produce a cache key, Performance Implications, the implementation of this
caching system takes a conservative approach towards including custom SQL
constructs and/or subclasses within the caching system. This includes that
any user-defined SQL constructs, including all the examples for this
extension, will not participate in caching by default unless they positively
assert that they are able to do so. The HasCacheKey.inherit_cache
attribute when set to True
at the class level of a specific subclass
will indicate that instances of this class may be safely cached, using the
cache key generation scheme of the immediate superclass. This applies
for example to the “synopsis” example indicated previously:
class MyColumn(ColumnClause):
inherit_cache = True
@compiles(MyColumn)
def compile_mycolumn(element, compiler, **kw):
return "[%s]" % element.name
Above, the MyColumn
class does not include any new state that
affects its SQL compilation; the cache key of MyColumn
instances will
make use of that of the ColumnClause
superclass, meaning it will take
into account the class of the object (MyColumn
), the string name and
datatype of the object:
>>> MyColumn("some_name", String())._generate_cache_key()
CacheKey(
key=('0', <class '__main__.MyColumn'>,
'name', 'some_name',
'type', (<class 'sqlalchemy.sql.sqltypes.String'>,
('length', None), ('collation', None))
), bindparams=[])
For objects that are likely to be used liberally as components within many
larger statements, such as Column
subclasses and custom SQL
datatypes, it’s important that caching be enabled as much as possible, as
this may otherwise negatively affect performance.
An example of an object that does contain state which affects its SQL
compilation is the one illustrated at Compiling sub-elements of a custom expression construct;
this is an “INSERT FROM SELECT” construct that combines together a
Table
as well as a Select
construct, each of
which independently affect the SQL string generation of the construct. For
this class, the example illustrates that it simply does not participate in
caching:
class InsertFromSelect(Executable, ClauseElement):
inherit_cache = False
def __init__(self, table, select):
self.table = table
self.select = select
@compiles(InsertFromSelect)
def visit_insert_from_select(element, compiler, **kw):
return "INSERT INTO %s (%s)" % (
compiler.process(element.table, asfrom=True, **kw),
compiler.process(element.select, **kw)
)
While it is also possible that the above InsertFromSelect
could be made to
produce a cache key that is composed of that of the Table
and
Select
components together, the API for this is not at the moment
fully public. However, for an “INSERT FROM SELECT” construct, which is only
used by itself for specific operations, caching is not as critical as in the
previous example.
For objects that are used in relative isolation and are generally standalone, such as custom DML constructs like an “INSERT FROM SELECT”, caching is generally less critical as the lack of caching for such a construct will have only localized implications for that specific operation.
Further Examples¶
“UTC timestamp” function¶
A function that works like “CURRENT_TIMESTAMP” except applies the appropriate conversions so that the time is in UTC time. Timestamps are best stored in relational databases as UTC, without time zones. UTC so that your database doesn’t think time has gone backwards in the hour when daylight savings ends, without timezones because timezones are like character encodings - they’re best applied only at the endpoints of an application (i.e. convert to UTC upon user input, re-apply desired timezone upon display).
For PostgreSQL and Microsoft SQL Server:
from sqlalchemy.sql import expression
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.types import DateTime
class utcnow(expression.FunctionElement):
type = DateTime()
inherit_cache = True
@compiles(utcnow, 'postgresql')
def pg_utcnow(element, compiler, **kw):
return "TIMEZONE('utc', CURRENT_TIMESTAMP)"
@compiles(utcnow, 'mssql')
def ms_utcnow(element, compiler, **kw):
return "GETUTCDATE()"
Example usage:
from sqlalchemy import (
Table, Column, Integer, String, DateTime, MetaData
)
metadata = MetaData()
event = Table("event", metadata,
Column("id", Integer, primary_key=True),
Column("description", String(50), nullable=False),
Column("timestamp", DateTime, server_default=utcnow())
)
“GREATEST” function¶
The “GREATEST” function is given any number of arguments and returns the one
that is of the highest value - its equivalent to Python’s max
function. A SQL standard version versus a CASE based version which only
accommodates two arguments:
from sqlalchemy.sql import expression, case
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.types import Numeric
class greatest(expression.FunctionElement):
type = Numeric()
name = 'greatest'
inherit_cache = True
@compiles(greatest)
def default_greatest(element, compiler, **kw):
return compiler.visit_function(element)
@compiles(greatest, 'sqlite')
@compiles(greatest, 'mssql')
@compiles(greatest, 'oracle')
def case_greatest(element, compiler, **kw):
arg1, arg2 = list(element.clauses)
return compiler.process(case((arg1 > arg2, arg1), else_=arg2), **kw)
Example usage:
Session.query(Account).\
filter(
greatest(
Account.checking_balance,
Account.savings_balance) > 10000
)
“false” expression¶
Render a “false” constant expression, rendering as “0” on platforms that don’t have a “false” constant:
from sqlalchemy.sql import expression
from sqlalchemy.ext.compiler import compiles
class sql_false(expression.ColumnElement):
inherit_cache = True
@compiles(sql_false)
def default_false(element, compiler, **kw):
return "false"
@compiles(sql_false, 'mssql')
@compiles(sql_false, 'mysql')
@compiles(sql_false, 'oracle')
def int_false(element, compiler, **kw):
return "0"
Example usage:
from sqlalchemy import select, union_all
exp = union_all(
select(users.c.name, sql_false().label("enrolled")),
select(customers.c.name, customers.c.enrolled)
)
Object Name | Description |
---|---|
compiles(class_, *specs) |
Register a function as a compiler for a
given |
deregister(class_) |
Remove all custom compilers associated with a given
|
- function sqlalchemy.ext.compiler.compiles(class_: Type[Any], *specs: str) → Callable[[_F], _F]¶
Register a function as a compiler for a given
ClauseElement
type.
- function sqlalchemy.ext.compiler.deregister(class_: Type[Any]) → None¶
Remove all custom compilers associated with a given
ClauseElement
type.
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