SQLAlchemy 0.6 Documentation

Release: 0.6.9 | Release Date: May 5, 2012
SQLAlchemy 0.6 Documentation » SQLAlchemy Core » Custom SQL Constructs and Compilation Extension

Custom SQL Constructs and Compilation Extension

Custom SQL Constructs and Compilation Extension

Provides an API for creation of custom ClauseElements and compilers.


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):

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)


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):

    def __init__(self, column, cmd):
        self.column = column
        self.cmd = cmd

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):
    def __init__(self, table, select):
        self.table = table
        self.select = select

def visit_insert_from_select(element, compiler, **kw):
    return "INSERT INTO %s (%s)" % (
        compiler.process(element.table, asfrom=True),

insert = InsertFromSelect(t1, select([t1]).where(t1.c.x>5))
print insert


"INSERT INTO mytable (SELECT mytable.x, mytable.y, mytable.z FROM mytable WHERE mytable.x > :x_1)"

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:

def compile_my_constraint(constraint, ddlcompiler, **kw):
    return "CONSTRAINT %s CHECK (%s)" % (

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

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')"
        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 a type 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()
  • FunctionElement - This is a hybrid of a ColumnElement 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 a select() construct:

    from sqlalchemy.sql.expression import FunctionElement
    class coalesce(FunctionElement):
        name = 'coalesce'
    def compile(element, compiler, **kw):
        return "coalesce(%s)" % compiler.process(element.clauses)
    @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)
  • DDLElement - The root of all DDL expressions, like CREATE TABLE, ALTER TABLE, etc. Compilation of DDLElement subclasses is issued by a DDLCompiler instead of a SQLCompiler. DDLElement also features Table and MetaData event hooks via the execute_at() method, allowing the construct to be invoked during CREATE TABLE and DROP TABLE sequences.

  • 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 an execute() method. It is already implicit within DDLElement and FunctionElement.