# SQLAlchemy 0.6 Documentation

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Release: 0.6.9 | Release Date: May 5, 2012

# 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):
        pass

    @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):

        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):
        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),
            compiler.process(element.select)
        )

    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)"

### 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):
        return "CONSTRAINT %s CHECK (%s)" % (
            constraint.name,
            ddlcompiler.sql_compiler.process(constraint.expression)
        )

## 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 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'

      @compiles(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`.