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# ext/compiler.py # Copyright (C) 2005-2021 the SQLAlchemy authors and contributors # <see AUTHORS file> # # This module is part of SQLAlchemy and is released under # the MIT License: http://www.opensource.org/licenses/mit-license.php r"""Provides an API for creation of custom ClauseElements and compilers. Synopsis ======== Usage involves the creation of one or more :class:`~sqlalchemy.sql.expression.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 :class:`~sqlalchemy.sql.expression.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 :class:`~sqlalchemy.engine.interfaces.Compiled` object in use. This object can be inspected for any information about the in-progress compilation, including ``compiler.dialect``, ``compiler.statement`` etc. The :class:`~sqlalchemy.sql.compiler.SQLCompiler` and :class:`~sqlalchemy.sql.compiler.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, **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 :meth:`_expression.Insert.from_select` method. .. note:: The above ``InsertFromSelect`` construct probably wants to have "autocommit" enabled. See :ref:`enabling_compiled_autocommit` for this step. 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 :meth:`.SQLCompiler.process`, which is the ``literal_binds`` flag. This indicates that any SQL expression which refers to a :class:`.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. .. _enabling_compiled_autocommit: Enabling Autocommit on a Construct ================================== Recall from the section :ref:`autocommit` that the :class:`_engine.Engine`, when asked to execute a construct in the absence of a user-defined transaction, detects if the given construct represents DML or DDL, that is, a data modification or data definition statement, which requires (or may require, in the case of DDL) that the transaction generated by the DBAPI be committed (recall that DBAPI always has a transaction going on regardless of what SQLAlchemy does). Checking for this is actually accomplished by checking for the "autocommit" execution option on the construct. When building a construct like an INSERT derivation, a new DDL type, or perhaps a stored procedure that alters data, the "autocommit" option needs to be set in order for the statement to function with "connectionless" execution (as described in :ref:`dbengine_implicit`). Currently a quick way to do this is to subclass :class:`.Executable`, then add the "autocommit" flag to the ``_execution_options`` dictionary (note this is a "frozen" dictionary which supplies a generative ``union()`` method):: from sqlalchemy.sql.expression import Executable, ClauseElement class MyInsertThing(Executable, ClauseElement): _execution_options = \ Executable._execution_options.union({'autocommit': True}) More succinctly, if the construct is truly similar to an INSERT, UPDATE, or DELETE, :class:`.UpdateBase` can be used, which already is a subclass of :class:`.Executable`, :class:`_expression.ClauseElement` and includes the ``autocommit`` flag:: from sqlalchemy.sql.expression import UpdateBase class MyInsertThing(UpdateBase): def __init__(self, ...): ... DDL elements that subclass :class:`.DDLElement` already have the "autocommit" flag turned on. 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. .. _type_compilation_extension: 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: * :class:`~sqlalchemy.sql.expression.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. * :class:`~sqlalchemy.sql.expression.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. :class:`~sqlalchemy.sql.expression.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() * :class:`~sqlalchemy.sql.functions.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, **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) * :class:`~sqlalchemy.schema.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. * :class:`~sqlalchemy.sql.expression.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``. 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() @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' @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): pass @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]) ) """ from .. import exc from .. import util from ..sql import visitors def compiles(class_, *specs): """Register a function as a compiler for a given :class:`_expression.ClauseElement` type.""" def decorate(fn): # get an existing @compiles handler existing = class_.__dict__.get("_compiler_dispatcher", None) # get the original handler. All ClauseElement classes have one # of these, but some TypeEngine classes will not. existing_dispatch = getattr(class_, "_compiler_dispatch", None) if not existing: existing = _dispatcher() if existing_dispatch: def _wrap_existing_dispatch(element, compiler, **kw): try: return existing_dispatch(element, compiler, **kw) except exc.UnsupportedCompilationError as uce: util.raise_( exc.UnsupportedCompilationError( compiler, type(element), message="%s construct has no default " "compilation handler." % type(element), ), from_=uce, ) existing.specs["default"] = _wrap_existing_dispatch # TODO: why is the lambda needed ? setattr( class_, "_compiler_dispatch", lambda *arg, **kw: existing(*arg, **kw), ) setattr(class_, "_compiler_dispatcher", existing) if specs: for s in specs: existing.specs[s] = fn else: existing.specs["default"] = fn return fn return decorate def deregister(class_): """Remove all custom compilers associated with a given :class:`_expression.ClauseElement` type.""" if hasattr(class_, "_compiler_dispatcher"): # regenerate default _compiler_dispatch visitors._generate_dispatch(class_) # remove custom directive del class_._compiler_dispatcher class _dispatcher(object): def __init__(self): self.specs = {} def __call__(self, element, compiler, **kw): # TODO: yes, this could also switch off of DBAPI in use. fn = self.specs.get(compiler.dialect.name, None) if not fn: try: fn = self.specs["default"] except KeyError as ke: util.raise_( exc.UnsupportedCompilationError( compiler, type(element), message="%s construct has no default " "compilation handler." % type(element), ), replace_context=ke, ) return fn(element, compiler, **kw)