[3] | 1 | """Provides an API for creation of custom ClauseElements and compilers. |
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| 2 | |
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| 3 | Synopsis |
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| 4 | ======== |
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| 5 | |
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| 6 | Usage involves the creation of one or more :class:`~sqlalchemy.sql.expression.ClauseElement` |
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| 7 | subclasses and one or more callables defining its compilation:: |
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| 8 | |
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| 9 | from sqlalchemy.ext.compiler import compiles |
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| 10 | from sqlalchemy.sql.expression import ColumnClause |
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| 11 | |
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| 12 | class MyColumn(ColumnClause): |
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| 13 | pass |
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| 14 | |
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| 15 | @compiles(MyColumn) |
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| 16 | def compile_mycolumn(element, compiler, **kw): |
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| 17 | return "[%s]" % element.name |
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| 18 | |
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| 19 | Above, ``MyColumn`` extends :class:`~sqlalchemy.sql.expression.ColumnClause`, the |
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| 20 | base expression element for column objects. The ``compiles`` decorator registers |
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| 21 | itself with the ``MyColumn`` class so that it is invoked when the object |
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| 22 | is compiled to a string:: |
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| 23 | |
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| 24 | from sqlalchemy import select |
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| 25 | |
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| 26 | s = select([MyColumn('x'), MyColumn('y')]) |
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| 27 | print str(s) |
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| 28 | |
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| 29 | Produces:: |
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| 30 | |
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| 31 | SELECT [x], [y] |
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| 32 | |
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| 33 | Compilers can also be made dialect-specific. The appropriate compiler will be invoked |
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| 34 | for the dialect in use:: |
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| 35 | |
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| 36 | from sqlalchemy.schema import DDLElement # this is a SQLA 0.6 construct |
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| 37 | |
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| 38 | class AlterColumn(DDLElement): |
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| 39 | |
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| 40 | def __init__(self, column, cmd): |
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| 41 | self.column = column |
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| 42 | self.cmd = cmd |
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| 43 | |
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| 44 | @compiles(AlterColumn) |
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| 45 | def visit_alter_column(element, compiler, **kw): |
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| 46 | return "ALTER COLUMN %s ..." % element.column.name |
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| 47 | |
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| 48 | @compiles(AlterColumn, 'postgres') |
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| 49 | def visit_alter_column(element, compiler, **kw): |
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| 50 | return "ALTER TABLE %s ALTER COLUMN %s ..." % (element.table.name, element.column.name) |
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| 51 | |
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| 52 | The second ``visit_alter_table`` will be invoked when any ``postgres`` dialect is used. |
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| 53 | |
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| 54 | The ``compiler`` argument is the :class:`~sqlalchemy.engine.base.Compiled` object |
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| 55 | in use. This object can be inspected for any information about the in-progress |
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| 56 | compilation, including ``compiler.dialect``, ``compiler.statement`` etc. |
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| 57 | The :class:`~sqlalchemy.sql.compiler.SQLCompiler` and :class:`~sqlalchemy.sql.compiler.DDLCompiler` (DDLCompiler is 0.6. only) |
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| 58 | both include a ``process()`` method which can be used for compilation of embedded attributes:: |
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| 59 | |
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| 60 | class InsertFromSelect(ClauseElement): |
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| 61 | def __init__(self, table, select): |
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| 62 | self.table = table |
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| 63 | self.select = select |
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| 64 | |
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| 65 | @compiles(InsertFromSelect) |
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| 66 | def visit_insert_from_select(element, compiler, **kw): |
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| 67 | return "INSERT INTO %s (%s)" % ( |
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| 68 | compiler.process(element.table, asfrom=True), |
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| 69 | compiler.process(element.select) |
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| 70 | ) |
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| 71 | |
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| 72 | insert = InsertFromSelect(t1, select([t1]).where(t1.c.x>5)) |
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| 73 | print insert |
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| 74 | |
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| 75 | Produces:: |
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| 76 | |
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| 77 | "INSERT INTO mytable (SELECT mytable.x, mytable.y, mytable.z FROM mytable WHERE mytable.x > :x_1)" |
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| 78 | |
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| 79 | |
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| 80 | """ |
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| 81 | |
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| 82 | def compiles(class_, *specs): |
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| 83 | def decorate(fn): |
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| 84 | existing = getattr(class_, '_compiler_dispatcher', None) |
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| 85 | if not existing: |
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| 86 | existing = _dispatcher() |
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| 87 | |
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| 88 | # TODO: why is the lambda needed ? |
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| 89 | setattr(class_, '_compiler_dispatch', lambda *arg, **kw: existing(*arg, **kw)) |
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| 90 | setattr(class_, '_compiler_dispatcher', existing) |
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| 91 | |
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| 92 | if specs: |
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| 93 | for s in specs: |
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| 94 | existing.specs[s] = fn |
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| 95 | else: |
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| 96 | existing.specs['default'] = fn |
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| 97 | return fn |
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| 98 | return decorate |
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| 99 | |
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| 100 | class _dispatcher(object): |
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| 101 | def __init__(self): |
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| 102 | self.specs = {} |
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| 103 | |
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| 104 | def __call__(self, element, compiler, **kw): |
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| 105 | # TODO: yes, this could also switch off of DBAPI in use. |
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| 106 | fn = self.specs.get(compiler.dialect.name, None) |
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| 107 | if not fn: |
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| 108 | fn = self.specs['default'] |
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| 109 | return fn(element, compiler, **kw) |
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| 110 | |
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