我想在SQLAlchemy中创建一个不区分大小写的唯一约束，该约束可以与Postgres和SQLite一起使用。

在Postgres中可以实现：

• SQL： CREATE UNIQUE INDEX my_index ON my_table (lower(my_field));
• SQLAlchemy： Index('my_index', func.lower(my_field), unique=True)

在SQLite中，可以使用以下方法实现：

• SQL： CREATE UNIQUE INDEX my_index ON my_table (my_field COLLATE NOCASE);
• SQLAlchemy： Index('my_index', collate(my_field, 'nocase'), unique=True)

因此，我试图将其子类化FunctionElement，以创建自己的类似于函数的构造，该构造可以编译为DBMSmy_field COLLATE NOCASE或lower(my_field)根据DBMS编译。我尝试遵循编译扩展指南，但遇到错误（请参见下面的追溯），可以通过SQLAlchemy源代码进行跟踪，但是我无法理解。我试着子类其他的事情，比如ColumnElement，ColumnClause，ClauseElement，但我得到类似的错误。

这是重现该错误的代码。我使用了SQLAlchemy的0.8.2版本。

from sqlalchemy import create_engine, Column, String, Integer
from sqlalchemy.types import TypeDecorator, VARCHAR
from sqlalchemy import func, Index, collate
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.expression import (
    # Not sure which one to subclass
    FunctionElement, ColumnElement, ColumnClause, ClauseElement
)


engine = create_engine('sqlite:///:memory:', echo=True)
Base = declarative_base()


class CaseInsensitive(FunctionElement):
    name = 'CaseInsensitive'
    type = VARCHAR()


@compiles(CaseInsensitive, 'sqlite')
def case_insensitive_sqlite(element, compiler, **kw):
    arg1, = list(element.clauses)
    return collate(compiler.process(arg1), 'nocase')


@compiles(CaseInsensitive, 'postgresql')
def case_insensitive_postgresql(element, compiler, **kw):
    arg1, = list(element.clauses)
    return func.lower(compiler.process(arg1))


class MyTable(Base):

    __tablename__ = 'my_table'

    id = Column(Integer, primary_key=True)

    my_field = Column(String)

    __table_args__ = (
        Index('idx', CaseInsensitive(my_field), unique=True),
    )

Traceback (most recent call last):
  File "tmp.py", line 33, in <module>
    class MyTable(Base):
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/ext/declarative/api.py", line 50, in __init__
    _as_declarative(cls, classname, cls.__dict__)
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 222, in _as_declarative
    **table_kw)
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/schema.py", line 332, in __new__
    table._init(name, metadata, *args, **kw)
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/schema.py", line 400, in _init
    self._init_items(*args)
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/schema.py", line 65, in _init_items
    item._set_parent_with_dispatch(self)
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/events.py", line 236, in _set_parent_with_dispatch
    self._set_parent(parent)
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/schema.py", line 2421, in _set_parent
    ColumnCollectionMixin._set_parent(self, table)
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/schema.py", line 2035, in _set_parent
    self.columns.add(col)
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/sql/expression.py", line 2477, in add
    self[column.key] = column
  File "/Users/vladimir/git/wb/lib/python2.7/site-packages/sqlalchemy/sql/expression.py", line 2296, in __getattr__
    key)
AttributeError: Neither 'CaseInsensitive' object nor 'Comparator' object has an attribute 'key'