Compilation extension in SQLAlchemy: functions used in indexes

Question

I want to make a case-insensitive unique constraint in SQLAlchemy, which would work with both Postgres and SQLite.

In Postgres it is achieved:

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

In SQLite it is achieved using:

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

So I'm trying to subclass the FunctionElement in order to create my own function-like construct, which would compile either to my_field COLLATE NOCASE or lower(my_field) depending on DBMS. I tried to follow the guide to compilation extension, but I'm getting an error (see traceback below), which I can follow through SQLAlchemy source code, but which I cannot understand. I tried subclassing other things, such as ColumnElement, ColumnClause, ClauseElement, but I'm getting similar errors.

Here is the code that reproduces the error. I used version 0.8.2 of SQLAlchemy.

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'

Answer 1

the @compiles method has to return a string. Also, there's some glitch in how Index is traversing the functionelement here so we need one little workaround:

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


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


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