upsert.py

#!/usr/bin/env python
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"""
This module provides an "Upsert" statement for SQL Alchemy which uses 
ON DUPLICATE KEY UPDATE to implement the insert or update semantics. It
supports doing a bulk insert.
"""

import sqlalchemy as sa
from sqlalchemy.ext.compiler import compiles
import sqlalchemy.sql.expression as expr

@compiles(expr.Insert, "mysql")
def compile_upsert(insert_stmt, compiler, **kwargs):
    keys = list(compiler.column_keys)
    pk = insert_stmt.table.primary_key
    if len(pk.columns) == 1:
        pkColumn = pk.columns.keys()[0]
        if pkColumn in keys:
            keys.remove(pkColumn)
    insert = compiler.visit_insert(insert_stmt, **kwargs)
    ondup = 'ON DUPLICATE KEY UPDATE'
    updates = ', '.join(
        '%s = VALUES(%s)' % (c.name, c.name)
        for c in insert_stmt.table.columns
        if c.name in keys
    )
    upsert = ' '.join((insert, ondup, updates))
    return upsert

This simple Gist overrides the INSERT statement for SQLAlchemy ORM models, and returns an INSERT .. ON DUPLICATE KEY UPDATE .. instead. It rewrites all columns except for the primary key.

Won't work if the PK is more than 1 column long.

Generally, I didn't put up too much effort into making this work - so YMMV.