Frequently Asked Questions

The Frequently Asked Questions section is a growing collection of commonly observed questions to well-known issues.

• Installation
  • I’m getting an error about greenlet not being installed when I try to use asyncio
• Connections / Engines
  • How do I configure logging?
  • How do I pool database connections? Are my connections pooled?
  • How do I pass custom connect arguments to my database API?
  • “MySQL Server has gone away”
  • “Commands out of sync; you can’t run this command now” / “This result object does not return rows. It has been closed automatically”
  • How Do I “Retry” a Statement Execution Automatically?
  • Why does SQLAlchemy issue so many ROLLBACKs?
  • I am using multiple connections with a SQLite database (typically to test transaction operation), and my test program is not working!
  • How do I get at the raw DBAPI connection when using an Engine?
  • How do I use engines / connections / sessions with Python multiprocessing, or os.fork()?
• MetaData / Schema
  • My program is hanging when I say table.drop() / metadata.drop_all()
  • Does SQLAlchemy support ALTER TABLE, CREATE VIEW, CREATE TRIGGER, Schema Upgrade Functionality?
  • How can I sort Table objects in order of their dependency?
  • How can I get the CREATE TABLE/ DROP TABLE output as a string?
  • How can I subclass Table/Column to provide certain behaviors/configurations?
• SQL Expressions
  • How do I render SQL expressions as strings, possibly with bound parameters inlined?
  • Why are percent signs being doubled up when stringifying SQL statements?
  • I’m using op() to generate a custom operator and my parenthesis are not coming out correctly
• ORM Configuration
  • How do I map a table that has no primary key?
  • How do I configure a Column that is a Python reserved word or similar?
  • How do I get a list of all columns, relationships, mapped attributes, etc. given a mapped class?
  • I’m getting a warning or error about “Implicitly combining column X under attribute Y”
  • I’m using Declarative and setting primaryjoin/secondaryjoin using an and_() or or_(), and I am getting an error message about foreign keys.
  • Why is ORDER BY recommended with LIMIT (especially with subqueryload())?
• Performance
  • Why is my application slow after upgrading to 1.4 and/or 2.x?
  • How can I profile a SQLAlchemy powered application?
  • I’m inserting 400,000 rows with the ORM and it’s really slow!
• Sessions / Queries
  • I’m re-loading data with my Session but it isn’t seeing changes that I committed elsewhere
  • “This Session’s transaction has been rolled back due to a previous exception during flush.” (or similar)
  • How do I make a Query that always adds a certain filter to every query?
  • My Query does not return the same number of objects as query.count() tells me - why?
  • I’ve created a mapping against an Outer Join, and while the query returns rows, no objects are returned. Why not?
  • I’m using joinedload() or lazy=False to create a JOIN/OUTER JOIN and SQLAlchemy is not constructing the correct query when I try to add a WHERE, ORDER BY, LIMIT, etc. (which relies upon the (OUTER) JOIN)
  • Query has no __len__(), why not?
  • How Do I use Textual SQL with ORM Queries?
  • I’m calling Session.delete(myobject) and it isn’t removed from the parent collection!
  • why isn’t my __init__() called when I load objects?
  • how do I use ON DELETE CASCADE with SA’s ORM?
  • I set the “foo_id” attribute on my instance to “7”, but the “foo” attribute is still None - shouldn’t it have loaded Foo with id #7?
  • How do I walk all objects that are related to a given object?
  • Is there a way to automagically have only unique keywords (or other kinds of objects) without doing a query for the keyword and getting a reference to the row containing that keyword?
  • Why does post_update emit UPDATE in addition to the first UPDATE?
• Third Party Integration Issues
  • I’m getting errors related to “numpy.int64”, “numpy.bool_”, etc.
  • SQL expression for WHERE/HAVING role expected, got True