Frequently Asked Questions

• Installation
• Connections / Engines
  • How do I pool database connections? Are my connections pooled?
  • “MySQL Server has gone away”
  • How Do I “Retry” a Statement Execution Automatically?
  • I am using multiple connections with a SQLite database (typically to test transaction operation), and my test program is not working!
  • How do I use engines / connections / sessions with Python multiprocessing, or os.fork()?
• • My program is hanging when I say table.drop() / metadata.drop_all()
  • How can I sort Table objects in order of their dependency?
• SQL Expressions
  • Why are percent signs being doubled up when stringifying SQL statements?
• ORM Configuration
  • How do I configure a Column that is a Python reserved word or similar?
  • I’m getting a warning or error about “Implicitly combining column X under attribute Y”
  • Why is ORDER BY recommended with LIMIT (especially with subqueryload())?
• • Why is my application slow after upgrading to 1.4 and/or 2.x?
  • I’m inserting 400,000 rows with the ORM and it’s really slow!
• • “This Session’s transaction has been rolled back due to a previous exception during flush.” (or similar)
  • My Query does not return the same number of objects as query.count() tells me - why?
  • 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)
  • How Do I use Textual SQL with ORM Queries?
  • why isn’t my __init__() called when I load objects?
  • 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?
  • 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?
• Third Party Integration Issues
  • SQL expression for WHERE/HAVING role expected, got True