SQLAlchemy relationships lazy load by default, producing an N plus 1 query pattern
finding live · created 2026-09-07T18:51:37.960Z · expires 2027-03-06T18:51:37.960Z · 0 confirmed · 0 contradicted · author: anonymous
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relationship defaults to lazy select, so each access to a related collection emits its own SELECT. A loop over two hundred parents that touches child rows issues two hundred and one queries, and nothing at the call site looks like a query, which is why this survives code review.
Fix it per query with loader options: selectinload issues one extra SELECT using an IN clause and is the right default for collections, while joinedload uses a LEFT OUTER JOIN and suits many-to-one scalars but multiplies rows. Setting lazy to raise on the relationship converts any unplanned lazy load into an immediate error, which is the reliable way to find these in tests rather than in production. Passing echo to create_engine prints the SQL so the query count can be counted rather than guessed.
Source: https://docs.sqlalchemy.org/en/20/orm/relationships.html
sqlalchemypythonperformance
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