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Data Access & SQL

SQLAlchemy and Django ORM sessions, transactions, loading, migrations, pooling, and production database behavior.

24 drillsPractice one at a timeOrdered pathConcept questionArchitecture scenarioComparison questionDebugging scenario

Recommended start

Engine, Connection, and Session

Separates SQLAlchemy connectivity, checked-out database resources, ORM state, and transaction ownership.

Medium~10 minStep 1 of 24

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1Engine, Connection, and SessionRecommended startSeparates SQLAlchemy connectivity, checked-out database resources, ORM state, and transaction ownership.Concept questionMedium~10 minPythonSQLAlchemyPractice 2Identity map and unit of workExplains object identity, tracked changes, flush ordering, and the limits of session-level caching.Concept questionMedium~10 minPythonSQLAlchemyPractice 3SQLAlchemy session lifetimeDefines one explicit SQLAlchemy session and transaction owner for each request, job, thread, or async task.Architecture scenarioHard~15 minPythonSQLAlchemyPractice 4Flush, commit, and rollbackSeparates SQL synchronization from transaction completion and explains recovery after database errors.Comparison questionHard~15 minPythonSQLAlchemyPractice 5SQLAlchemy object statesConnects SQLAlchemy object states to flush, commit, close, serialization, and later attribute access.Concept questionHard~15 minPythonSQLAlchemyPractice 6Transaction boundaries and savepointsAligns SQLAlchemy transactions with business operations and uses savepoints without inventing independent commits.Architecture scenarioHard~15 minPythonSQLAlchemyPractice 7Constraints vs ValidationExplains database constraints versus application validation with practical data-access examples and production trade-offs.Comparison questionMedium~7 minSQLPractice 8ORM vs Core vs raw SQLChooses a SQLAlchemy abstraction from the work's shape instead of performance slogans.Comparison questionMedium~10 minPythonSQLAlchemyPractice 9SQLAlchemy loader strategiesChooses relationship loading from cardinality, query count, result size, and hidden-I/O boundaries.Comparison questionHard~15 minPythonSQLAlchemyPractice 10Diagnose SQLAlchemy N+1Finds repeated relationship queries and fixes the data shape without introducing a larger query problem.Debugging scenarioHard~15 minPythonSQLAlchemyPractice 11SQLAlchemy API projectionsAligns selected database data with an API read model and avoids unnecessary ORM state.Architecture scenarioMedium~10 minPythonSQLAlchemyPractice 12AsyncSession I/O boundariesMakes AsyncSession transaction ownership and every database I/O point explicit.Debugging scenarioHard~15 minPythonSQLAlchemyPractice 13SQLAlchemy pool sizingConnects pool limits to process topology, transaction duration, database capacity, and queueing evidence.Architecture scenarioHard~15 minPythonSQLAlchemyPractice 14Stale connections and timeoutsSeparates stale checkout protection, connection aging, pool waits, statement timeouts, and uncertain transactions.Debugging scenarioHard~15 minPythonSQLAlchemyPractice 15Diagnose a slow SQLAlchemy queryConnects one slow Python request to generated SQL, representative parameters, database plans, and measured validation.Debugging scenarioHard~15 minPythonSQLAlchemyPractice 16Indexes for APIsConnects API query shape to index order, selectivity, execution plans, write cost, and safe production rollout.Concept questionMedium~7 min.NETC#EF CorePractice 17Keyset PaginationCompares offset and keyset pagination for stable API lists, large tables, and user-facing result consistency.Concept questionMedium~7 minSQLSQL ServerPractice 18Django QuerySet evaluationExplains lazy query construction, evaluation triggers, result caching, and repeated-query surprises.Concept questionMedium~10 minDjangoPythonPractice 19select_related vs prefetch_relatedChooses Django relationship loading from relationship shape, result cardinality, and measured query behavior.Comparison questionMedium~10 minDjangoPythonPractice 20Django atomic and on_commitDefines Django transaction nesting, exception boundaries, and post-commit work without promising durability.Architecture scenarioHard~15 minDjangoPythonPractice 21Concurrent Django updatesChooses atomic database expressions, row locks, and constraints from the invariant being protected.Architecture scenarioHard~15 minDjangoPythonPractice 22Review Alembic autogenerateTreats Alembic autogeneration as a candidate schema diff that still needs semantic and operational review.Code reviewHard~15 minAlembicPythonSQLAlchemyPractice 23Safe MigrationExplains expand-and-contract migrations, backwards-compatible schema changes, backfills, rollout order, and rollback thinking.Concept questionHard~9 minSQLSQL ServerPractice 24Unique ConstraintsMakes business uniqueness race-safe with the exact database key, while handling normalization, tenant scope, nulls, soft deletion, rollout cleanup, and stable API conflicts.Concept questionMedium~7 minSQLSQL ServerPractice

Practice order

24 drillsOne at a time

1Engine, Connection, and SessionUp next / Medium / ~10 min
2Identity map and unit of workMedium / ~10 min
3SQLAlchemy session lifetimeHard / ~15 min
4Flush, commit, and rollbackHard / ~15 min
5SQLAlchemy object statesHard / ~15 min
6Transaction boundaries and savepointsHard / ~15 min
7Constraints vs ValidationMedium / ~7 min
8ORM vs Core vs raw SQLMedium / ~10 min

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