Data access.
WHAT IT PROVIDES
Models defining tables, with relationships and constraints.
WHAT MIGRATIONS DO
Generate and apply schema changes from model definitions.
WHAT TO ALWAYS DO
Review generated migrations before applying them.
WHY
Generated changes occasionally do something destructive you did not intend.
WHAT THE COMMONEST PERFORMANCE FAULT IS
Accessing a related object for each item in a loop, producing a query per item.
WHAT SOLVES IT
Selecting or prefetching related data with the original query.
WHICH TO USE
One for single-valued relationships, joining in the same query The other for multi-valued relationships, using a second query
WHAT TO MONITOR
Query count per request, during development.
WHAT ELSE TO BE CAREFUL WITH
Loading entire tables into memory Querying without indexes on filtered fields Counting by loading all records
WHAT TO USE INSTEAD OF THAT LAST
The database's own counting.
WHAT TO KNOW ABOUT QUERYSETS
They are lazy, evaluated only when used.
WHY THAT MATTERS
The place a query executes may not be where it was written.
WHAT TO USE FOR MONEY
Exact decimal fields.
