QuerySets Evaluation in Django

  • Creating a QuerySet does not involve any Database(DB) activity until it is evaluated. 
  • QuerySets usually return another unevaluated QuerySet. 
  • You can concatenate as many filters as you like to a QuerySet, and you will not hit the database until the QuerySet is evaluated. 
  • When a QuerySet is evaluated, it translates into an SQL query to the database.
  • QuerySets are only evaluated in the following scenarios:
    • The first time you iterate over them
    • When you pickle or cache them
    • When you slice them, for Ex:, Posts.objects.all()[:5]
    • When you call repr() or len() on them
    • When you explicitly call list() on them
    • When you test them in a statement, such as bool(), or, and, or if

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