Class Case

django/db/models/expressions.py:1717–1813

An SQL searched CASE expression: CASE WHEN n > 0 THEN 'positive' WHEN n < 0 THEN 'negative' ELSE 'zero' END

@deconstructible(path="django.db.models.Case")
class Case(SQLiteNumericMixin, Expression):
    """
    An SQL searched CASE expression:

        CASE
            WHEN n > 0
                THEN 'positive'
            WHEN n < 0
                THEN 'negative'
            ELSE 'zero'
        END
    """

    template = "CASE %(cases)s ELSE %(default)s END"
    case_joiner = " "

    def __init__(self, *cases, default=None, output_field=None, **extra):
        if not all(isinstance(case, When) for case in cases):
            raise TypeError("Positional arguments must all be When objects.")
        super().__init__(output_field)
        self.cases = list(cases)
        self.default = self._parse_expressions(default)[0]
        self.extra = extra

    def __str__(self):
        return "CASE %s, ELSE %r" % (
            ", ".join(str(c) for c in self.cases),
            self.default,
        )

    def __repr__(self):
        return "<%s: %s>" % (self.__class__.__name__, self)

    def get_source_expressions(self):
        return [*self.cases, self.default]

    def set_source_expressions(self, exprs):
        *self.cases, self.default = exprs

    def copy(self):
        c = super().copy()
        c.cases = c.cases[:]
        return c

    def as_sql(
        self, compiler, connection, template=None, case_joiner=None, **extra_context
    ):
        connection.ops.check_expression_support(self)
        if not self.cases:
            return compiler.compile(self.default)
        template_params = {**self.extra, **extra_context}
        case_parts = []
        sql_params = []
        for case in self.cases:
            try:
                case_sql, case_params = compiler.compile(case)
            except EmptyResultSet:
                continue