Class ColPairs

django/db/models/expressions.py:1390–1444

class ColPairs(Expression):
    def __init__(self, alias, targets, sources, output_field):
        super().__init__(output_field=output_field)
        self.alias = alias
        self.targets = targets
        self.sources = sources

    def __len__(self):
        return len(self.targets)

    def __iter__(self):
        return iter(self.get_cols())

    def __repr__(self):
        return (
            f"{self.__class__.__name__}({self.alias!r}, {self.targets!r}, "
            f"{self.sources!r}, {self.output_field!r})"
        )

    def get_cols(self):
        return [
            Col(self.alias, target, source)
            for target, source in zip(self.targets, self.sources)
        ]

    def get_source_expressions(self):
        return self.get_cols()

    def set_source_expressions(self, exprs):
        assert all(isinstance(expr, Col) and expr.alias == self.alias for expr in exprs)
        self.targets = [col.target for col in exprs]
        self.sources = [col.field for col in exprs]

    def as_sql(self, compiler, connection):
        cols_sql = []
        cols_params = []
        cols = self.get_cols()

        for col in cols:
            sql, params = col.as_sql(compiler, connection)
            cols_sql.append(sql)
            cols_params.extend(params)

        return ", ".join(cols_sql), tuple(cols_params)

    def relabeled_clone(self, relabels):
        return self.__class__(
            relabels.get(self.alias, self.alias), self.targets, self.sources, self.field
        )

    def resolve_expression(self, *args, **kwargs):
        return self

    def select_format(self, compiler, sql, params):
        return sql, params


class Ref(Expression):