Method join

django/db/models/sql/query.py:1134–1186

Return an alias for the 'join', either reusing an existing alias for that join or creating a new one. 'join' is either a base_table_class or join_class. The 'reuse' parameter can be either None which means all joins are reusable, or it can be a set containin

(self, join, reuse=None)

        return len([1 for count in self.alias_refcount.values() if count])

    def join(self, join, reuse=None):
        class="st">"""
        Return an alias for the &class="cm">#x27;join', either reusing an existing alias for
        that join or creating a new one. &class="cm">#x27;join' is either a base_table_class or
        join_class.

        The &class="cm">#x27;reuse' parameter can be either None which means all joins are
        reusable, or it can be a set containing the aliases that can be reused.

        A join is always created as LOUTER if the lhs alias is LOUTER to make
        sure chains like t1 LOUTER t2 INNER t3 aren&class="cm">#x27;t generated. All new
        joins are created as LOUTER if the join is nullable.
        class="st">"""
        reuse_aliases = [
            a
            for a, j in self.alias_map.items()
            if (reuse is None or a in reuse) and j == join
        ]
        if reuse_aliases:
            if join.table_alias in reuse_aliases:
                reuse_alias = join.table_alias
            else:
                class="cm"># Reuse the most recent alias of the joined table
                class="cm"># (a many-to-many relation may be joined multiple times).
                reuse_alias = reuse_aliases[-1]
            self.ref_alias(reuse_alias)
            return reuse_alias

        class="cm"># No reuse is possible, so we need a new alias.
        alias, _ = self.table_alias(
            join.table_name, create=True, filtered_relation=join.filtered_relation
        )
        if join.join_type:
            if self.alias_map[join.parent_alias].join_type == LOUTER or join.nullable:
                join_type = LOUTER
            else:
                join_type = INNER
            join.join_type = join_type
        join.table_alias = alias
        self.alias_map[alias] = join
        if filtered_relation := join.filtered_relation:
            resolve_reuse = reuse
            if resolve_reuse is not None:
                resolve_reuse = set(reuse) | {alias}
            joins_len = len(self.alias_map)
            join.filtered_relation = filtered_relation.resolve_expression(
                self, reuse=resolve_reuse
            )
            class="cm"># Some joins were during expression resolving, they must be present
            class="cm"># before the one we just added.
            if joins_len < len(self.alias_map):
                self.alias_map[alias] = self.alias_map.pop(alias)
        return alias

    def join_parent_model(self, opts, model, alias, seen):
        class="st">"""