起步

上一篇讨论了如何用数据库存储无限分级的数据。对于数据量大的情况(比如用户之间有邀请链,有点三级分销的意思),就要用到 closure table 的结构来进行存储。那么在 Django 中如何处理这个结构的模型呢?

定义模型

至少是要两个模型的,一个是存储分类,一个储存分类之间的关系:

class Category(models.Model):
    name = models.CharField(max_length=31)
    def __str__(self):
        return self.name

class CategoryRelation(models.Model):
    ancestor = models.ForeignKey(Category, null=True, related_name='ancestors', on_delete=models.SET_NULL, db_constraint=False, verbose_name='祖先')
    descendant = models.ForeignKey(Category,null=True,  related_name='descendants', on_delete=models.SET_NULL,
                                   db_constraint=False, verbose_name='子孙')
    distance = models.IntegerField()

    class Meta:
        unique_together = ("ancestor", "descendant")

数据操作

获得所有后代节点

class Category(models.Model):
    ...
    def get_descendants(self, include_self=False):
        """获得所有后代节点"""
        kw = {
            'descendants__ancestor' : self
        }
        if not include_self:
            kw['descendants__distance__gt'] = 0
        qs = Category.objects.filter(**kw).order_by('descendants__distance')
        return qs

获得直属下级

class Category(models.Model):
    ...
    def get_children(self):
        """获得直属下级"""
        qs = Category.objects.filter(descendants__ancestor=self, descendants__distance=1)
        return qs

节点的移动

节点的移动是比较难的,在 [https://www.percona.com/blog/2011/02/14/moving-subtrees-in-closure-table/][1] 中讲述了,利用django能够执行原生的sql语句进行:

    def add_child(self, child):
        """将某个分类加入本分类,"""
        if CategoryRelation.objects.filter(ancestor=child, descendant=self).exists() \
                or CategoryRelation.objects.filter(ancestor=self, descendant=child, distance=1).exists():
            """child不能是self的祖先节点 or 它们已经是父子节点"""
            return

        # 如果表中不存在节点自身数据
        if not CategoryRelation.objects.filter(ancestor=child, descendant=child).exists():
            CategoryRelation.objects.create(ancestor=child, descendant=child, distance=0)
        table_name = CategoryRelation._meta.db_table
        cursor = connection.cursor()

        cursor.execute(f"""
            DELETE a
            FROM
                {table_name} AS a
            JOIN {table_name} AS d ON a.descendant_id = d.descendant_id
            LEFT JOIN {table_name} AS x ON x.ancestor_id = d.ancestor_id
            AND x.descendant_id = a.ancestor_id
            WHERE
                d.ancestor_id = {child.id}
            AND x.ancestor_id IS NULL;
        """)

        cursor.execute(f"""
        INSERT INTO {table_name} (ancestor_id, descendant_id, distance)
        SELECT supertree.ancestor_id, subtree.descendant_id,
        supertree.distance+subtree.distance+1
        FROM {table_name} AS supertree JOIN {table_name} AS subtree
        WHERE subtree.ancestor_id = {child.id}
        AND supertree.descendant_id = {self.id};
        """)

节点删除

节点删除有两种操作,一个是将所有子节点也删除,另一个是将自己点移到上级节点中。

扩展阅读

  • [https://www.percona.com/blog/2011/02/14/moving-subtrees-in-closure-table/][2]
  • [http://technobytz.com/closure_table_store_hierarchical_data.html][3]

    完整代码

    
    class Category(models.Model):
    name = models.CharField(max_length=31)
    def __str__(self):
        return self.name
    
    def get_descendants(self, include_self=False):
        """获得所有后代节点"""
        kw = {
            'descendants__ancestor' : self
        }
        if not include_self:
            kw['descendants__distance__gt'] = 0
        qs = Category.objects.filter(**kw).order_by('descendants__distance')
    
        return qs
    
    def get_children(self):
        """获得直属下级"""
        qs = Category.objects.filter(descendants__ancestor=self, descendants__distance=1)
        return qs
    
    def get_ancestors(self, include_self=False):
        """获得所有祖先节点"""
        kw = {
            'ancestors__descendant': self
        }
        if not include_self:
            kw['ancestors__distance__gt'] = 0
        qs = Category.objects.filter(**kw).order_by('ancestors__distance')
        return qs
    
    def get_parent(self):
        """分类仅有一个父节点"""
        parent = Category.objects.get(ancestors__descendant=self, ancestors__distance=1)
        return parent
    
    def get_parents(self):
        """分类仅有一个父节点"""
        qs = Category.objects.filter(ancestors__descendant=self, ancestors__distance=1)
        return qs
    
    def remove(self, delete_subtree=False):
        """删除节点"""
        if delete_subtree:
            # 删除所有子节点
            children_queryset = self.get_descendants(include_self=True)
            for child in children_queryset:
                CategoryRelation.objects.filter(Q(ancestor=child) | Q(descendant=child)).delete()
                child.delete()
        else:
            # 所有子节点移到上级
            parent = self.get_parent()
            children = self.get_children()
            for child in children:
                parent.add_chile(child)
    
            # CategoryRelation.objects.filter(descendant=self, distance=0).delete()
            CategoryRelation.objects.filter(Q(ancestor=self) | Q(descendant=self)).delete()
            self.delete()
    
    def add_child(self, child):
        """将某个分类加入本分类,"""
        if CategoryRelation.objects.filter(ancestor=child, descendant=self).exists() \
                or CategoryRelation.objects.filter(ancestor=self, descendant=child, distance=1).exists():
            """child不能是self的祖先节点 or 它们已经是父子节点"""
            return
    
        # 如果表中不存在节点自身数据
        if not CategoryRelation.objects.filter(ancestor=child, descendant=child).exists():
            CategoryRelation.objects.create(ancestor=child, descendant=child, distance=0)
        table_name = CategoryRelation._meta.db_table
        cursor = connection.cursor()
    
        cursor.execute(f"""
            DELETE a
            FROM
                {table_name} AS a
            JOIN {table_name} AS d ON a.descendant_id = d.descendant_id
            LEFT JOIN {table_name} AS x ON x.ancestor_id = d.ancestor_id
            AND x.descendant_id = a.ancestor_id
            WHERE
                d.ancestor_id = {child.id}
            AND x.ancestor_id IS NULL;
        """)
    
        cursor.execute(f"""
        INSERT INTO {table_name} (ancestor_id, descendant_id, distance)
        SELECT supertree.ancestor_id, subtree.descendant_id,
        supertree.distance+subtree.distance+1
        FROM {table_name} AS supertree JOIN {table_name} AS subtree
        WHERE subtree.ancestor_id = {child.id}
        AND supertree.descendant_id = {self.id};
        """)

class CategoryRelation(models.Model): ancestor = models.ForeignKey(Category, null=True, related_name='ancestors', on_delete=models.SET_NULL, db_constraint=False, verbose_name='祖先') descendant = models.ForeignKey(Category,null=True, related_name='descendants', on_delete=models.SET_NULL, db_constraint=False, verbose_name='子孙') distance = models.IntegerField()

class Meta:
    unique_together = ("ancestor", "descendant")


  [1]: https://www.percona.com/blog/2011/02/14/moving-subtrees-in-closure-table/
  [2]: https://www.percona.com/blog/2011/02/14/moving-subtrees-in-closure-table/
  [3]: http://technobytz.com/closure_table_store_hierarchical_data.html

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