Peewee ForeignKey filtering

import peewee

db = peewee.SqliteDatabase('/somepath')

class BaseModel(peewee.Model):
    class Meta:
        database = db

class Page(BaseModel):
    url = peewee.CharField(unique=True)

class Tag(BaseModel):
    name = peewee.CharField(index=True)

class PageTag(BaseModel):
    page = peewee.ForeignKeyField(Page, related_name='_pagetags', index=True)
    tag = peewee.ForeignKeyField(Tag, index=True)

query = Page.select(Page.url)
tag = 'some_tag'
# this works:
query = query.where(
    orm.Page.id << [
        i.page.id for i in
        orm.PageTag.select(orm.PageTag.page)
        .where(
        orm.PageTag.tag ==
            orm.Tag.get(orm.Tag.name == tag))])

# this doesn't:
query = query.where(tag << orm.Page._pagetags)