SQLAlchemy：通过辅助表以自定义条件进行外部联接

Question

I'm trying to write a query with an outer join using a custom condition, through a secondary (many-to-many) table. I'm having trouble figuring out the right syntax.

Let's say I have those Item and Tag tables, with a many-to-many relationship between them :

item_to_tag_table = Table('item_to_tag', Base.metadata,
    Column('item_id', Integer, ForeignKey('item.id')),
    Column('tag_id', Integer, ForeignKey('tag.id')))

With the following relationship in Item :

class Item(Base):
    id = Column(Integer, primary_key=True)

    tags = relationship("Tag",
        secondary = item_to_tag_table,
        backref = "items",
        )

And I want to query items that do not contain a specific tag, so I need to use an outer join.

If a tag was linked to only 1 item I could do something like this :

DBSession.query(Items)\
    .outerjoin(Tag, and_(Tag.itemId == Item.id, Tag.name == "foo"))\
    .having(func.count(Tag.id) < 1)

However, for a many-to many relationship, I can't determine the right syntax:

DBSession.query(Items)\
    .outerjoin(Tag, and_( ?? , Tag.name == "foo"))\
    .having(func.count(Tag.id) < 1)

Any ideas? Do I have to make 2 outer joins, one to the secondary table, and the other to the Tag table?

Thanks in advance!

Answer 1

In your example you don't need to use outerjoin, NOT expression is all you need ("~" before condition).

To filter many-to-many relationship use .any()

DBSession.query(Items)\
    .join(Tag)\ 
    .filter(~Item.tags.any(name="foo"))