动态（自引用）关系应用过滤器

Question

I have a model that I would like to query with a filter. I also want this filter to be applied on its children (one-to-many self-referential relationship). Example:

class Item(Model):
    __tablename__ = 'item'

    id = Column(Integer, primary_key=True)
    color = Column(String)

    parent_id = Column(Integer, ForeignKey('item.id'))
    children = relationship(
        'Item',
        backref=backref('parent', remote_side=[id]),
        lazy='dynamic'
    )

    item = session.query(Item).filter(Item.color == 'blue').one()

Now I want to apply the same filter on the children collection once, and be able to access the filtered collection in a transparent way later.

I could imagine something like:

item.children.apply_filter(Item.color == 'blue')  # `children` is dynamic

and then accessing item.children would be equivalent to accessing item.children.filter(Item.color == 'blue').all() now. Note that with this solution, the fact that the relationship is self-referential doesn't matter.

SQLAlchemy's API is huge and I can't figure out if there is a proper way to do this (or something equivalent).

(Is it possible to modify the primaryjoin of a relationship at query time ? Or give it a parameter ?)

Answer 1

Please note that if you have multiple items with the same color, your original query item = session.query(Item).filter(Item.color == 'blue').one() will fail.

When you get past that and get your (parent) Item instance:

my_item = session.query(Item).filter(Item.color == 'blue')[0]
# my_item = session.query(Item).filter(Item.color == 'blue').filter(Item.parent == None).one() # assume this is only one root item per color

because children is dynamic , my_item.children will return you a query. In this case you simply apply another filter and you are done:

my_blue_children = my_item.children.filter(Item.color == 'blue').all()

The filter clause will now apply a filter to the select statement loading children.