SQLAlchemy - 在一对多关系中过滤子项

Question

I have declared my models using the style described in the flask docs to have One-To-many relationships. The models are nested twice - I have Articles with Segments with Items.

Both Segments and Items have a visibility attribute which can be True or False. When editing, I just select all items, but when displaying I'd like to only select those which the visibility attribute is True .

Is there an idiom in SQLAlchemy for filtering children? I've tried with subqueries but could not achieve what I wanted.

Eg. I want to achieve:

A single article -> Visible Segment 1 -> This segment's Visible Item 1
                                      -> This segment's Visible Item 2
                 -> Visible Segment 2 -> This segment's Visible Item 1
                                      -> This segment's Visible Item 2

Answer 1

if the relationships refer to small numbers of objects I typically just do this in Python:

class Article(Base):
    # ...

    segments = relationship(Segment)

    @property
    def visible_items(self):
        return [item for segment in self.segments 
                     for item in segment.items
                     if segment.visible and item.visible]

class Segment(Base):
    # ...

    items = relationship(Item)

now if you really want to go full relationship for this, you can define an alternate relationship with that primaryjoin:

class Article(Base):
    # ...

    segments = relationship(Segment)

    visible_segments = relationship(Segment, 
            primaryjoin="and_(Segment.article_id == Article.id, Segment.visible == True)")

    @property
    def visible_items(self):
        return [item for segment in self.visible_segments 
                     for item in segment.visible_items]

class Segment(Base):
    # ...

    items = relationship(Item)
    visible_items = relationship(Item, 
               primaryjoin="and_(Item.segment_id == Segment.id, Item.visible==True)")