Flask-SQLAlchemy best approach to taking out parent after removing all children

Question

You are able to delete all children after their associate parent gets deleted cascadingly, but what about the opposite way? In this many-to-one relationship:

class RecipeTag(db.Model):

    __tablename__ = "recipe_tags"
    id = db.Column(db.Integer, primary_key=True)
    recipe_id = db.Column(db.Integer, db.ForeignKey("recipes.id", ondelete="CASCADE"), nullable=False)
    tag_id = db.Column(db.Integer, db.ForeignKey("tags.id", ondelete="CASCADE"), nullable=False)

    parent = db.relationship("Tag", cascade="all, delete", single_parent=True, back_populates="children")

class Tag(db.Model):

    __tablename__  = "tags"
    id = db.Column(db.Integer, primary_key=True)
    tag = db.Column(db.String, nullable=False)

    children = db.relationship("RecipeTag", cascade="all, delete, delete-orphan", back_populates="parent")

The RecipeTag will connect the Recipe model with the Tag model via foreign keys. However, many recipes will use the same tag, thus a Tag can have multiple references from RecipeTag. So when a recipe gets deleted, all its associated recipe_tags get deleted as well automatically through cascade deletes, leaving some tags to be childless. So it doesn't need to exist anymore.

I was thinking of querying all of the tags and pick ones out that have no children left like so:

class Tag(db.Model):
    ...
    @staticmethod
    def delete_after_no_children():
        tags = Tag.query.all()
        for tag in tags:
            if not tag.children:
                db.session.delete(tag)
        db.session.commit()

This works, but I think it would be wildly inefficient. What would be the best approach to do this?