QuerySelectField query parameter ignored when the query returns no result.

[TrilceAC]

According to the documentation:

The query property on the field can be set from within a view to assign a query per-instance to the field. If the property is not set, the query_factory callable passed to the field constructor will be called to obtain a query

But this fails if the query returns no results. In those cases, the query_factory is used. If there is one, the choices of the QuerySelectField are those returned by the query_factory. Otherwise, when no query_factory has been passed, rendering the field raises the following exception: TypeError: 'NoneType' object is not callable.

This is unfortunate in some cases, like for example when dealing with forms to handle trees, which is my case. Since a Tree has no cycle, I want to avoid that the edition of an element sets its parent to itself or one of its children. Therefore, if there is a single tree, when editing a root element, it should have no candidate for being it parent.

I provide a simplified code which is not tested, i.e. some variables have been renamed and mistakes can occur:

class Node(Model):
    MAXLEN_NAME = 128

    __tablename__ = 'node'
    id = Column(Integer, primary_key=True, index=True)
    name = Column(
        Unicode(MAXLEN_NAME),
        nullable=False,
        index=True,
    )

    parent_id = Column(
        Integer,
        ForeignKey('node.id'),
        index=True
    )
    parent = relationship(
        'Node',
        remote_side='Node.id',
        backref='children'
    )

    def descendants(self):
        """Somehow get the descendants."""
        # OPTIMIZE !!!
        return (
            frozenset(self.children) if self.children else frozenset() | {
                desc.descendant() for desc in self.children
            }
        )

This is the form:

class NodeForm(OrderFormMixin, ModelForm):

    class Meta:

        model = Node
        strip_string_fields = True
        order = (
            'name',
            'parent',
            'submit',
            '*'
        )
    parent = QuerySelectField(
        label='Parent',
        query_factory=lambda: (
            Node.query
            .order_by(Node.name)
            .all()
        ),
        allow_blank=True,
        description='direct parent of the node.'
    )
    enviar = SubmitField()

And finally the view...

@blueprint.route('/edit/<int:node_id>', methods=('GET', 'POST'))
def edit(node_id):
    node = Node.query.options(
        joinedload(Node.parent)
    ).get_or_404(node_id)

    form = NodeForm(obj=node)

    # Do not put invalid options on the QuerySelectField
    # THIS FAILS if there is no candidate, i.e. the query returns []
    form.parent.query = (
        Node.query
        .filter(
            Node.id != node.id,
            Node.id.notin_(
                desc.id for desc in node.descendant()
            )
        )
        .order_by(Node.name)
        .all()
    )

    if form.validate_on_submit():
        form.populate_obj(node)
        db.session.commit()
        return redirect(url_for('.index'))

    return render_template(
        'node/alta.html.j2',
        title='Editar ubicación {0}'.format(node),
        form=form
    )

[TrilceAC]

I've found the bug. The problem occurs when deciding whether either query or query_factory should be used: _get_object_list should be fixed as follows:

    def _get_object_list(self):
        if self._object_list is None:
            # query = self.query or self.query_factory() # <--- FAILS when query returns [].
            query = self.query if (self.query or self.query==[]) else self.query_factory() # <--- FIX!
            get_pk = self.get_pk
            self._object_list = list(
                (text_type(get_pk(obj)), obj) for obj in query
            )
        return self._object_list