patch_data should work with empty arrays

[zcmander]

Patch_data ignores empty arrays. This becomes issue when trying to remove all elements from specific model, like in the example, remove all items from order. I think patch_data should be in this situation {"items": []}, but it's actually just an empty dictionary.

$ python example.py 
wtforms version: 2.0.1
wtforms_json version: 0.2.10
form.data {'items': [{'name': 'Foo'}, {'name': 'Bar'}]}
form.patch_data {}
Traceback (most recent call last):
  File "example.py", line 67, in <module>
    assert(form.patch_data == {"items": []})
AssertionError

Here is a working example to reproduce the issue:

# -*- coding: utf-8 -*-
from sqlalchemy import Column, ForeignKey, Integer, Unicode
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
import wtforms
from wtforms import Form
from wtforms.fields import (
    FormField,
    StringField,
    FieldList,
)
import wtforms_json
wtforms_json.init()

Base = declarative_base()

import pprint
print "wtforms version:", wtforms.__version__
print "wtforms_json version:", wtforms_json.__version__


#1. Setup SQLAlchemy Models

class Item(Base):
    __tablename__ = 'item'
    id = Column(Integer, primary_key=True)
    order_id = Column(Integer, ForeignKey("order.id"))
    order = relationship("Order", backref="items")
    name = Column(Unicode())


class Order(Base):
    __tablename__ = 'order'
    id = Column(Integer, primary_key=True)


#2. Forms definition

class ItemForm(Form):
    name = StringField()


class OrderForm(Form):
    items = FieldList(FormField(ItemForm))


#3. Create existing instance and fill the form

instance = Order(items=[
    Item(name="Foo"),
    Item(name="Bar"),
])

json_data = {
    "items": []
}

form = OrderForm.from_json(json_data, obj=instance)

print u"form.data", pprint.pformat(form.data)
print u"form.patch_data", pprint.pformat(form.patch_data)


#4. patch_data should contain an empty list

assert(form.patch_data == {"items": []})

Is there anything that can be done to resolve this issue?

[gaetan-petit]

Same here, it is clearly a bug.

[zcmander]

See PR: #45

[drimer]

Is there any update on this? Or a work around to use for now? Tried specifying None, but that has the same result.