Selected value issue with `QuerySelectMultipleField` in edit form

[aminalaee]

When working with QuerySelectMultipleField , the empty form works ok,
but when the edit form is built from an object like form = MyForm(obj=obj) then the multiple select field does not show the selected field.

In other words it always shows none of the options to be selected but in reality there could be some of them selected.

I think the fix is in the data property here:

https://github.com/wtforms/wtforms-sqlalchemy/blob/c0d741429407f0bbd2e66e28528e0bada4769ac4/wtforms_sqlalchemy/fields.py#L166-L179

Which instead should be like:

    def _get_data(...):
        formdata = self._formdata
        if formdata is not None:
            data = []
            for pk, obj in self._object_list:
                if not formdata:
                    break
                elif pk in formdata:
                    formdata.remove(pk)
                    data.append(obj)
            if formdata:
                self._invalid_formdata = True
            self.data = data or self._data  # This change here fixed it
        return self._data

I don't know if a PR for this would be accepted or not, I could do that anyway.

[aminalaee]

BTW, thanks a lot for the work here, I really appreciate that.
I've used most of this code in my project (which I need to put a reference it was used from this project), the reason I didn't directly use wtforms-sqlalchemy was that I wanted to support SQLAlchemy > 1.4 .