Multiple choices in flask-admin form_choices

Question

Well, I recently approached to flask-admin and I cannot figure out how to solve this issue. I know that I can use form_choices to restrict the possible values for a text-field by specifying a list of tuples. Anyway, form_choices allows to select only one value at a time. How can I specify that in some cases I may need of a comma-separated list of values?

I tried this workaround:

form_args = {
    'FIELD': {
        'render_kw': {"multiple": "multiple"},
    }
}

but, even though a multiselect input actually appears on the webpage, only the first value is saved.

EIDT 05/13/2017

By playing a bit with flask-admin I found two possible (partial-)solution for my question, both of them with specific drawbacks.

1) The first deals with the use of Select2TagsField

from flask_admin.form.fields import Select2TagsField
...
form_extra_fields = {
    'muri': Select2TagsField()
}

With this method is it possible to easily implement select menu for normal input text, even though at present I do not understand how to pass choices to Select2TagsField. It works well as a sort of multiple free text input. However, as far as I understand, it is not possible to pair Select2TagsField and form_choices

2) The second is a bit longer but it offers some more control on code (at least I presume). Still it implies the use of form_choices, but this time paired with on_model_change

form_args = {
    'FIELD': {
        'render_kw': {"multiple": "multiple"},
    }
}
form_choices = {'FIELD': [
    ('1', 'M1'), ('2', 'M2'), ('3', 'M3'), ('4', 'M4')
]}
...
def on_model_change(self, form, model, is_created):
    if len(form.FIELD.raw_data) > 1:
        model.FIELD = ','.join(form.FIELD.raw_data)

This solution, despite the former one, allows to map choices and works well when adding data to the model, but in editing it gives some problems. Any time I open the edit dialog the FIELD is empty. If I look at the data sent to the form (with on_form_prefill by printing form.FIELD.data) I get a comma separated string in the terminal but nothing appear in the pertinent select field on the webpage.

Answer 1

Maybe this is already outdated but I managed to change it and make it work with a multiple choice array field from postgres. To make it work I extended Select2Field to know how to deal with the list:

class MultipleSelect2Field(Select2Field):
    """Extends select2 field to make it work with postgresql arrays and using choices.

    It is far from perfect and it should be tweaked it a bit more.
    """

    def iter_choices(self):
        """Iterate over choices especially to check if one of the values is selected."""
        if self.allow_blank:
            yield (u'__None', self.blank_text, self.data is None)

        for value, label in self.choices:
            yield (value, label, self.coerce(value) in self.data)

    def process_data(self, value):
        """This is called when you create the form with existing data."""
        if value is None:
            self.data = []
        else:
            try:
                self.data = [self.coerce(value) for value in value]
            except (ValueError, TypeError):
                self.data = []

    def process_formdata(self, valuelist):
        """Process posted data."""
        if not valuelist:
            return

        if valuelist[0] == '__None':
            self.data = []
        else:
            try:
                self.data = [self.coerce(value) for value in valuelist]
            except ValueError:
                raise ValueError(self.gettext(u'Invalid Choice: could not coerce'))

    def pre_validate(self, form):
        """Validate sent keys to make sure user don't post data that is not a valid choice."""
        sent_data = set(self.data)
        valid_data = {k for k, _ in self.choices}
        invalid_keys = sent_data - valid_data
        if invalid_keys:
            raise ValueError('These values are invalid {}'.format(','.join(invalid_keys)))

and to use it do this on the ModelView

class SomeView(ModelView):
    form_args = dict(FIELD=dict(render_kw=dict(multiple="multiple"), choices=CHOICES_TUPLE))
    form_overrides = dict(FIELD=MultipleSelect2Field)

Answer 2

For this approach to work you would need to use a column that can store a list of elements. At least with sqlite this is not possible using Flask-Admin. However it would be better for you to store your choices in a separate data model and use constraints to link the two models. See a working example here.

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_admin import Admin
from flask_admin.contrib.sqla import ModelView


app = Flask(__name__)
app.config['SECRET_KEY'] = '8e12c91677b3b3df266a770b22c82f2f'
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
db = SQLAlchemy(app)
admin = Admin(app)


item_tag_relation = db.Table('item_tag_relation',
    db.Column('item_id', db.Integer, db.ForeignKey('item.id')),
    db.Column('tag_id', db.Integer, db.ForeignKey('tag.id'))
)


class Item(db.Model):
    id = db.Column(db.Integer(), primary_key=True)
    name = db.Column(db.String())
    tags = db.relationship("Tag",
                               secondary=item_tag_relation,
                               backref="items")

    def __repr__(self):
        return self.name


class Tag(db.Model):
    id = db.Column(db.Integer(), primary_key=True)
    name = db.Column(db.String())

    def __repr__(self):
        return self.name


class ItemModelView(ModelView):
    pass


db.create_all()
admin.add_view(ItemModelView(Item, db.session))
admin.add_view(ModelView(Tag, db.session))

if __name__ == '__main__':
    app.run(debug=True)