"""
Helper functions for creating Form classes from Django models
and database field objects.
"""

from django.utils.translation import ugettext
from django.utils.encoding import smart_unicode


from util import ValidationError
from forms import BaseForm, SortedDictFromList
from fields import Field, ChoiceField
from widgets import Select, SelectMultiple, MultipleHiddenInput

__all__ = (
    'save_instance', 'form_for_model', 'form_for_instance', 'form_for_fields',
    'ModelChoiceField', 'ModelMultipleChoiceField'
)

def save_instance(form, instance, fields=None, fail_message='saved', commit=True):
    """
    Saves bound Form ``form``'s cleaned_data into model instance ``instance``.

    If commit=True, then the changes to ``instance`` will be saved to the
    database. Returns ``instance``.
    """
    from django.db import models
    opts = instance.__class__._meta
    if form.errors:
        raise ValueError("The %s could not be %s because the data didn't validate." % (opts.object_name, fail_message))
    cleaned_data = form.cleaned_data
    for f in opts.fields:
        if not f.editable or isinstance(f, models.AutoField) or not f.name in cleaned_data:
            continue
        if fields and f.name not in fields:
            continue
        setattr(instance, f.name, cleaned_data[f.name])
    if commit:
        instance.save()
        for f in opts.many_to_many:
            if fields and f.name not in fields:
                continue
            if f.name in cleaned_data:
                setattr(instance, f.attname, cleaned_data[f.name])
    # GOTCHA: If many-to-many data is given and commit=False, the many-to-many
    # data will be lost. This happens because a many-to-many options cannot be
    # set on an object until after it's saved. Maybe we should raise an
    # exception in that case.
    return instance

def make_model_save(model, fields, fail_message):
    "Returns the save() method for a Form."
    def save(self, commit=True):
        return save_instance(self, model(), fields, fail_message, commit)
    return save
    
def make_instance_save(instance, fields, fail_message):
    "Returns the save() method for a Form."
    def save(self, commit=True):
        return save_instance(self, instance, fields, fail_message, commit)
    return save

def form_for_model(model, form=BaseForm, fields=None, formfield_callback=lambda f: f.formfield()):
    """
    Returns a Form class for the given Django model class.

    Provide ``form`` if you want to use a custom BaseForm subclass.

    Provide ``formfield_callback`` if you want to define different logic for
    determining the formfield for a given database field. It's a callable that
    takes a database Field instance and returns a form Field instance.
    """
    opts = model._meta
    field_list = []
    for f in opts.fields + opts.many_to_many:
        if not f.editable:
            continue
        if fields and not f.name in fields:
            continue
        formfield = formfield_callback(f)
        if formfield:
            field_list.append((f.name, formfield))
    base_fields = SortedDictFromList(field_list)
    return type(opts.object_name + 'Form', (form,), 
        {'base_fields': base_fields, '_model': model, 'save': make_model_save(model, fields, 'created')})

def form_for_instance(instance, form=BaseForm, fields=None, formfield_callback=lambda f, **kwargs: f.formfield(**kwargs)):
    """
    Returns a Form class for the given Django model instance.

    Provide ``form`` if you want to use a custom BaseForm subclass.

    Provide ``formfield_callback`` if you want to define different logic for
    determining the formfield for a given database field. It's a callable that
    takes a database Field instance, plus **kwargs, and returns a form Field
    instance with the given kwargs (i.e. 'initial').
    """
    model = instance.__class__
    opts = model._meta
    field_list = []
    for f in opts.fields + opts.many_to_many:
        if not f.editable:
            continue
        if fields and not f.name in fields:
            continue
        current_value = f.value_from_object(instance)
        formfield = formfield_callback(f, initial=current_value)
        if formfield:
            field_list.append((f.name, formfield))
    base_fields = SortedDictFromList(field_list)
    return type(opts.object_name + 'InstanceForm', (form,),
        {'base_fields': base_fields, '_model': model, 'save': make_instance_save(instance, fields, 'changed')})

def form_for_fields(field_list):
    "Returns a Form class for the given list of Django database field instances."
    fields = SortedDictFromList([(f.name, f.formfield()) for f in field_list if f.editable])
    return type('FormForFields', (BaseForm,), {'base_fields': fields})

class QuerySetIterator(object):
    def __init__(self, queryset, empty_label, cache_choices):
        self.queryset, self.empty_label, self.cache_choices = queryset, empty_label, cache_choices

    def __iter__(self):
        if self.empty_label is not None:
            yield (u"", self.empty_label)
        for obj in self.queryset:
            yield (obj._get_pk_val(), smart_unicode(obj))
        # Clear the QuerySet cache if required.
        if not self.cache_choices:
            self.queryset._result_cache = None

class ModelChoiceField(ChoiceField):
    "A ChoiceField whose choices are a model QuerySet."
    # This class is a subclass of ChoiceField for purity, but it doesn't
    # actually use any of ChoiceField's implementation.
    def __init__(self, queryset, empty_label=u"---------", cache_choices=False,
            required=True, widget=Select, label=None, initial=None, help_text=None):
        self.queryset = queryset
        self.empty_label = empty_label
        self.cache_choices = cache_choices
        # Call Field instead of ChoiceField __init__() because we don't need
        # ChoiceField.__init__().
        Field.__init__(self, required, widget, label, initial, help_text)
        self.widget.choices = self.choices

    def _get_choices(self):
        # If self._choices is set, then somebody must have manually set
        # the property self.choices. In this case, just return self._choices.
        if hasattr(self, '_choices'):
            return self._choices
        # Otherwise, execute the QuerySet in self.queryset to determine the
        # choices dynamically. Return a fresh QuerySetIterator that has not
        # been consumed. Note that we're instantiating a new QuerySetIterator
        # *each* time _get_choices() is called (and, thus, each time
        # self.choices is accessed) so that we can ensure the QuerySet has not
        # been consumed.
        return QuerySetIterator(self.queryset, self.empty_label, self.cache_choices)

    def _set_choices(self, value):
        # This method is copied from ChoiceField._set_choices(). It's necessary
        # because property() doesn't allow a subclass to overwrite only
        # _get_choices without implementing _set_choices.
        self._choices = self.widget.choices = list(value)

    choices = property(_get_choices, _set_choices)

    def clean(self, value):
        Field.clean(self, value)
        if value in ('', None):
            return None
        try:
            value = self.queryset.model._default_manager.get(pk=value)
        except self.queryset.model.DoesNotExist:
            raise ValidationError(ugettext(u'Select a valid choice. That choice is not one of the available choices.'))
        return value

class ModelMultipleChoiceField(ModelChoiceField):
    "A MultipleChoiceField whose choices are a model QuerySet."
    hidden_widget = MultipleHiddenInput
    def __init__(self, queryset, cache_choices=False, required=True,
            widget=SelectMultiple, label=None, initial=None, help_text=None):
        super(ModelMultipleChoiceField, self).__init__(queryset, None, cache_choices,
            required, widget, label, initial, help_text)

    def clean(self, value):
        if self.required and not value:
            raise ValidationError(ugettext(u'This field is required.'))
        elif not self.required and not value:
            return []
        if not isinstance(value, (list, tuple)):
            raise ValidationError(ugettext(u'Enter a list of values.'))
        final_values = []
        for val in value:
            try:
                obj = self.queryset.model._default_manager.get(pk=val)
            except self.queryset.model.DoesNotExist:
                raise ValidationError(ugettext(u'Select a valid choice. %s is not one of the available choices.') % val)
            else:
                final_values.append(obj)
        return final_values