"""

Various data structures used in query construction.

Factored out from django.db.models.query to avoid making the main module very

large and/or so that they can be used by other modules without getting into

circular import difficulties.

"""

import weakref

from django.utils import tree

from django.utils.datastructures import SortedDict

class CyclicDependency(Exception):

    """

    An error when dealing with a collection of objects that have a cyclic

    dependency, i.e. when deleting multiple objects.

    """

    pass

class CollectedObjects(object):

    """

    A container that stores keys and lists of values along with remembering the

    parent objects for all the keys.

    This is used for the database object deletion routines so that we can

    calculate the 'leaf' objects which should be deleted first.

    previously_seen is an optional argument. It must be a CollectedObjects

    instance itself; any previously_seen collected object will be blocked from

    being added to this instance.

    """

    def __init__(self, previously_seen=None):

        self.data = {}

        self.children = {}

        if previously_seen:

            self.blocked = previously_seen.blocked

            for cls, seen in previously_seen.data.items():

                self.blocked.setdefault(cls, SortedDict()).update(seen)

        else:

            self.blocked = {}

        """

        Adds an item to the container.

        Arguments:

        * model - the class of the object being added.

        * pk - the primary key.

        * obj - the object itself.

        * parent_model - the model of the parent object that this object was

          reached through.

        * nullable - should be True if this relation is nullable.

        Returns True if the item already existed in the structure and

        False otherwise.

        """

        if pk in self.blocked.get(model, {}):

            return True

        d = self.data.setdefault(model, SortedDict())

        retval = pk in d

        d[pk] = obj

        # Nullable relationships can be ignored -- they are nulled out before

        # deleting, and therefore do not affect the order in which objects

        # have to be deleted.

        if parent_model is not None and not nullable:

            self.children.setdefault(parent_model, []).append(model)

        return retval

    def __contains__(self, key):

        return self.data.__contains__(key)

    def __getitem__(self, key):

        return self.data[key]

    def __nonzero__(self):

        return bool(self.data)

    def iteritems(self):

        for k in self.ordered_keys():

            yield k, self[k]

    def items(self):

        return list(self.iteritems())

    def keys(self):

        return self.ordered_keys()

    def ordered_keys(self):

        """

        Returns the models in the order that they should be dealt with (i.e.

        models with no dependencies first).

        """

        dealt_with = SortedDict()

        # Start with items that have no children

        models = self.data.keys()

        while len(dealt_with) < len(models):

            found = False

            for model in models:

                if model in dealt_with:

                    continue

                children = self.children.setdefault(model, [])

                if len([c for c in children if c not in dealt_with]) == 0:

                    dealt_with[model] = None

                    found = True

            if not found:

                raise CyclicDependency(

                    "There is a cyclic dependency of items to be processed.")

        return dealt_with.keys()

    def unordered_keys(self):

        """

        Fallback for the case where is a cyclic dependency but we don't  care.

        """

        return self.data.keys()

class QueryWrapper(object):

    """

    A type that indicates the contents are an SQL fragment and the associate

    parameters. Can be used to pass opaque data to a where-clause, for example.

    """

    def __init__(self, sql, params):

        self.data = sql, params

        return self.data

class Q(tree.Node):

    """

    Encapsulates filters as objects that can then be combined logically (using

    & and |).

    """

    # Connection types

    AND = 'AND'

    OR = 'OR'

    default = AND

    def __init__(self, *args, **kwargs):

        super(Q, self).__init__(children=list(args) + kwargs.items())

    def _combine(self, other, conn):

        if not isinstance(other, Q):

            raise TypeError(other)

        obj.add(other, conn)

        return obj

    def __or__(self, other):

        return self._combine(other, self.OR)

    def __and__(self, other):

        return self._combine(other, self.AND)

    def __invert__(self):

        obj.negate()

        return obj

class DeferredAttribute(object):

    """

    A wrapper for a deferred-loading field. When the value is read from this

    object the first time, the query is executed.

    """

    def __init__(self, field_name, model):

        self.field_name = field_name

        self.model_ref = weakref.ref(model)

        self.loaded = False

    def __get__(self, instance, owner):

        """

        Retrieves and caches the value from the datastore on the first lookup.

        Returns the cached value.

        """

        assert instance is not None

        cls = self.model_ref()

        data = instance.__dict__

        if data.get(self.field_name, self) is self:

        return data[self.field_name]

    def __set__(self, instance, value):

        """

        Deferred loading attributes can be set normally (which means there will

        never be a database lookup involved.

        """

        instance.__dict__[self.field_name] = value

    """

    Returns True if this field should be used to descend deeper for

    select_related() purposes. Used by both the query construction code

    (sql.query.fill_related_selections()) and the model instance creation code

    (query.get_cached_row()).

    """

    if not field.rel:

        return False

        return False

    if not restricted and field.null:

        return False

    return True

# This function is needed because data descriptors must be defined on a class

# object, not an instance, to have any effect.

def deferred_class_factory(model, attrs):

    """

    Returns a class object that is a copy of "model" with the specified "attrs"

    being replaced with DeferredAttribute objects. The "pk_value" ties the

    deferred attributes to a particular instance of the model.

    """

    class Meta:

        pass

    setattr(Meta, "proxy", True)

    setattr(Meta, "app_label", model._meta.app_label)

    # The app_cache wants a unique name for each model, otherwise the new class

    # won't be created (we get an old one back). Therefore, we generate the

    # name using the passed in attrs. It's OK to reuse an old case if the attrs

    # are identical.

    name = "%s_Deferred_%s" % (model.__name__, '_'.join(sorted(list(attrs))))

    overrides = dict([(attr, DeferredAttribute(attr, model))

            for attr in attrs])

    overrides["Meta"] = Meta

    overrides["__module__"] = model.__module__

    overrides["_deferred"] = True

    return type(name, (model,), overrides)

# The above function is also used to unpickle model instances with deferred

# fields.

deferred_class_factory.__safe_for_unpickling__ = True