# Source code for django.db.models.query_utils

    """
    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 copy
    import functools
    import inspect
    import warnings
    from collections import namedtuple

    from django.core.exceptions import FieldDoesNotExist, FieldError
    from django.db.models.constants import LOOKUP_SEP
    from django.utils import tree
    from django.utils.deprecation import RemovedInDjango40Warning

    # PathInfo is used when converting lookups (fk__somecol). The contents
    # describe the relation in Model terms (model Options and Fields for both
    # sides of the relation. The join_field is the field backing the relation.
    PathInfo = namedtuple('PathInfo', 'from_opts to_opts target_fields join_field m2m direct filtered_relation')

    class InvalidQueryType(type):
        @property
        def _subclasses(self):
            return (FieldDoesNotExist, FieldError)

        def __warn(self):
            warnings.warn(
                'The InvalidQuery exception class is deprecated. Use '
                'FieldDoesNotExist or FieldError instead.',
                category=RemovedInDjango40Warning,
                stacklevel=4,
            )

        def __instancecheck__(self, instance):
            self.__warn()
            return isinstance(instance, self._subclasses) or super().__instancecheck__(instance)

        def __subclasscheck__(self, subclass):
            self.__warn()
            return issubclass(subclass, self._subclasses) or super().__subclasscheck__(subclass)

    class InvalidQuery(Exception, metaclass=InvalidQueryType):
        pass

    def subclasses(cls):
        yield cls
        for subclass in cls.__subclasses__():
            yield from subclasses(subclass)

    class Q(tree.Node):
        """
        Encapsulate filters as objects that can then be combined logically (using
        `&` and `|`).
        """
        # Connection types
        AND = 'AND'
        OR = 'OR'
        default = AND
        conditional = True

        def __init__(self, *args, _connector=None, _negated=False, **kwargs):
            super().__init__(children=[*args, *sorted(kwargs.items())], connector=_connector, negated=_negated)

        def _combine(self, other, conn):
            if not isinstance(other, Q):
                raise TypeError(other)

            # If the other Q() is empty, ignore it and just use `self`.
            if not other:
                return copy.deepcopy(self)
            # Or if this Q is empty, ignore it and just use `other`.
            elif not self:
                return copy.deepcopy(other)

            obj = type(self)()
            obj.connector = conn
            obj.add(self, conn)
            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 = type(self)()
            obj.add(self, self.AND)
            obj.negate()
            return obj

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            # We must promote any new joins to left outer joins so that when Q is
            # used as an expression, rows aren't filtered due to joins.
            clause, joins = query._add_q(
                self, reuse, allow_joins=allow_joins, split_subq=False,
                check_filterable=False,
            )
            query.promote_joins(joins)
            return clause

        def deconstruct(self):
            path = '%s.%s' % (self.__class__.__module__, self.__class__.__name__)
            if path.startswith('django.db.models.query_utils'):
                path = path.replace('django.db.models.query_utils', 'django.db.models')
            args, kwargs = (), {}
            if len(self.children) == 1 and not isinstance(self.children[0], Q):
                child = self.children[0]
                kwargs = {child[0]: child[1]}
            else:
                args = tuple(self.children)
                if self.connector != self.default:
                    kwargs = {'_connector': self.connector}
            if self.negated:
                kwargs['_negated'] = True
            return path, args, kwargs

    class DeferredAttribute:
        """
        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):
            self.field = field

        def __get__(self, instance, cls=None):
            """
            Retrieve and caches the value from the datastore on the first lookup.
            Return the cached value.
            """
            if instance is None:
                return self
            data = instance.__dict__
            field_name = self.field.attname
            if field_name not in data:
                # Let's see if the field is part of the parent chain. If so we
                # might be able to reuse the already loaded value. Refs #18343.
                val = self._check_parent_chain(instance)
                if val is None:
                    instance.refresh_from_db(fields=[field_name])
                    val = getattr(instance, field_name)
                data[field_name] = val
            return data[field_name]

        def _check_parent_chain(self, instance):
            """
            Check if the field value can be fetched from a parent field already
            loaded in the instance. This can be done if the to-be fetched
            field is a primary key field.
            """
            opts = instance._meta
            link_field = opts.get_ancestor_link(self.field.model)
            if self.field.primary_key and self.field != link_field:
                return getattr(instance, link_field.attname)
            return None

    class RegisterLookupMixin:

        @classmethod
        def _get_lookup(cls, lookup_name):
            return cls.get_lookups().get(lookup_name, None)

        @classmethod
        @functools.lru_cache(maxsize=None)
        def get_lookups(cls):
            class_lookups = [parent.__dict__.get('class_lookups', {}) for parent in inspect.getmro(cls)]
            return cls.merge_dicts(class_lookups)

        def get_lookup(self, lookup_name):
            from django.db.models.lookups import Lookup
            found = self._get_lookup(lookup_name)
            if found is None and hasattr(self, 'output_field'):
                return self.output_field.get_lookup(lookup_name)
            if found is not None and not issubclass(found, Lookup):
                return None
            return found

        def get_transform(self, lookup_name):
            from django.db.models.lookups import Transform
            found = self._get_lookup(lookup_name)
            if found is None and hasattr(self, 'output_field'):
                return self.output_field.get_transform(lookup_name)
            if found is not None and not issubclass(found, Transform):
                return None
            return found

        @staticmethod
        def merge_dicts(dicts):
            """
            Merge dicts in reverse to preference the order of the original list. e.g.,
            merge_dicts([a, b]) will preference the keys in 'a' over those in 'b'.
            """
            merged = {}
            for d in reversed(dicts):
                merged.update(d)
            return merged

        @classmethod
        def _clear_cached_lookups(cls):
            for subclass in subclasses(cls):
                subclass.get_lookups.cache_clear()

        @classmethod
        def register_lookup(cls, lookup, lookup_name=None):
            if lookup_name is None:
                lookup_name = lookup.lookup_name
            if 'class_lookups' not in cls.__dict__:
                cls.class_lookups = {}
            cls.class_lookups[lookup_name] = lookup
            cls._clear_cached_lookups()
            return lookup

        @classmethod
        def _unregister_lookup(cls, lookup, lookup_name=None):
            """
            Remove given lookup from cls lookups. For use in tests only as it's
            not thread-safe.
            """
            if lookup_name is None:
                lookup_name = lookup.lookup_name
            del cls.class_lookups[lookup_name]

    def select_related_descend(field, restricted, requested, load_fields, reverse=False):
        """
        Return 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_klass_info()).

        Arguments:
         * field - the field to be checked
         * restricted - a boolean field, indicating if the field list has been
           manually restricted using a requested clause)
         * requested - The select_related() dictionary.
         * load_fields - the set of fields to be loaded on this model
         * reverse - boolean, True if we are checking a reverse select related
        """
        if not field.remote_field:
            return False
        if field.remote_field.parent_link and not reverse:
            return False
        if restricted:
            if reverse and field.related_query_name() not in requested:
                return False
            if not reverse and field.name not in requested:
                return False
        if not restricted and field.null:
            return False
        if load_fields:
            if field.attname not in load_fields:
                if restricted and field.name in requested:
                    msg = (
                        'Field %s.%s cannot be both deferred and traversed using '
                        'select_related at the same time.'
                    ) % (field.model._meta.object_name, field.name)
                    raise FieldError(msg)
        return True

    def refs_expression(lookup_parts, annotations):
        """
        Check if the lookup_parts contains references to the given annotations set.
        Because the LOOKUP_SEP is contained in the default annotation names, check
        each prefix of the lookup_parts for a match.
        """
        for n in range(1, len(lookup_parts) + 1):
            level_n_lookup = LOOKUP_SEP.join(lookup_parts[0:n])
            if level_n_lookup in annotations and annotations[level_n_lookup]:
                return annotations[level_n_lookup], lookup_parts[n:]
        return False, ()

    def check_rel_lookup_compatibility(model, target_opts, field):
        """
        Check that self.model is compatible with target_opts. Compatibility
        is OK if:
          1) model and opts match (where proxy inheritance is removed)
          2) model is parent of opts' model or the other way around
        """
        def check(opts):
            return (
                model._meta.concrete_model == opts.concrete_model or
                opts.concrete_model in model._meta.get_parent_list() or
                model in opts.get_parent_list()
            )
        # If the field is a primary key, then doing a query against the field's
        # model is ok, too. Consider the case:
        # class Restaurant(models.Model):
        #     place = OneToOneField(Place, primary_key=True):
        # Restaurant.objects.filter(pk__in=Restaurant.objects.all()).
        # If we didn't have the primary key check, then pk__in (== place__in) would
        # give Place's opts as the target opts, but Restaurant isn't compatible
        # with that. This logic applies only to primary keys, as when doing __in=qs,
        # we are going to turn this into __in=qs.values('pk') later on.
        return (
            check(target_opts) or
            (getattr(field, 'primary_key', False) and check(field.model._meta))
        )

    class FilteredRelation:
        """Specify custom filtering in the ON clause of SQL joins."""

        def __init__(self, relation_name, *, condition=Q()):
            if not relation_name:
                raise ValueError('relation_name cannot be empty.')
            self.relation_name = relation_name
            self.alias = None
            if not isinstance(condition, Q):
                raise ValueError('condition argument must be a Q() instance.')
            self.condition = condition
            self.path = []

        def __eq__(self, other):
            if not isinstance(other, self.__class__):
                return NotImplemented
            return (
                self.relation_name == other.relation_name and
                self.alias == other.alias and
                self.condition == other.condition
            )

        def clone(self):
            clone = FilteredRelation(self.relation_name, condition=self.condition)
            clone.alias = self.alias
            clone.path = self.path[:]
            return clone

        def resolve_expression(self, *args, **kwargs):
            """
            QuerySet.annotate() only accepts expression-like arguments
            (with a resolve_expression() method).
            """
            raise NotImplementedError('FilteredRelation.resolve_expression() is unused.')

        def as_sql(self, compiler, connection):
            # Resolve the condition in Join.filtered_relation.
            query = compiler.query
            where = query.build_filtered_relation_q(self.condition, reuse=set(self.path))
            return compiler.compile(where)