reverse_related.py

"""
"Rel objects" for related fields.

"Rel objects" (for lack of a better name) carry information about the relation
modeled by a related field and provide some utility functions. They're stored
in the ``remote_field`` attribute of the field.

They also act as reverse fields for the purposes of the Meta API because
they're the closest concept currently available.
"""

from django.core import exceptions
from django.utils.functional import cached_property

from . import BLANK_CHOICE_DASH
from .mixins import FieldCacheMixin


class ForeignObjectRel(FieldCacheMixin):
    """
    Used by ForeignObject to store information about the relation.

    ``_meta.get_fields()`` returns this class to provide access to the field
    flags for the reverse relation.
    """

    # Field flags
    auto_created = True
    concrete = False
    editable = False
    is_relation = True

    # Reverse relations are always nullable (Django can't enforce that a
    # foreign key on the related model points to this model).
    null = True

    def __init__(self, field, to, related_name=None, related_query_name=None,
                 limit_choices_to=None, parent_link=False, on_delete=None):
        self.field = field
        self.model = to
        self.related_name = related_name
        self.related_query_name = related_query_name
        self.limit_choices_to = {} if limit_choices_to is None else limit_choices_to
        self.parent_link = parent_link
        self.on_delete = on_delete

        self.symmetrical = False
        self.multiple = True

    # Some of the following cached_properties can't be initialized in
    # __init__ as the field doesn't have its model yet. Calling these methods
    # before field.contribute_to_class() has been called will result in
    # AttributeError
    @cached_property
    def hidden(self):
        return self.is_hidden()

    @cached_property
    def name(self):
        return self.field.related_query_name()

    @property
    def remote_field(self):
        return self.field

    @property
    def target_field(self):
        """
        When filtering against this relation, return the field on the remote
        model against which the filtering should happen.
        """
        target_fields = self.get_path_info()[-1].target_fields
        if len(target_fields) > 1:
            raise exceptions.FieldError("Can't use target_field for multicolumn relations.")
        return target_fields[0]

    @cached_property
    def related_model(self):
        if not self.field.model:
            raise AttributeError(
                "This property can't be accessed before self.field.contribute_to_class has been called.")
        return self.field.model

    @cached_property
    def many_to_many(self):
        return self.field.many_to_many

    @cached_property
    def many_to_one(self):
        return self.field.one_to_many

    @cached_property
    def one_to_many(self):
        return self.field.many_to_one

    @cached_property
    def one_to_one(self):
        return self.field.one_to_one

    def get_lookup(self, lookup_name):
        return self.field.get_lookup(lookup_name)

    def get_internal_type(self):
        return self.field.get_internal_type()

    @property
    def db_type(self):
        return self.field.db_type

    def __repr__(self):
        return '<%s: %s.%s>' % (
            type(self).__name__,
            self.related_model._meta.app_label,
            self.related_model._meta.model_name,
        )

    def get_choices(self, include_blank=True, blank_choice=BLANK_CHOICE_DASH, ordering=()):
        """
        Return choices with a default blank choices included, for use
        as <select> choices for this field.

        Analog of django.db.models.fields.Field.get_choices(), provided
        initially for utilization by RelatedFieldListFilter.
        """
        qs = self.related_model._default_manager.all()
        if ordering:
            qs = qs.order_by(*ordering)
        return (blank_choice if include_blank else []) + [
            (x.pk, str(x)) for x in qs
        ]

    def is_hidden(self):
        """Should the related object be hidden?"""
        return bool(self.related_name) and self.related_name[-1] == '+'

    def get_joining_columns(self):
        return self.field.get_reverse_joining_columns()

    def get_extra_restriction(self, where_class, alias, related_alias):
        return self.field.get_extra_restriction(where_class, related_alias, alias)

    def set_field_name(self):
        """
        Set the related field's name, this is not available until later stages
        of app loading, so set_field_name is called from
        set_attributes_from_rel()
        """
        # By default foreign object doesn't relate to any remote field (for
        # example custom multicolumn joins currently have no remote field).
        self.field_name = None

    def get_accessor_name(self, model=None):
        # This method encapsulates the logic that decides what name to give an
        # accessor descriptor that retrieves related many-to-one or
        # many-to-many objects. It uses the lowercased object_name + "_set",
        # but this can be overridden with the "related_name" option. Due to
        # backwards compatibility ModelForms need to be able to provide an
        # alternate model. See BaseInlineFormSet.get_default_prefix().
        opts = model._meta if model else self.related_model._meta
        model = model or self.related_model
        if self.multiple:
            # If this is a symmetrical m2m relation on self, there is no reverse accessor.
            if self.symmetrical and model == self.model:
                return None
        if self.related_name:
            return self.related_name
        return opts.model_name + ('_set' if self.multiple else '')

    def get_path_info(self, filtered_relation=None):
        return self.field.get_reverse_path_info(filtered_relation)

    def get_cache_name(self):
        """
        Return the name of the cache key to use for storing an instance of the
        forward model on the reverse model.
        """
        return self.get_accessor_name()


class ManyToOneRel(ForeignObjectRel):
    """
    Used by the ForeignKey field to store information about the relation.

    ``_meta.get_fields()`` returns this class to provide access to the field
    flags for the reverse relation.

    Note: Because we somewhat abuse the Rel objects by using them as reverse
    fields we get the funny situation where
    ``ManyToOneRel.many_to_one == False`` and
    ``ManyToOneRel.one_to_many == True``. This is unfortunate but the actual
    ManyToOneRel class is a private API and there is work underway to turn
    reverse relations into actual fields.
    """

    def __init__(self, field, to, field_name, related_name=None, related_query_name=None,
                 limit_choices_to=None, parent_link=False, on_delete=None):
        super().__init__(
            field, to,
            related_name=related_name,
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
            parent_link=parent_link,
            on_delete=on_delete,
        )

        self.field_name = field_name

    def __getstate__(self):
        state = self.__dict__.copy()
        state.pop('related_model', None)
        return state

    def get_related_field(self):
        """
        Return the Field in the 'to' object to which this relationship is tied.
        """
        field = self.model._meta.get_field(self.field_name)
        if not field.concrete:
            raise exceptions.FieldDoesNotExist("No related field named '%s'" % self.field_name)
        return field

    def set_field_name(self):
        self.field_name = self.field_name or self.model._meta.pk.name


class OneToOneRel(ManyToOneRel):
    """
    Used by OneToOneField to store information about the relation.

    ``_meta.get_fields()`` returns this class to provide access to the field
    flags for the reverse relation.
    """

    def __init__(self, field, to, field_name, related_name=None, related_query_name=None,
                 limit_choices_to=None, parent_link=False, on_delete=None):
        super().__init__(
            field, to, field_name,
            related_name=related_name,
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
            parent_link=parent_link,
            on_delete=on_delete,
        )

        self.multiple = False


class ManyToManyRel(ForeignObjectRel):
    """
    Used by ManyToManyField to store information about the relation.

    ``_meta.get_fields()`` returns this class to provide access to the field
    flags for the reverse relation.
    """

    def __init__(self, field, to, related_name=None, related_query_name=None,
                 limit_choices_to=None, symmetrical=True, through=None,
                 through_fields=None, db_constraint=True):
        super().__init__(
            field, to,
            related_name=related_name,
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
        )

        if through and not db_constraint:
            raise ValueError("Can't supply a through model and db_constraint=False")
        self.through = through

        if through_fields and not through:
            raise ValueError("Cannot specify through_fields without a through model")
        self.through_fields = through_fields

        self.symmetrical = symmetrical
        self.db_constraint = db_constraint

    def get_related_field(self):
        """
        Return the field in the 'to' object to which this relationship is tied.
        Provided for symmetry with ManyToOneRel.
        """
        opts = self.through._meta
        if self.through_fields:
            field = opts.get_field(self.through_fields[0])
        else:
            for field in opts.fields:
                rel = getattr(field, 'remote_field', None)
                if rel and rel.model == self.model:
                    break
        return field.foreign_related_fields[0]