Django 1.9.5 : FieldError for reverse lookup fields occasionally

Question

当我尝试为预取的 Queryset 运行 objects.all() 时，我在 Django 中遇到了一个令人沮丧的间歇性错误。有一个问题，在查询集的实例化和通过它运行迭代之间，model._meta 偶尔似乎缺少字段。这几乎就像查询集的预取实际上没有及时运行对象列表的迭代。

在此示例中，data.service_log 只是一个查询集，其中包含一些名为 servicelog 的预取项。当我在 shell 中运行查询集时，我可以查看查询集上 self.names_to_path(lookup_splitted, self.get_meta()) 方法中的所有字段。它们都在那里，特别是“服务日志”。

请注意此错误的回溯，它表示“servicelog”不是可用字段，但它在字段列表中列出了可供选择的字段。这似乎是一个 Django 错误，但我不能确定，因为我无法解释或隔离该行为。我不可能是唯一遇到此错误的人。它似乎在 django/db/models/sql/query.py 中的 names_to_paths() 方法中。这是无法解析的代码:

query.py names_to_paths():

field = None
try:
    field = opts.get_field(name)
except FieldDoesNotExist:
    if name in self.annotation_select:
        field = self.annotation_select[name].output_field

if field is not None:
    # Fields that contain one-to-many relations with a generic
    # model (like a GenericForeignKey) cannot generate reverse
    # relations and therefore cannot be used for reverse querying.
    if field.is_relation and not field.related_model:
        raise FieldError(
            "Field %r does not generate an automatic reverse "
            "relation and therefore cannot be used for reverse "
            "querying. If it is a GenericForeignKey, consider "
            "adding a GenericRelation." % name
        )
    try:
        model = field.model._meta.concrete_model
    except AttributeError:
        model = None
else:
    # We didn't find the current field, so move position back
    # one step.
    pos -= 1
    if pos == -1 or fail_on_missing:
        field_names = list(get_field_names_from_opts(opts))
        available = sorted(field_names + list(self.annotation_select))
        raise FieldError("Cannot resolve keyword %r into field. "
                         "Choices are: %s" % (name, ", .join(available)))
    break

字段在第一次尝试时没有设置，然后条件语句if field is not None失败所以我们进入else block 。那里的 pos 减少了 1，但由于该字段是“servicelog”，它已经为 0。但是，当我在 shell 中尝试此操作时，它总是会找到 field用opts.get_field('servicelog')`。只有从 WSGI 和 Apache2 运行时才会出现此故障。同样，并非所有时间都如此，这使得测试极其困难。我对此感到困惑，并且不确定在哪里寻找线索。如果有人对要探索的内容有任何想法，我将不胜感激。

Traceback (most recent call last):

  File "/var/www/fast/services/views/edit.py", line 12897, in service_log
    for service in data.service_log:

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 258, in __iter__
    self._fetch_all()

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 1076, in _fetch_all
    self._prefetch_related_objects()

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 656, in _prefetch_related_objects
    prefetch_related_objects(self._result_cache, self._prefetch_related_lookups)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 1457, in prefetch_related_objects
    obj_list, additional_lookups = prefetch_one_level(obj_list, prefetcher, lookup, level)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 1556, in prefetch_one_level
    prefetcher.get_prefetch_queryset(instances, lookup.get_current_queryset(level)))

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/related_descriptors.py", line 802, in get_prefetch_queryset
    queryset = queryset._next_is_sticky().filter(**query)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 790, in filter
    return self._filter_or_exclude(False, *args, **kwargs)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 808, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1243, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1269, in _add_q
    allow_joins=allow_joins, split_subq=split_subq,

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1149, in build_filter
    lookups, parts, reffed_expression = self.solve_lookup_type(arg)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1035, in solve_lookup_type
    _, field, _, lookup_parts = self.names_to_path(lookup_splitted, self.get_meta())

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1330, in names_to_path
    "Choices are: %s" % (name, ", ".join(available)))

FieldError: Cannot resolve keyword u'servicelog' into field. Choices are: additional_county_worker_notes, adoption_disrupted, adoption_first_name, adoption_last_name, adoption_middle_name, adoption_placement_date, adoption_placement_reason, adoption_placement_reason_id, adoption_placement_type, adoption_risk_level, adoption_risk_level_id, adoption_termination_date, adoption_termination_destination, adoption_termination_reason, adoption_termination_reason_id, adoptive_placement, agency, agency_id, all_items_outstanding, all_items_past_due, appeal_process_date, attached_file, attends_college, attorney_email_address, attorney_email_address_id, attorney_fax, attorney_fax_id, attorney_investigator_email_address, attorney_investigator_email_address_id, attorney_investigator_fax, attorney_investigator_fax_id, attorney_investigator_name, attorney_investigator_phone, attorney_investigator_phone_id, attorney_name, attorney_phone, attorney_phone_id, blood_related_to_applicants, blood_relationship, casa_email_address, casa_email_address_id, casa_fax, casa_fax_id, casa_name, casa_phone, casa_phone_id, certification_items_outstanding, certification_items_past_due, classification, classification_id, client, client_id, county_adoption_worker, county_adoption_worker_cell, county_adoption_worker_cell_id, county_adoption_worker_email_address, county_adoption_worker_email_address_id, county_adoption_worker_fax, county_adoption_worker_fax_id, county_adoption_worker_office, county_adoption_worker_office_id, county_adoption_worker_phone, county_adoption_worker_phone_id, county_adoption_worker_title, county_case_number, county_worker, county_worker_cell, county_worker_cell_id, county_worker_email_address, county_worker_email_address_id, county_worker_fax, county_worker_fax_id, county_worker_office, county_worker_office_id, county_worker_phone, county_worker_phone_id, county_worker_title, court, court_case_name, court_case_number, court_department, court_id, created, current_grade, date_identified_adoptive, date_placed_with_home, deleted, discharge_summary, eligibility_worker, eligibility_worker_email_address, eligibility_worker_email_address_id, eligibility_worker_phone, eligibility_worker_phone_id, emergency_placement, employed_80_hours, enable_discharge_summary, expected_type, expected_type_id, extended_family_contact_allowed, final_payment_amount, finalization_date, foreign_placement, hearing_36626_date, homestudy, id, incident_placement_1, incident_placement_2, incident_placement_3, incident_placement_4, individualized_plan_review, inhousemove, interpretive_summary, item_due, items_approved, items_pending, items_rejected, items_update_requested, la_county_id, medi_cal, medi_cal_eligibility_phone, medi_cal_eligibility_phone_id, medi_cal_eligibility_worker, medi_cal_id, modified, monthly_monitored_visit_hours, mother_child, move_in_type, move_out_type, new_protective_custody_petition, non_minor_dependent, non_truant, notes, number_of_files_required, other_school_contact, other_school_contact_first_name, other_school_contact_last_name, parent_payment_override_annually, parent_payment_override_daily, parent_payment_override_monthly, parental_contact_allowed, parental_group, parental_group_id, payment_amount, percent_certified, percent_items_complete, person_number, placement, placement_date, placement_id, placement_payment_override_annually, placement_payment_override_daily, placement_payment_override_monthly, placement_reason, placement_reason_details, placement_reason_id, placement_self, placer_shelter_bed, prior_placement, progress_summary, projected_adoption_36626_date, projected_adoption_finalization_date, projected_adoption_placement_date, recordreview, requires_educational_support, requires_mental_health_services, respite, school, school_different, school_id, school_liaison_email, school_liaison_first_name, school_liaison_last_name, school_liaison_phone, school_liaison_phone_extension, school_notes, serial_number, servicecontact_onbehalf, servicedeliverylog, servicelog, social_worker_at_termination, social_worker_at_termination_id, special_health_care_needs, state_case_number, teachers, termination_date, termination_destination, termination_reason, termination_reason_details, termination_reason_id, therapist, therapy_code, therapy_supervision_requirements, treatment_abilities, treatment_needs, treatment_preferences, treatment_strengths, treatmentneed, update_requested, update_requested_by, update_requested_by_id, update_requested_date, update_requested_note, updateable, use_number_required, uses_psychotropic_medication, visit_frequency_override, visit_frequency_override_id, visitation_restrictions, who_can_pickup_at_home, who_can_pickup_at_school, who_can_visit

更新 - 添加模型/违规 View 代码

models.py
class ParentalGroup(models.Model):
    many fields...

class Placement(models.Model):
    parental_group = models.ForeignKey(ParentalGroup, null=True, blank=True)
    many more fields...

class ServiceLog(models.Model):
    parental_group = models.ForeignKey(ParentalGroup, null=True, blank=True)
    placement = models.ManyToManyField(Placement, blank=True)
    many more fields...


views.py:
data.service_log = ServiceLog.objects.filter(
    parental_group=data.pg,
).prefetch_related(
    Prefetch(
        'placement',
        queryset=Placement.objects.all(),
        to_attr='placements'
    ),
)

for service in data.service_log:
    some code to generate data to pass to template...

return render_to_response(...)

Answer 1

我在 Gunicorn/Django runserver 上使用 Django 1.8.6 时遇到了类似的问题。我也没有设法在 shell/notebook 环境中重现错误。

我通过添加 related_name 解决了随机出现的 FieldError到 ManyToManyField。我用了a through model不过在 ManyToManyField 中。

在你的情况下:

class ServiceLog(models.Model):
    parental_group = models.ForeignKey(ParentalGroup, null=True, blank=True)
    placement = models.ManyToManyField(Placement, blank=True, related_name='servicelog')

一月