在我的 Django 模型中，有一个 CharField 作为主键，具有 250 个字符作为 max_length 和一个 ManyToManyField:
class GenericRoom(models.Model):
key = models.CharField(primary_key=True, max_length=250)
name = models.CharField(max_length=500, default='')
extension = models.IntegerField(null=True)
owner = models.ForeignKey(settings.AUTH_USER_MODEL, null=True, related_name="%(app_label)s_%(class)s_owner",
on_delete=models.CASCADE)
moderators = models.ManyToManyField(settings.AUTH_USER_MODEL, related_name="%(app_label)s_%(class)s_moderators",
blank=True)
type = models.CharField(max_length=50, choices=ROOM_TYPE_CHOICES, default='meeting')
class Room(GenericRoom):
pin = models.IntegerField(null=True)
此外，在 Postgres 中，key 列的最大长度为 250，如下所示:
Column | Type | Collation | Nullable | Default
---------------------+------------------------+-----------+----------+---------
key | character varying(250) | | not null |
但是，当我给出的 key 超过 50 个字符时，我正在做:
r=Room.objects.create(key='a_veeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeery_long_key')
r.moderators.add(User.objects.get(id=123))
我得到:
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
return self.cursor.execute(sql, params)
psycopg2.DataError: value too long for type character varying(50)
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/models/fields/related_descriptors.py", line 898, in add
self._add_items(self.source_field_name, self.target_field_name, *objs)
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/models/fields/related_descriptors.py", line 1065, in _add_items
for obj_id in new_ids
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/models/query.py", line 466, in bulk_create
ids = self._batched_insert(objs_without_pk, fields, batch_size)
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/models/query.py", line 1142, in _batched_insert
inserted_id = self._insert(item, fields=fields, using=self.db, return_id=True)
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/models/query.py", line 1125, in _insert
return query.get_compiler(using=using).execute_sql(return_id)
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/models/sql/compiler.py", line 1283, in execute_sql
cursor.execute(sql, params)
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 100, in execute
return super().execute(sql, params)
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 68, in execute
return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 77, in _execute_with_wrappers
return executor(sql, params, many, context)
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
return self.cursor.execute(sql, params)
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/utils.py", line 89, in __exit__
raise dj_exc_value.with_traceback(traceback) from exc_value
File "/home/ilias/.virtualenvs/vsceneapi/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
return self.cursor.execute(sql, params)
django.db.utils.DataError: value too long for type character varying(50)
对于短于 50 的 key ，一切正常。
因此 key 列必须有 50 个字符的隐式限制，即使该限制看起来是 250。
但是为什么？我该如何改变它？
编辑:
来自 Postgres 的表格描述是:
Column | Type | Collation | Nullable | Default
---------------------+------------------------+-----------+----------+---------
key | character varying(250) | | not null |
owner_id | integer | | |
extension | integer | | |
name | character varying(500) | | not null |
type | character varying(100) | | not null |
pexip_conference_id | integer | | |
pin | integer | | |
Indexes:
"rooms_room_pkey" PRIMARY KEY, btree (key)
"rooms_room_key_ca48250b_like" btree (key varchar_pattern_ops)
"rooms_room_owner_id_13734245" btree (owner_id)
Foreign-key constraints:
"rooms_room_owner_id_13734245_fk_users_user_id" FOREIGN KEY (owner_id) REFERENCES users_user(id) DEFERRABLE INITIALLY DEFERRED
Referenced by:
TABLE "meeting_spaces_meetingspace" CONSTRAINT "meeting_spaces_meetingspace_room_id_10905900_fk" FOREIGN KEY (room_id) REFERENCES rooms_room(key) DEFERRABLE INITIALLY DEFERRED
TABLE "meetings_meeting" CONSTRAINT "meetings_meeting_room_id_02a205a4_fk" FOREIGN KEY (room_id) REFERENCES rooms_room(key) DEFERRABLE INITIALLY DEFERRED
TABLE "recordings_recording" CONSTRAINT "recordings_recording_room_id_799f40af_fk" FOREIGN KEY (room_id) REFERENCES rooms_room(key) DEFERRABLE INITIALLY DEFERRED
TABLE "rooms_room_moderators" CONSTRAINT "rooms_room_moderators_room_id_abd2abc5_fk_rooms_room_key" FOREIGN KEY (room_id) REFERENCES rooms_room(key) DEFERRABLE INITIALLY DEFERRED
TABLE "users_user" CONSTRAINT "users_user_room_id_06ed4152_fk" FOREIGN KEY (room_id) REFERENCES rooms_room(key) DEFERRABLE INITIALLY DEFERRED
最佳答案
不同的数据库后端对索引列中字符串的长度有不同的限制。
对于 postgres，您可以找到限制 here (这是一个长度为2000的字符串)
对于mysql，你可以找到限制here [还有一篇 SO 帖子更好地解释了它 here , 是一个长度为255的字符串)
所以从理论上讲，没有理由不这样做(Django 还添加了一个限制来处理数据库的可移植性，allowing custom primary_keys, with max_length=255，但您的 key 的长度在该范围内)。
我怀疑原因在于您管理数据库的方式，因为我发现您的模型与实际数据库表不匹配(键 type 是 varchar(100) 在数据库中，但在模型中为 max_length=50。
我建议您查看您的迁移并仔细检查它们，也可能重新创建表会起作用，也许那里出了点问题。
希望对您有所帮助。
关于Django-Postgres : length of CharField as primary key，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50762562/