检查空查询集 [英] Checking for empty queryset

问题描述

我想确认这是检查空查询集的正确方法，如果这就是为什么我有一个UNIQUE约束错误.

I want to confirm is this the right way to check for empty queryset, and if it is why am I having a UNIQUE constraint error.

syn_check= Synonym.objects.filter(MD.objects.get(**filter_dict), synonym_type=Stype.objects.filter(description=values.capitalize().strip()), synonym_name=key)
if not syn_check:
    print 'unavailable synonym', key, values
    syn = Synonym()
    syn.synonym_type=Stype.objects.filter(description=values.capitalize().strip()
    syn.synonym_name = key.strip()
    syn.save()

我在做任何奇怪的事情吗?我还使用了 if not syn_check.count()，但遇到了同样的问题.

Am I doing anything that is weird? I also used if not syn_check.count() and I had the same problem.

这是回溯:

    Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
    utility.execute()
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 377, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/core/management/base.py", line 338, in execute
    output = self.handle(*args, **options)
  File "/home/project/PycharmProjects/project/project/management/commands/back_populate_data.py", line 53, in handle
    loaded_syn = data_loaded_syn4_pubchem(synonym_decoded, filter_dict)
  File "/home/project/PycharmProjects/project/project/lookup_data.py", line 203, in data_loaded_syn4_pubchem
    mol_syn_ob.save()
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/models/base.py", line 589, in save
    force_update=force_update, update_fields=update_fields)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/models/base.py", line 617, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/models/base.py", line 698, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/models/base.py", line 731, in _do_insert
    using=using, raw=raw)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/models/manager.py", line 92, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/models/query.py", line 921, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 921, in execute_sql
    cursor.execute(sql, params)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/backends/utils.py", line 81, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File "/home/project/.virtualenvs/project/local/lib/python2.7/site-packages/django/db/backends/sqlite3/base.py", line 485, in execute
    return Database.Cursor.execute(self, query, params)
django.db.utils.IntegrityError: UNIQUE constraint failed: project_synonym.molecule_id, project_synonym.synonym_name, project_synonym.synonym_type_id

谢谢

推荐答案

按需检查查询集就可以了.

Checking the queryset as you are doing is fine.

if not syn_check:

但是，当您只想知道查询集是否为空时，您正在从数据库中加载所有对象，因此效率不尽如人意.

However, it is not as efficent as it could be, you are loading all the objects from the database, when all you want to know is whether the queryset is empty or not.

最好使用 count()，

if not syn_check.count():

，甚至更好地使用 exists()

if not syn_check.exists():

代码中的问题是在哪里定义查询集

The problem in your code is where you define the queryset

syn_check = Synonym.objects.filter(MD.objects.get(**filter_dict), synonym_type=Stype.objects.filter(description=values.capitalize().strip()), synonym_name=key)

filter()方法接受

The filter() method accepts either Q objects as args, or keyword arguments. It doesn't make sense to pass the model instance from MD.objects.get(...) as a positional argument.

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