Checking for empty queryset - django

Question

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

Answer 1

Checking the queryset as you are doing is fine.
if not syn_check:
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.
It would be better to use count(),
if not syn_check.count():
and even better to use exists()
if not syn_check.exists():
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)
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.