环境：

win10 、python 3.6.5 django 1.11.8

背景：

1、数据库记录更新；2、数据库记录创建；3、创建一条已存在该主键的记录

        1、3实现了数据库记录的更新；2实现了记录的创建； save实现的原理是什么？

分析：

1、模型：

class DelayTest(models.Model):id = models.AutoField(auto_created=True, primary_key=True)name = models.CharField(max_length=20, null=True, verbose_name="名称")createdate = models.DateTimeField(auto_now_add=True)updatedate = models.DateTimeField(auto_now=True)class Meta:db_table = "delaytest"

2、视图函数

from datetime import datetime
"创建一条数据"
blogs = DelayTest(name="hello")
blogs.save()
"创建一条已存在的数据"
blogs = DelayTest(id=1, name="hello", createdate=datetime.now())
blogs.save()
"更改一条数据"
blogs = DelayTest.objects.get(id=2)
blogs.save()

源码分析：

        创建一条数据

        save调用了django\db\models\base.py内的save（）函数，源码如下：

    def save(self, force_insert=False, force_update=False, using=None,update_fields=None):"""Saves the current instance. Override this in a subclass if you want tocontrol the saving process.The 'force_insert' and 'force_update' parameters can be used to insistthat the "save" must be an SQL insert or update (or equivalent fornon-SQL backends), respectively. Normally, they should not be set."""# Ensure that a model instance without a PK hasn't been assigned to# a ForeignKey or OneToOneField on this model. If the field is# nullable, allowing the save() would result in silent data loss.for field in self._meta.concrete_fields:if field.is_relation:# If the related field isn't cached, then an instance hasn't# been assigned and there's no need to worry about this check.try:getattr(self, field.get_cache_name())except AttributeError:continueobj = getattr(self, field.name, None)# A pk may have been assigned manually to a model instance not# saved to the database (or auto-generated in a case like# UUIDField), but we allow the save to proceed and rely on the# database to raise an IntegrityError if applicable. If# constraints aren't supported by the database, there's the# unavoidable risk of data corruption.if obj and obj.pk is None:# Remove the object from a related instance cache.if not field.remote_field.multiple:delattr(obj, field.remote_field.get_cache_name())raise ValueError("save() prohibited to prevent data loss due to ""unsaved related object '%s'." % field.name)using = using or router.db_for_write(self.__class__, instance=self)if force_insert and (force_update or update_fields):raise ValueError("Cannot force both insert and updating in model saving.")deferred_fields = self.get_deferred_fields()if update_fields is not None:# If update_fields is empty, skip the save. We do also check for# no-op saves later on for inheritance cases. This bailout is# still needed for skipping signal sending.if len(update_fields) == 0:returnupdate_fields = frozenset(update_fields)field_names = set()for field in self._meta.fields:if not field.primary_key:field_names.add(field.name)if field.name != field.attname:field_names.add(field.attname)non_model_fields = update_fields.difference(field_names)if non_model_fields:raise ValueError("The following fields do not exist in this ""model or are m2m fields: %s"% ', '.join(non_model_fields))# If saving to the same database, and this model is deferred, then# automatically do a "update_fields" save on the loaded fields.elif not force_insert and deferred_fields and using == self._state.db:field_names = set()for field in self._meta.concrete_fields:if not field.primary_key and not hasattr(field, 'through'):field_names.add(field.attname)loaded_fields = field_names.difference(deferred_fields)if loaded_fields:update_fields = frozenset(loaded_fields)self.save_base(using=using, force_insert=force_insert,force_update=force_update, update_fields=update_fields)save.alters_data = True

这里注意：外键、many_to_many、one_to_one这些情况，注释如下：

        # Ensure that a model instance without a PK hasn't been assigned to# a ForeignKey or OneToOneField on this model. If the field is# nullable, allowing the save() would result in silent data loss.

最终调用了save_base函数，源码如下：

    def save_base(self, raw=False, force_insert=False,force_update=False, using=None, update_fields=None):"""Handles the parts of saving which should be done only once per save,yet need to be done in raw saves, too. This includes some sanitychecks and signal sending.The 'raw' argument is telling save_base not to save any parentmodels and not to do any changes to the values before save. Thisis used by fixture loading."""using = using or router.db_for_write(self.__class__, instance=self)assert not (force_insert and (force_update or update_fields))assert update_fields is None or len(update_fields) > 0cls = origin = self.__class__# Skip proxies, but keep the origin as the proxy model.if cls._meta.proxy:cls = cls._meta.concrete_modelmeta = cls._metaif not meta.auto_created:pre_save.send(sender=origin, instance=self, raw=raw, using=using,update_fields=update_fields,)with transaction.atomic(using=using, savepoint=False):if not raw:self._save_parents(cls, using, update_fields)updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)# Store the database on which the object was savedself._state.db = using# Once saved, this is no longer a to-be-added instance.self._state.adding = False# Signal that the save is completeif not meta.auto_created:post_save.send(sender=origin, instance=self, created=(not updated),update_fields=update_fields, raw=raw, using=using,)save_base.alters_data = True

此处先是返送pre_save信号，然后调用支持事务with方式去修改数据库，之后发送post_save信号，存储完成，其核心处理代码是：

updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)

此处调用了_save_table方法，源码如下：

    def _save_table(self, raw=False, cls=None, force_insert=False,force_update=False, using=None, update_fields=None):"""Does the heavy-lifting involved in saving. Updates or inserts the datafor a single table."""meta = cls._metanon_pks = [f for f in meta.local_concrete_fields if not f.primary_key]if update_fields:non_pks = [f for f in non_pksif f.name in update_fields or f.attname in update_fields]pk_val = self._get_pk_val(meta)if pk_val is None:pk_val = meta.pk.get_pk_value_on_save(self)setattr(self, meta.pk.attname, pk_val)pk_set = pk_val is not Noneif not pk_set and (force_update or update_fields):raise ValueError("Cannot force an update in save() with no primary key.")updated = False# If possible, try an UPDATE. If that doesn't update anything, do an INSERT.if pk_set and not force_insert:base_qs = cls._base_manager.using(using)values = [(f, None, (getattr(self, f.attname) if raw else f.pre_save(self, False)))for f in non_pks]forced_update = update_fields or force_updateupdated = self._do_update(base_qs, using, pk_val, values, update_fields,forced_update)if force_update and not updated:raise DatabaseError("Forced update did not affect any rows.")if update_fields and not updated:raise DatabaseError("Save with update_fields did not affect any rows.")if not updated:if meta.order_with_respect_to:# If this is a model with an order_with_respect_to# autopopulate the _order fieldfield = meta.order_with_respect_tofilter_args = field.get_filter_kwargs_for_object(self)order_value = cls._base_manager.using(using).filter(**filter_args).count()self._order = order_valuefields = meta.local_concrete_fieldsif not pk_set:fields = [f for f in fields if f is not meta.auto_field]update_pk = meta.auto_field and not pk_setresult = self._do_insert(cls._base_manager, using, fields, update_pk, raw)if update_pk:setattr(self, meta.pk.attname, result)return updated

base_qs是该table内所有数据的查询集；pk_val是一个主键id；updated是标志位，说明该语句是update还是其他类型；先进行更新（_do_update），若是失败，再进行插入（_do_insert），其注释内容也说明了：

# If possible, try an UPDATE. If that doesn't update anything, do an INSERT.

此处调用了_do_update，其源码如下：

    def _do_update(self, base_qs, using, pk_val, values, update_fields, forced_update):"""This method will try to update the model. If the model was updated (inthe sense that an update query was done and a matching row was foundfrom the DB) the method will return True."""filtered = base_qs.filter(pk=pk_val)if not values:# We can end up here when saving a model in inheritance chain where# update_fields doesn't target any field in current model. In that# case we just say the update succeeded. Another case ending up here# is a model with just PK - in that case check that the PK still# exists.return update_fields is not None or filtered.exists()if self._meta.select_on_save and not forced_update:if filtered.exists():# It may happen that the object is deleted from the DB right after# this check, causing the subsequent UPDATE to return zero matching# rows. The same result can occur in some rare cases when the# database returns zero despite the UPDATE being executed# successfully (a row is matched and updated). In order to# distinguish these two cases, the object's existence in the# database is again checked for if the UPDATE query returns 0.return filtered._update(values) > 0 or filtered.exists()else:return Falsereturn filtered._update(values) > 0

此处将之前的base_qs进行过滤，过滤条件为pk（主键），很明显，创建的时候主键是自动生成的，若是过滤，则返回为空集合，最后执行

return filtered._update(values) > 0

此处调用_update方法，该方法路径为：django\db\models\query.py下的_update方法，源码：

    def _update(self, values):"""A version of update that accepts field objects instead of field names.Used primarily for model saving and not intended for use by generalcode (it requires too much poking around at model internals to beuseful at that level)."""assert self.query.can_filter(), \"Cannot update a query once a slice has been taken."query = self.query.clone(sql.UpdateQuery)query.add_update_fields(values)self._result_cache = Nonereturn query.get_compiler(self.db).execute_sql(CURSOR)_update.alters_data = True_update.queryset_only = False

显然，此处存在执行sql语句的函数：execute_sql，

路径：django\db\models\sql\compiler.py

源码：

    def execute_sql(self, result_type=MULTI, chunked_fetch=False):"""Run the query against the database and returns the result(s). Thereturn value is a single data item if result_type is SINGLE, or aniterator over the results if the result_type is MULTI.result_type is either MULTI (use fetchmany() to retrieve all rows),SINGLE (only retrieve a single row), or None. In this last case, thecursor is returned if any query is executed, since it's used bysubclasses such as InsertQuery). It's possible, however, that no queryis needed, as the filters describe an empty set. In that case, None isreturned, to avoid any unnecessary database interaction."""if not result_type:result_type = NO_RESULTStry:sql, params = self.as_sql()if not sql:raise EmptyResultSetexcept EmptyResultSet:if result_type == MULTI:return iter([])else:returnif chunked_fetch:cursor = self.connection.chunked_cursor()else:cursor = self.connection.cursor()try:cursor.execute(sql, params)except Exception:try:# Might fail for server-side cursors (e.g. connection closed)cursor.close()except Exception:# Ignore clean up errors and raise the original error instead.# Python 2 doesn't chain exceptions. Remove this error# silencing when dropping Python 2 compatibility.passraiseif result_type == CURSOR:# Caller didn't specify a result_type, so just give them back the# cursor to process (and close).return cursorif result_type == SINGLE:try:val = cursor.fetchone()if val:return val[0:self.col_count]return valfinally:# done with the cursorcursor.close()if result_type == NO_RESULTS:cursor.close()returnresult = cursor_iter(cursor, self.connection.features.empty_fetchmany_value,self.col_count)if not chunked_fetch and not self.connection.features.can_use_chunked_reads:try:# If we are using non-chunked reads, we return the same data# structure as normally, but ensure it is all read into memory# before going any further. Use chunked_fetch if requested.return list(result)finally:# done with the cursorcursor.close()return result

此处result_type是CURSOR，调用时已经说明了。

显然，执行语句未成功（原因是没有符合条件的id），直接执行sql语句

update delaytest set name="hello" where id=100;

因为该id不存在，返回结果如下：

mysql> update delaytest set name="hello" where id=100;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0  Changed: 0  Warnings: 0

显然，更新是不行的，之前的注释：

If possible, try an UPDATE. If that doesn't update anything, do an INSERT.

先来尝试update，不行的话再去执行insert。此处返回curser，最终返回到

return filtered._update(values) > 0，显然0 > 0是False，所以，再返回给updated也是False，然后只能去执行

result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)

OK，创建执行成功后，发送post_save信号，save执行结束

创建一条已存在的数据

与之前代码运行不同之处在于_do_update中的过滤，存在一条数据，然后在execute_sql中执行成功，并返回1，显然 1 > 0，其值为True，即updated=True，返回True，然后发送post_save信号，save执行结束。

更新数据

同创建一条已存在的数据

结论：save不能辨别是更新还是新建，都是先以update做，如果是0条，则再去执行创建

PS：

同事问我save的使用方式，我说针对对象可以创建和更新的时候使用，我当时认为save可以辨别是创建还是更新，他感觉我说的不对。结果我给测试失败了，原因是没有添加createdate的问题（测试时是：blogs = DelayTest(id=1, name="hello")，写博文的时候：blogs = DelayTest(id=1, name="hello", createdate=datetime.now())），测试失败意味着我说的有问题，所以才有了源码分析save这个行为。