现在MVC架构开发中。m部分是提供底层数据。无论是cs还是bs越来越看重数据对象的业务处理，而不是以前原生的sql得到的数据。
因此，1个通用的关系数据库字段对应对模型对象的框架就比较重要了。有了他可以节省大量的开发时间。
本篇侧重简要分析django中的orm。
对于orm，既然是通用，那么就存在5个重要问题。
1：如何多数据库的支持
2：对象字段类型的提供
3：sql->object如何实现转换
4：object->sql如何存储
5：数据库字段关联关系在对象中的对应。
11分析如上几个问题。
1：对于多数据库支持，这个问题比较好解决，因为sql通用的操作代码比较多。差别在细微和一些数据库特性。数据库联接上。基于分层思想。
高层封装sql通用方法类。特性方法。和数据库操作方法类。在具体对应的数据库实现文件中针对性改写。
根据猜想，分析django.db.backends模块下文件。得到
backends
….
|----sqlite3
|----__init__.py
|----base.py 基于sqlite3构造继承数据库基类。填充sqlte3特性，完善sqlte3的运算操作与游标
|----client.py 本地执行sqlite3 构造db文件
|----creation.py 提供构造数据库的model与数据库字段对应关系，填充创建方法
|----introspection.py 提供自省对应的model与数据库字段关系，填充自省方法
__init__.py 数据库基础，特性，运算操作，自省，本地执行，验证 等基类封装
creation.py 构造数据库基类封装
signals.py 数据库构造信号
util.py 游标，调试游标（记录时间）封装问题2：对象字段类型的提供。
sql语句与iphone的视图与android的控件一样。都是1个提供比较同样方法或者属性的集合。细微差别在子类实现时候差异化。
对于sql提供的字段，有整形，布尔型，字符，浮点，日期等。。。
这段比较不熟悉，因此分析models的模块得到如下目录
models
|----fields
|----__init__.py 对象的基类与对象类
|----fies.py 文件对象
|----proxy.py 代理类，暂时还不知道用途
|----related.py 关联关系（1对多，多对多，反向，主见，1对1）
|----subclassing.py 暂时不知道
|----sql
|----__init__.py
|----aggregates.py 集合类，用于组合sql字段模版，重点是as_sql
|----complier.py sql语句组合类，最终查询是通过此类实现
|----constants.py 常量
|----datastructures.py 暂时不清楚
|----expressions.py 评估程序？暂时不清楚
|----query.py 查询
|----subqueries.py 增删改查表达式（从Qeury文件中继承）
|----where.py
__init__.py
aggregates.py 集合类，平均数，统计，最大，最小，stdDev，求和，方差
base.py 模型基类
constants.py
deletion.py 删除的集合类collecor
expression.py 逻辑运算表达式
loading.py 加载 加载应用程序于模型，注册模型，等方法的封装
manager.py 对象管理类。对象管理描述符类
options.py 模型对象支持的选项
query.py 查询集合类（基类，值类，值列表，时间，空）
query_utils.py 查询包装
related.py
signals.py 操作相关的信号定义大致根据如上的代码详细查看源代码会方便理解。
有了sql的连接管理，有了model提供字段，如何管理对象。。
从django的使用上可以看出。
模型.管理.操作()[切片处理]得到QuerySet。其中QuerySet封装在models.query.py中。
下面就剩下3个问题。
3：sql->object，从QuerySet入手。分析iterator()
def iterator(self):
"""
An iterator over the results from applying this QuerySet to the
database.
"""
...
# Cache db and model outside the loop
db = self.db
model = self.model
compiler = self.query.get_compiler(using=db)
if fill_cache:
klass_info = get_klass_info(model, max_depth=max_depth,
requested=requested, only_load=only_load) 得到类信息
for row in compiler.results_iter(): 从数据库遍历字段
if fill_cache:
obj, _ = get_cached_row(row, index_start, db, klass_info,
offset=len(aggregate_select)) 获取字段
else:
# Omit aggregates in object creation.
row_data = row[index_start:aggregate_start]
if skip:
obj = model_cls(**dict(zip(init_list, row_data)))
else:
obj = model(*row_data) 根据字段生成模型对象
# Store the source database of the object
obj._state.db = db
# This object came from the database; it's not being added.
obj._state.adding = False
if extra_select:
for i, k in enumerate(extra_select):
setattr(obj, k, row[i])
# Add the aggregates to the model
if aggregate_select:
for i, aggregate in enumerate(aggregate_select):
setattr(obj, aggregate, row[i + aggregate_start])
# Add the known related objects to the model, if there are any
if self._known_related_objects:
for field, rel_objs in self._known_related_objects.items():
pk = getattr(obj, field.get_attname())
try:
rel_obj = rel_objs[pk]
except KeyError:
pass # may happen in qs1 | qs2 scenarios
else:
setattr(obj, field.name, rel_obj)
yield objdef get_cached_row(row, index_start, using, klass_info, offset=0):
"""
Helper function that recursively returns an object with the specified
related attributes already populated.
This method may be called recursively to populate deep select_related()
clauses.
Arguments:
* row - the row of data returned by the database cursor
* index_start - the index of the row at which data for this
object is known to start
* offset - the number of additional fields that are known to
exist in row for `klass`. This usually means the number of
annotated results on `klass`.
* using - the database alias on which the query is being executed.
* klass_info - result of the get_klass_info function
"""
if klass_info is None:
return None
klass, field_names, field_count, related_fields, reverse_related_fields, pk_idx = klass_info
fields = row[index_start : index_start + field_count]
# If the pk column is None (or the Oracle equivalent ''), then the related
# object must be non-existent - set the relation to None.
if fields[pk_idx] == None or fields[pk_idx] == '':
obj = None
elif field_names:
obj = klass(**dict(zip(field_names, fields)))
else:
obj = klass(*fields)生成对象
# If an object was retrieved, set the database state.
if obj:
obj._state.db = using
obj._state.adding = False
# Instantiate related fields
index_end = index_start + field_count + offset
# Iterate over each related object, populating any
# select_related() fields
for f, klass_info in related_fields:
# Recursively retrieve the data for the related object
cached_row = get_cached_row(row, index_end, using, klass_info)
# If the recursive descent found an object, populate the
# descriptor caches relevant to the object
if cached_row:
rel_obj, index_end = cached_row
if obj is not None:
# If the base object exists, populate the
# descriptor cache
setattr(obj, f.get_cache_name(), rel_obj)
if f.unique and rel_obj is not None:
# If the field is unique, populate the
# reverse descriptor cache on the related object
setattr(rel_obj, f.related.get_cache_name(), obj)
# Now do the same, but for reverse related objects.
# Only handle the restricted case - i.e., don't do a depth
# descent into reverse relations unless explicitly requested
for f, klass_info in reverse_related_fields:
# Recursively retrieve the data for the related object
cached_row = get_cached_row(row, index_end, using, klass_info)
# If the recursive descent found an object, populate the
# descriptor caches relevant to the object
if cached_row:
rel_obj, index_end = cached_row
if obj is not None:
# If the field is unique, populate the
# reverse descriptor cache
setattr(obj, f.related.get_cache_name(), rel_obj)
if rel_obj is not None:
# If the related object exists, populate
# the descriptor cache.
setattr(rel_obj, f.get_cache_name(), obj)
# Now populate all the non-local field values on the related
# object. If this object has deferred fields, we need to use
# the opts from the original model to get non-local fields
# correctly.
opts = rel_obj._meta
if getattr(rel_obj, '_deferred'):
opts = opts.proxy_for_model._meta
for rel_field, rel_model in opts.get_fields_with_model():
if rel_model is not None:
setattr(rel_obj, rel_field.attname, getattr(obj, rel_field.attname))
# populate the field cache for any related object
# that has already been retrieved
if rel_field.rel:
try:
cached_obj = getattr(obj, rel_field.get_cache_name())
setattr(rel_obj, rel_field.get_cache_name(), cached_obj)
except AttributeError:
# Related object hasn't been cached yet
pass
return obj, index_end以上是简要分析。
下面还有如何把object->sql
object.save()方法，执行以后会将数据存储到db。因此分析model基类的save方法。分析到save_base方法
def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
force_update=False, using=None, update_fields=None):
...
manager = cls._base_manager
...
result = manager._insert([self], fields=fields, return_id=update_pk, using=using, raw=raw)这里可以看出是插入部分manager内部
def _insert(self, objs, fields, **kwargs):
return insert_query(self.model, objs, fields, **kwargs)insert_query中
def insert_query(model, objs, fields, return_id=False, raw=False, using=None):
query = sql.InsertQuery(model)生成插入查询对象
query.insert_values(fields, objs, raw=raw)插入值
return query.get_compiler(using=using).execute_sql(return_id)获得生成sql对象，并执行sql语句插入值部分
def insert_values(self, fields, objs, raw=False):
self.fields = fields
# Check that no Promise object reaches the DB. Refs #10498.
for field in fields:
for obj in objs:
value = getattr(obj, field.attname)
if isinstance(value, Promise):
setattr(obj, field.attname, force_text(value))
self.objs = objs
self.raw = raw获取sql编译对象上面已经说过。
查看执行语句
def execute_sql(self, return_id=False):
assert not (return_id and len(self.query.objs) != 1)
self.return_id = return_id
cursor = self.connection.cursor()
for sql, params in self.as_sql():
cursor.execute(sql, params)
if not (return_id and cursor):
return
if self.connection.features.can_return_id_from_insert:
return self.connection.ops.fetch_returned_insert_id(cursor)
return self.connection.ops.last_insert_id(cursor,
self.query.model._meta.db_table, self.query.model._meta.pk.column)看到这里，看as_sql()方法
def as_sql(self):
qn = self.connection.ops.quote_name
opts = self.query.model._meta
result = ['INSERT INTO %s' % qn(opts.db_table)]
has_fields = bool(self.query.fields)
fields = self.query.fields if has_fields else [opts.pk]
result.append('(%s)' % ', '.join([qn(f.column) for f in fields]))
if has_fields:
params = values = [
[
f.get_db_prep_save(getattr(obj, f.attname) if self.query.raw else f.pre_save(obj, True), connection=self.connection)
for f in fields
]
for obj in self.query.objs
]
else:
values = [[self.connection.ops.pk_default_value()] for obj in self.query.objs]
params = [[]]
fields = [None]
can_bulk = (not any(hasattr(field, "get_placeholder") for field in fields) and
not self.return_id and self.connection.features.has_bulk_insert)
if can_bulk:
placeholders = [["%s"] * len(fields)]
else:
placeholders = [
[self.placeholder(field, v) for field, v in zip(fields, val)]
for val in values
]
# Oracle Spatial needs to remove some values due to #10888
params = self.connection.ops.modify_insert_params(placeholders, params)
if self.return_id and self.connection.features.can_return_id_from_insert:
params = params[0]
col = "%s.%s" % (qn(opts.db_table), qn(opts.pk.column))
result.append("VALUES (%s)" % ", ".join(placeholders[0]))
r_fmt, r_params = self.connection.ops.return_insert_id()
# Skip empty r_fmt to allow subclasses to customize behaviour for
# 3rd party backends. Refs #19096.
if r_fmt:
result.append(r_fmt % col)
params += r_params
return [(" ".join(result), tuple(params))]
if can_bulk:
result.append(self.connection.ops.bulk_insert_sql(fields, len(values)))
return [(" ".join(result), tuple([v for val in values for v in val]))]
else:
return [
(" ".join(result + ["VALUES (%s)" % ", ".join(p)]), vals)
for p, vals in zip(placeholders, params)
]因此object->sql部分逻辑是
object->得到表名，字段名，和值
格式化到 insert into 表 (字段) values (值) 并执行
最后1个问题。。有点复杂的说。要深入学习下。