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<meta name="twitter:description" content="Django是Python最流行的Web框架，功能强大。虽然学习入门门槛较高，但是后期使用起来，避免了很多不必要的造轮子。 其中ORM是Django最主要的特点之一，它将数据库表定义映射为Python对象，避免了SQL语句的字符串编码，使代码变得清晰且易于维护。 Django的ORM代码，是Python元类编程的很好应用，下面一起看看。 from django.db import models class Student(models.Model): name = models.CharField(max_length=20) age = models.SmallIntegerField() 上面代码是定义表的基本例子。我们继承models.Model对象，models.Model对象部分源码如下 class Model(metaclass=ModelBase): def __init__(self, *args, **kwargs): # Alias some things as …">
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Django Model源码解析
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<p>Django是Python最流行的Web框架，功能强大。虽然学习入门门槛较高，但是后期使用起来，避免了很多不必要的造轮子。
其中ORM是Django最主要的特点之一，它将数据库表定义映射为Python对象，避免了SQL语句的字符串编码，使代码变得清晰且易于维护。
Django的ORM代码，是Python元类编程的很好应用，下面一起看看。</p>
<div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db</span> <span class="kn">import</span> <span class="n">models</span>
<span class="k">class</span> <span class="nc">Student</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">20</span><span class="p">)</span>
<span class="n">age</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">SmallIntegerField</span><span class="p">()</span>
</pre></div>
<p>上面代码是定义表的基本例子。我们继承<code>models.Model</code>对象，<code>models.Model</code>对象部分源码如下</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Model</span><span class="p">(</span><span class="n">metaclass</span><span class="o">=</span><span class="n">ModelBase</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="c1"># Alias some things as locals to avoid repeat global lookups</span>
<span class="bp">cls</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="vm">__class__</span>
<span class="n">opts</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_meta</span>
<span class="n">_setattr</span> <span class="o">=</span> <span class="nb">setattr</span>
<span class="n">_DEFERRED</span> <span class="o">=</span> <span class="n">DEFERRED</span>
<span class="n">pre_init</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">sender</span><span class="o">=</span><span class="bp">cls</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="n">args</span><span class="p">,</span> <span class="n">kwargs</span><span class="o">=</span><span class="n">kwargs</span><span class="p">)</span>
<span class="c1"># Set up the storage for instance state</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_state</span> <span class="o">=</span> <span class="n">ModelState</span><span class="p">()</span>
<span class="c1"># There is a rather weird disparity here; if kwargs, it's set, then args</span>
<span class="c1"># overrides it. It should be one or the other; don't duplicate the work</span>
<span class="c1"># The reason for the kwargs check is that standard iterator passes in by</span>
<span class="c1"># args, and instantiation for iteration is 33% faster.</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">args</span><span class="p">)</span> <span class="o">></span> <span class="nb">len</span><span class="p">(</span><span class="n">opts</span><span class="o">.</span><span class="n">concrete_fields</span><span class="p">):</span>
<span class="c1"># Daft, but matches old exception sans the err msg.</span>
<span class="k">raise</span> <span class="ne">IndexError</span><span class="p">(</span><span class="s2">"Number of args exceeds number of fields"</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">kwargs</span><span class="p">:</span>
<span class="n">fields_iter</span> <span class="o">=</span> <span class="nb">iter</span><span class="p">(</span><span class="n">opts</span><span class="o">.</span><span class="n">concrete_fields</span><span class="p">)</span>
<span class="c1"># The ordering of the zip calls matter - zip throws StopIteration</span>
<span class="c1"># when an iter throws it. So if the first iter throws it, the second</span>
<span class="c1"># is *not* consumed. We rely on this, so don't change the order</span>
<span class="c1"># without changing the logic.</span>
<span class="k">for</span> <span class="n">val</span><span class="p">,</span> <span class="n">field</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">args</span><span class="p">,</span> <span class="n">fields_iter</span><span class="p">):</span>
<span class="k">if</span> <span class="n">val</span> <span class="ow">is</span> <span class="n">_DEFERRED</span><span class="p">:</span>
<span class="k">continue</span>
<span class="n">_setattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">field</span><span class="o">.</span><span class="n">attname</span><span class="p">,</span> <span class="n">val</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="c1"># Slower, kwargs-ready version. fields_iter = iter(opts.fields)</span>
<span class="k">for</span> <span class="n">val</span><span class="p">,</span> <span class="n">field</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">args</span><span class="p">,</span> <span class="n">fields_iter</span><span class="p">):</span>
<span class="k">if</span> <span class="n">val</span> <span class="ow">is</span> <span class="n">_DEFERRED</span><span class="p">:</span>
<span class="k">continue</span>
<span class="n">_setattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">field</span><span class="o">.</span><span class="n">attname</span><span class="p">,</span> <span class="n">val</span><span class="p">)</span>
<span class="n">kwargs</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="n">field</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
</pre></div>
<p>可以看到<code>Model</code>的定义使用了元类<code>ModelBase</code>。<code>ModelBase</code>主要处理表映射的Fields字段，具体怎么处理，后面再具体分析。所谓元类，就是普通类的类，也就是普通类是元类的实例。
可以用<code>类名.__class__</code>查看普通类的元类，默认元类是<code>type</code>。《流畅的Python》第21章很透彻的分析了元类，下面这幅图是里面的一张插图，很好的表示了元类与普通类的关系。</p>
<p><img alt="Python元类图" src="./images/meta_class.jpg"></p>
<p>我们回来，继续看<code>models.Model</code>源码。
我们看到<code>__init__</code>初始化方法第二行<code>opts=self._meta</code>。这个<code>_meta</code>属性，也是通过元类<code>ModelBase</code>附加给<code>Model</code>类的。它是一个<code>options.Options</code>对象，是管理ORM的核心。
<code>__init__</code>方法剩余的代码主要是处理域和值的映射关系，也是依赖<code>_meta</code>属性管理。完整的代码这里就不粘贴了。
<code>Model</code>对象除了<code>__init__</code>方法外，还有负责验证的<code>clean_fields</code>, <code>clean</code>, <code>validate_unique</code>以及负责保存对象到库的<code>save_base</code>方法，也都依赖<code>_meta</code>属性。
下面是<code>save_base</code>的代码，它通过<code>_meta</code>处理proxy，parents，auto_created，db_name然后创建或更新数据库表。</p>
<div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">save_base</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">raw</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">force_insert</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span>
<span class="n">force_update</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">using</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">update_fields</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="sd">"""</span>
<span class="sd"> Handle the parts of saving which should be done only once per save,</span>
<span class="sd"> yet need to be done in raw saves, too. This includes some sanity</span>
<span class="sd"> checks and signal sending.</span>
<span class="sd"> The 'raw' argument is telling save_base not to save any parent</span>
<span class="sd"> models and not to do any changes to the values before save. This</span>
<span class="sd"> is used by fixture loading.</span>
<span class="sd"> """</span>
<span class="n">using</span> <span class="o">=</span> <span class="n">using</span> <span class="ow">or</span> <span class="n">router</span><span class="o">.</span><span class="n">db_for_write</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="vm">__class__</span><span class="p">,</span> <span class="n">instance</span><span class="o">=</span><span class="bp">self</span><span class="p">)</span>
<span class="k">assert</span> <span class="ow">not</span> <span class="p">(</span><span class="n">force_insert</span> <span class="ow">and</span> <span class="p">(</span><span class="n">force_update</span> <span class="ow">or</span> <span class="n">update_fields</span><span class="p">))</span>
<span class="k">assert</span> <span class="n">update_fields</span> <span class="ow">is</span> <span class="kc">None</span> <span class="ow">or</span> <span class="n">update_fields</span>
<span class="bp">cls</span> <span class="o">=</span> <span class="n">origin</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="vm">__class__</span>
<span class="c1"># Skip proxies, but keep the origin as the proxy model.</span>
<span class="k">if</span> <span class="bp">cls</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">proxy</span><span class="p">:</span>
<span class="bp">cls</span> <span class="o">=</span> <span class="bp">cls</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">concrete_model</span>
<span class="n">meta</span> <span class="o">=</span> <span class="bp">cls</span><span class="o">.</span><span class="n">_meta</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">meta</span><span class="o">.</span><span class="n">auto_created</span><span class="p">:</span>
<span class="n">pre_save</span><span class="o">.</span><span class="n">send</span><span class="p">(</span>
<span class="n">sender</span><span class="o">=</span><span class="n">origin</span><span class="p">,</span> <span class="n">instance</span><span class="o">=</span><span class="bp">self</span><span class="p">,</span> <span class="n">raw</span><span class="o">=</span><span class="n">raw</span><span class="p">,</span> <span class="n">using</span><span class="o">=</span><span class="n">using</span><span class="p">,</span>
<span class="n">update_fields</span><span class="o">=</span><span class="n">update_fields</span><span class="p">,</span>
<span class="p">)</span>
<span class="c1"># A transaction isn't needed if one query is issued.</span>
<span class="k">if</span> <span class="n">meta</span><span class="o">.</span><span class="n">parents</span><span class="p">:</span>
<span class="n">context_manager</span> <span class="o">=</span> <span class="n">transaction</span><span class="o">.</span><span class="n">atomic</span><span class="p">(</span><span class="n">using</span><span class="o">=</span><span class="n">using</span><span class="p">,</span> <span class="n">savepoint</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">context_manager</span> <span class="o">=</span> <span class="n">transaction</span><span class="o">.</span><span class="n">mark_for_rollback_on_error</span><span class="p">(</span><span class="n">using</span><span class="o">=</span><span class="n">using</span><span class="p">)</span>
<span class="k">with</span> <span class="n">context_manager</span><span class="p">:</span>
<span class="n">parent_inserted</span> <span class="o">=</span> <span class="kc">False</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">raw</span><span class="p">:</span>
<span class="n">parent_inserted</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_save_parents</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">using</span><span class="p">,</span> <span class="n">update_fields</span><span class="p">)</span>
<span class="n">updated</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_save_table</span><span class="p">(</span>
<span class="n">raw</span><span class="p">,</span> <span class="bp">cls</span><span class="p">,</span> <span class="n">force_insert</span> <span class="ow">or</span> <span class="n">parent_inserted</span><span class="p">,</span>
<span class="n">force_update</span><span class="p">,</span> <span class="n">using</span><span class="p">,</span> <span class="n">update_fields</span><span class="p">,</span>
<span class="p">)</span>
<span class="c1"># Store the database on which the object was saved</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_state</span><span class="o">.</span><span class="n">db</span> <span class="o">=</span> <span class="n">using</span>
<span class="c1"># Once saved, this is no longer a to-be-added instance.</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_state</span><span class="o">.</span><span class="n">adding</span> <span class="o">=</span> <span class="kc">False</span>
<span class="c1"># Signal that the save is complete</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">meta</span><span class="o">.</span><span class="n">auto_created</span><span class="p">:</span>
<span class="n">post_save</span><span class="o">.</span><span class="n">send</span><span class="p">(</span>
<span class="n">sender</span><span class="o">=</span><span class="n">origin</span><span class="p">,</span> <span class="n">instance</span><span class="o">=</span><span class="bp">self</span><span class="p">,</span> <span class="n">created</span><span class="o">=</span><span class="p">(</span><span class="ow">not</span> <span class="n">updated</span><span class="p">),</span>
<span class="n">update_fields</span><span class="o">=</span><span class="n">update_fields</span><span class="p">,</span> <span class="n">raw</span><span class="o">=</span><span class="n">raw</span><span class="p">,</span> <span class="n">using</span><span class="o">=</span><span class="n">using</span><span class="p">,</span>
<span class="p">)</span>
</pre></div>
<p>简单看了<code>model.Model</code>的源码后，我们接下来简单分析下<code>ModelBase</code>代码。</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ModelBase</span><span class="p">(</span><span class="nb">type</span><span class="p">):</span>
<span class="sd">"""Metaclass for all models."""</span>
<span class="k">def</span> <span class="fm">__new__</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">bases</span><span class="p">,</span> <span class="n">attrs</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="n">super_new</span> <span class="o">=</span> <span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__new__</span>
<span class="c1"># Also ensure initialization is only performed for subclasses of Model</span>
<span class="c1"># (excluding Model class itself).</span>
<span class="n">parents</span> <span class="o">=</span> <span class="p">[</span><span class="n">b</span> <span class="k">for</span> <span class="n">b</span> <span class="ow">in</span> <span class="n">bases</span> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">b</span><span class="p">,</span> <span class="n">ModelBase</span><span class="p">)]</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">parents</span><span class="p">:</span>
<span class="k">return</span> <span class="n">super_new</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">bases</span><span class="p">,</span> <span class="n">attrs</span><span class="p">)</span>
<span class="c1"># Create the class.</span>
<span class="n">module</span> <span class="o">=</span> <span class="n">attrs</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'__module__'</span><span class="p">)</span>
<span class="n">new_attrs</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'__module__'</span><span class="p">:</span> <span class="n">module</span><span class="p">}</span>
<span class="n">classcell</span> <span class="o">=</span> <span class="n">attrs</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'__classcell__'</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<span class="k">if</span> <span class="n">classcell</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">new_attrs</span><span class="p">[</span><span class="s1">'__classcell__'</span><span class="p">]</span> <span class="o">=</span> <span class="n">classcell</span>
<span class="n">attr_meta</span> <span class="o">=</span> <span class="n">attrs</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'Meta'</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<span class="c1"># Pass all attrs without a (Django-specific) contribute_to_class()</span>
<span class="c1"># method to type.__new__() so that they're properly initialized</span>
<span class="c1"># (i.e. __set_name__()).</span>
<span class="n">contributable_attrs</span> <span class="o">=</span> <span class="p">{}</span>
<span class="k">for</span> <span class="n">obj_name</span><span class="p">,</span> <span class="n">obj</span> <span class="ow">in</span> <span class="nb">list</span><span class="p">(</span><span class="n">attrs</span><span class="o">.</span><span class="n">items</span><span class="p">()):</span>
<span class="k">if</span> <span class="n">_has_contribute_to_class</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span>
<span class="n">contributable_attrs</span><span class="p">[</span><span class="n">obj_name</span><span class="p">]</span> <span class="o">=</span> <span class="n">obj</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">new_attrs</span><span class="p">[</span><span class="n">obj_name</span><span class="p">]</span> <span class="o">=</span> <span class="n">obj</span>
<span class="n">new_class</span> <span class="o">=</span> <span class="n">super_new</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">bases</span><span class="p">,</span> <span class="n">new_attrs</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="n">abstract</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">attr_meta</span><span class="p">,</span> <span class="s1">'abstract'</span><span class="p">,</span> <span class="kc">False</span><span class="p">)</span>
<span class="n">meta</span> <span class="o">=</span> <span class="n">attr_meta</span> <span class="ow">or</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">new_class</span><span class="p">,</span> <span class="s1">'Meta'</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<span class="n">base_meta</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">new_class</span><span class="p">,</span> <span class="s1">'_meta'</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<span class="n">app_label</span> <span class="o">=</span> <span class="kc">None</span>
<span class="c1"># Look for an application configuration to attach the model to.</span>
<span class="n">app_config</span> <span class="o">=</span> <span class="n">apps</span><span class="o">.</span><span class="n">get_containing_app_config</span><span class="p">(</span><span class="n">module</span><span class="p">)</span>
<span class="k">if</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">meta</span><span class="p">,</span> <span class="s1">'app_label'</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">if</span> <span class="n">app_config</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">abstract</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span>
<span class="s2">"Model class </span><span class="si">%s</span><span class="s2">.</span><span class="si">%s</span><span class="s2"> doesn't declare an explicit "</span>
<span class="s2">"app_label and isn't in an application in "</span>
<span class="s2">"INSTALLED_APPS."</span> <span class="o">%</span> <span class="p">(</span><span class="n">module</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">app_label</span> <span class="o">=</span> <span class="n">app_config</span><span class="o">.</span><span class="n">label</span>
<span class="n">new_class</span><span class="o">.</span><span class="n">add_to_class</span><span class="p">(</span><span class="s1">'_meta'</span><span class="p">,</span> <span class="n">Options</span><span class="p">(</span><span class="n">meta</span><span class="p">,</span> <span class="n">app_label</span><span class="p">))</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">abstract</span><span class="p">:</span>
<span class="n">new_class</span><span class="o">.</span><span class="n">add_to_class</span><span class="p">(</span>
<span class="s1">'DoesNotExist'</span><span class="p">,</span>
<span class="n">subclass_exception</span><span class="p">(</span>
<span class="s1">'DoesNotExist'</span><span class="p">,</span>
<span class="nb">tuple</span><span class="p">(</span>
<span class="n">x</span><span class="o">.</span><span class="n">DoesNotExist</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">parents</span> <span class="k">if</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="s1">'_meta'</span><span class="p">)</span> <span class="ow">and</span> <span class="ow">not</span> <span class="n">x</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">abstract</span>
<span class="p">)</span> <span class="ow">or</span> <span class="p">(</span><span class="n">ObjectDoesNotExist</span><span class="p">,),</span>
<span class="n">module</span><span class="p">,</span>
<span class="n">attached_to</span><span class="o">=</span><span class="n">new_class</span><span class="p">))</span>
<span class="n">new_class</span><span class="o">.</span><span class="n">add_to_class</span><span class="p">(</span>
<span class="s1">'MultipleObjectsReturned'</span><span class="p">,</span>
<span class="n">subclass_exception</span><span class="p">(</span>
<span class="s1">'MultipleObjectsReturned'</span><span class="p">,</span>
<span class="nb">tuple</span><span class="p">(</span>
<span class="n">x</span><span class="o">.</span><span class="n">MultipleObjectsReturned</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">parents</span> <span class="k">if</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="s1">'_meta'</span><span class="p">)</span> <span class="ow">and</span> <span class="ow">not</span> <span class="n">x</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">abstract</span>
<span class="p">)</span> <span class="ow">or</span> <span class="p">(</span><span class="n">MultipleObjectsReturned</span><span class="p">,),</span>
<span class="n">module</span><span class="p">,</span>
<span class="n">attached_to</span><span class="o">=</span><span class="n">new_class</span><span class="p">))</span>
<span class="k">if</span> <span class="n">base_meta</span> <span class="ow">and</span> <span class="ow">not</span> <span class="n">base_meta</span><span class="o">.</span><span class="n">abstract</span><span class="p">:</span>
<span class="c1"># Non-abstract child classes inherit some attributes from their</span>
<span class="c1"># non-abstract parent (unless an ABC comes before it in the</span>
<span class="c1"># method resolution order).</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">meta</span><span class="p">,</span> <span class="s1">'ordering'</span><span class="p">):</span>
<span class="n">new_class</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">ordering</span> <span class="o">=</span> <span class="n">base_meta</span><span class="o">.</span><span class="n">ordering</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">meta</span><span class="p">,</span> <span class="s1">'get_latest_by'</span><span class="p">):</span>
<span class="n">new_class</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">get_latest_by</span> <span class="o">=</span> <span class="n">base_meta</span><span class="o">.</span><span class="n">get_latest_by</span>
<span class="n">is_proxy</span> <span class="o">=</span> <span class="n">new_class</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">proxy</span>
</pre></div>
<p>首先<code>ModelBase</code>继承<code>type</code>，所有的元类都继承<code>type</code>。接下来<code>__new__</code>方法，它才是Python真正的构造函数。由于普通类是元类的实例，当定义<code>Model</code>对象时，<code>ModelBase</code>的<code>__new__</code>方法将被调用。
注意是定义时，而不是调用时。就是类体本身属性和方法的初始化，都会有对应元类的<code>__new__</code>方法构建。<code>__new__</code>是一个类方法，接受的三个位置参数分别是要创建的类类名，继承的基类对象元组，以及类所属属性的映射。
下面是<code>__new__</code>部分代码注释。</p>
<div class="highlight"><pre><span></span><span class="c1"># Meta属性会再赋值给__meta</span>
<span class="n">attr_meta</span> <span class="o">=</span> <span class="n">attrs</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'Meta'</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<span class="c1"># 调用父类的__new__创建类对象，也就是Model对象</span>
<span class="n">new_class</span> <span class="o">=</span> <span class="n">super_new</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">bases</span><span class="p">,</span> <span class="n">new_attrs</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="c1"># 给Model增加_meta属性, 转化成了Options对象</span>
<span class="n">new_class</span><span class="o">.</span><span class="n">add_to_class</span><span class="p">(</span><span class="s1">'_meta'</span><span class="p">,</span> <span class="n">Options</span><span class="p">(</span><span class="n">meta</span><span class="p">,</span> <span class="n">app_label</span><span class="p">))</span>
<span class="c1"># 处理抽象父Model对应的域</span>
<span class="k">for</span> <span class="n">base</span> <span class="ow">in</span> <span class="nb">reversed</span><span class="p">([</span><span class="n">new_class</span><span class="p">]</span> <span class="o">+</span> <span class="n">parents</span><span class="p">):</span>
<span class="c1"># Conceptually equivalent to `if base is Model`.</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">base</span><span class="p">,</span> <span class="s1">'_meta'</span><span class="p">):</span>
<span class="k">continue</span>
<span class="c1"># Skip concrete parent classes.</span>
<span class="k">if</span> <span class="n">base</span> <span class="o">!=</span> <span class="n">new_class</span> <span class="ow">and</span> <span class="ow">not</span> <span class="n">base</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">abstract</span><span class="p">:</span>
<span class="k">continue</span>
<span class="c1"># Locate OneToOneField instances.</span>
<span class="k">for</span> <span class="n">field</span> <span class="ow">in</span> <span class="n">base</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">local_fields</span><span class="p">:</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">field</span><span class="p">,</span> <span class="n">OneToOneField</span><span class="p">):</span>
<span class="n">related</span> <span class="o">=</span> <span class="n">resolve_relation</span><span class="p">(</span><span class="n">new_class</span><span class="p">,</span> <span class="n">field</span><span class="o">.</span><span class="n">remote_field</span><span class="o">.</span><span class="n">model</span><span class="p">)</span>
<span class="n">parent_links</span><span class="p">[</span><span class="n">make_model_tuple</span><span class="p">(</span><span class="n">related</span><span class="p">)]</span> <span class="o">=</span> <span class="n">field</span>
<span class="c1"># 给Model对象追加objects作为manager</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">opts</span><span class="o">.</span><span class="n">managers</span><span class="p">:</span>
<span class="k">if</span> <span class="nb">any</span><span class="p">(</span><span class="n">f</span><span class="o">.</span><span class="n">name</span> <span class="o">==</span> <span class="s1">'objects'</span> <span class="k">for</span> <span class="n">f</span> <span class="ow">in</span> <span class="n">opts</span><span class="o">.</span><span class="n">fields</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span>
<span class="s2">"Model </span><span class="si">%s</span><span class="s2"> must specify a custom Manager, because it has a "</span>
<span class="s2">"field named 'objects'."</span> <span class="o">%</span> <span class="bp">cls</span><span class="o">.</span><span class="vm">__name__</span>
<span class="p">)</span>
<span class="n">manager</span> <span class="o">=</span> <span class="n">Manager</span><span class="p">()</span>
<span class="n">manager</span><span class="o">.</span><span class="n">auto_created</span> <span class="o">=</span> <span class="kc">True</span>
<span class="bp">cls</span><span class="o">.</span><span class="n">add_to_class</span><span class="p">(</span><span class="s1">'objects'</span><span class="p">,</span> <span class="n">manager</span><span class="p">)</span>
</pre></div>
<p>可以看到<code>ModelBase</code>主要是给<code>Model</code>对象追加<code>_meta</code>属性及objects manager，并利用<code>Options</code>对象校验Model和父Model域定义。<code>Options</code>代码这里不再分析，明天写写<code>Model</code>的<code>Field</code>对象。</p>
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