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<h1><a href="http://wbowam.github.io/i-am-just-testing.html"> I am just Testing </a></h1>
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<div class="highlight"><pre><span class="n">class</span> <span class="n">Poll</span><span class="p">(</span><span class="n">models</span><span class="p">.</span><span class="n">Model</span><span class="p">)</span><span class="o">:</span>
<span class="n">slug</span> <span class="o">=</span> <span class="n">models</span><span class="p">.</span><span class="n">SlugField</span><span class="p">(</span><span class="n">unique_for_month</span><span class="o">=</span><span class="err">'</span><span class="n">pub_date</span><span class="err">'</span><span class="p">)</span>
<span class="n">question</span> <span class="o">=</span> <span class="n">models</span><span class="p">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">maxlength</span><span class="o">=</span><span class="mi">255</span><span class="p">)</span>
<span class="n">pub_date</span> <span class="o">=</span> <span class="n">models</span><span class="p">.</span><span class="n">DateTimeField</span><span class="p">()</span>
<span class="n">expire_date</span> <span class="o">=</span> <span class="n">models</span><span class="p">.</span><span class="n">DateTimeField</span><span class="p">()</span>
<span class="n">def</span> <span class="n">__repr__</span><span class="p">(</span><span class="n">self</span><span class="p">)</span><span class="o">:</span>
<span class="k">return</span> <span class="n">self</span><span class="p">.</span><span class="n">question</span>
<span class="n">class</span> <span class="n">Meta</span><span class="o">:</span>
<span class="n">get_latest_by</span> <span class="o">=</span> <span class="err">'</span><span class="n">pub_date</span><span class="err">'</span>
<span class="n">class</span> <span class="n">Choice</span><span class="p">(</span><span class="n">models</span><span class="p">.</span><span class="n">Model</span><span class="p">)</span><span class="o">:</span>
<span class="n">poll</span> <span class="o">=</span> <span class="n">models</span><span class="p">.</span><span class="n">ForeignKey</span><span class="p">(</span><span class="n">Poll</span><span class="p">,</span> <span class="n">edit_inline</span><span class="o">=</span><span class="n">models</span><span class="p">.</span><span class="n">TABULAR</span><span class="p">,</span>
<span class="n">num_in_admin</span><span class="o">=</span><span class="mi">10</span><span class="p">,</span> <span class="n">min_num_in_admin</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span>
<span class="n">choice</span> <span class="o">=</span> <span class="n">models</span><span class="p">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">maxlength</span><span class="o">=</span><span class="mi">255</span><span class="p">,</span> <span class="n">core</span><span class="o">=</span><span class="n">True</span><span class="p">)</span>
<span class="n">votes</span> <span class="o">=</span> <span class="n">models</span><span class="p">.</span><span class="n">IntegerField</span><span class="p">(</span><span class="n">editable</span><span class="o">=</span><span class="n">False</span><span class="p">,</span> <span class="k">default</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="n">def</span> <span class="n">__repr__</span><span class="p">(</span><span class="n">self</span><span class="p">)</span><span class="o">:</span>
<span class="k">return</span> <span class="n">self</span><span class="p">.</span><span class="n">choice</span>
</pre></div>
<h3 id="p1">获得一个数据对象p1<a class="headerlink" href="#p1" title="Permanent link">¶</a></h3>
<div class="highlight"><pre><span class="n">from</span> <span class="n">datetime</span> <span class="n">import</span> <span class="n">datetime</span>
<span class="n">p1</span> <span class="o">=</span> <span class="n">Poll</span><span class="p">(</span><span class="n">slug</span><span class="o">=</span><span class="err">'</span><span class="n">whatsup</span><span class="err">'</span><span class="p">,</span> <span class="n">question</span><span class="o">=</span><span class="s">"What's up?"</span><span class="p">,</span>\
<span class="n">pub_date</span><span class="o">=</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">20</span><span class="p">),</span> <span class="n">expire_date</span><span class="o">=</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">20</span><span class="p">))</span>
<span class="n">p1</span><span class="p">.</span><span class="n">save</span><span class="p">()</span>
</pre></div>
<ul>
<li>数据对象有一个初始方法save()</li>
</ul>
<h3 id="_1">获取结果集对象<a class="headerlink" href="#_1" title="Permanent link">¶</a></h3>
<h4 id="p2">无限制获取对象集p2<a class="headerlink" href="#p2" title="Permanent link">¶</a></h4>
<div class="highlight"><pre><span class="n">p2</span><span class="o">=</span><span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">()</span>
<span class="o">>>></span><span class="n">p2</span>
<span class="p">[</span><span class="n">What</span><span class="err">'</span><span class="n">s</span> <span class="n">up</span><span class="o">?</span><span class="p">,</span> <span class="n">What</span><span class="err">'</span><span class="n">s</span> <span class="n">your</span> <span class="n">name</span><span class="o">?</span><span class="p">]</span>
</pre></div>
<h6 id="p2_1">注意：在这里p2是个对象集，自身也是个对象。<a class="headerlink" href="#p2_1" title="Permanent link">¶</a></h6>
<h4 id="_2">增加一些限制条件直到描述的子集满足你的需要。<a class="headerlink" href="#_2" title="Permanent link">¶</a></h4>
<p>最常用的两个定制结果集的方法是:</p>
<div class="highlight"><pre><span class="n">filter</span><span class="p">(</span><span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="err">返回一个匹配查询参数的新的结果集</span><span class="p">.</span>
<span class="n">exclude</span><span class="p">(</span><span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="err">返回一个不匹配查询参数的新的结果集</span><span class="p">.</span>
</pre></div>
<p>这两个方法的返回值都是结果集对象,因此结果集可以进行链式处理:</p>
<div class="highlight"><pre><span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">question__startswith</span><span class="o">=</span><span class="s">"What"</span><span class="p">)</span>\
<span class="p">.</span><span class="n">exclude</span><span class="p">(</span><span class="n">pub_date__gte</span><span class="o">=</span><span class="n">datetime</span><span class="p">.</span><span class="n">now</span><span class="p">())</span>\
<span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">pub_date__gte</span><span class="o">=</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">1</span><span class="p">))</span>
</pre></div>
<p>以一个初始结果集作为参数, 然后进行过滤, 再进行排除, 再进行另一个过滤. <br />
这样得到的最终结果就一个问题开头单词是 "What", 发布日期在 2005年1月1日至今的所有民意测验的集合. </p>
<p>每个结果集都是一个独一无二的对象. 以上操作的每一步都生成了一个新的结果集: </p>
<div class="highlight"><pre><span class="n">q1</span> <span class="o">=</span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">question__startswith</span><span class="o">=</span><span class="s">"What"</span><span class="p">)</span>
<span class="n">q2</span> <span class="o">=</span> <span class="n">q1</span><span class="p">.</span><span class="n">exclude</span><span class="p">(</span><span class="n">pub_date__gte</span><span class="o">=</span><span class="n">datetime</span><span class="p">.</span><span class="n">now</span><span class="p">())</span>
<span class="n">q3</span> <span class="o">=</span> <span class="n">q1</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">pub_date__gte</span><span class="o">=</span><span class="n">datetime</span><span class="p">.</span><span class="n">now</span><span class="p">())</span>
</pre></div>
<p>这三步生成了三个结果集; 一个初始结果集包含所有的以"What"开头的民意测验, 两个初始结果集的子集(一个排除条件,一个过滤条件). <br />
对原始结果集的改进过程并没有影响到原始的结果集.</p>
<h6 id="_3">值得注意的是结果集的创建根本没有访问数据库.只有当对结果集取值时才会访问数据库.<a class="headerlink" href="#_3" title="Permanent link">¶</a></h6>
<h3 id="_4">字段查询<a class="headerlink" href="#_4" title="Permanent link">¶</a></h3>
<p>以 field__lookuptype (注意是双下线)形式进行基本的字段查询,举例来说:</p>
<div class="highlight"><pre><span class="n">polls</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">pub_date__lte</span><span class="o">=</span><span class="n">datetime</span><span class="p">.</span><span class="n">now</span><span class="p">())</span>
</pre></div>
<p>该查询翻译成SQL就是:</p>
<div class="highlight"><pre><span class="n">SELECT</span> <span class="o">*</span> <span class="n">FROM</span> <span class="n">polls_polls</span> <span class="n">WHERE</span> <span class="n">pub_date</span> <span class="o"><=</span> <span class="n">NOW</span><span class="p">();</span>
</pre></div>
<h3 id="db-api">DB API 支持下列查找类型:<a class="headerlink" href="#db-api" title="Permanent link">¶</a></h3>
<div class="highlight"><pre><span class="err">类型</span> <span class="err">描述</span>
<span class="n">exact</span> <span class="err">精确匹配</span><span class="o">:</span> <span class="n">polls</span><span class="p">.</span><span class="n">get_object</span><span class="p">(</span><span class="n">id__exact</span><span class="o">=</span><span class="mi">14</span><span class="p">).</span>
<span class="n">iexact</span> <span class="err">忽略大小写的精确匹配</span><span class="o">:</span> <span class="n">polls</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">slug__iexact</span><span class="o">=</span><span class="s">"foo"</span><span class="p">)</span> <span class="err">匹配</span> <span class="n">foo</span><span class="p">,</span> <span class="n">FOO</span><span class="p">,</span> <span class="n">fOo</span><span class="p">,</span> <span class="err">等等</span><span class="p">.</span>
<span class="n">contains</span> <span class="err">大小写敏感的内容包含测试</span><span class="o">:</span> <span class="n">polls</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">question__contains</span><span class="o">=</span><span class="s">"spam"</span><span class="p">)</span> <span class="err">返回</span><span class="n">question</span> <span class="err">中包含</span> <span class="s">"spam"</span> <span class="err">的所有民意测验</span><span class="p">.(</span><span class="err">仅</span><span class="n">PostgreSQL</span> <span class="err">和</span> <span class="n">MySQL</span><span class="err">支持</span><span class="p">.</span> <span class="n">SQLite</span> <span class="err">的</span><span class="n">LIKE</span> <span class="err">语句不支持大小写敏感特性</span><span class="p">.</span> <span class="err">对</span><span class="n">Sqlite</span> <span class="err">来说</span><span class="p">,</span> <span class="n">contains</span> <span class="err">等于</span> <span class="n">icontains</span><span class="p">.)</span>
<span class="n">icontains</span> <span class="err">大小写不敏感的内容包含测试</span><span class="o">:</span>
<span class="n">gt</span> <span class="err">大于</span><span class="o">:</span> <span class="n">polls</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">id__gt</span><span class="o">=</span><span class="mi">4</span><span class="p">).</span>
<span class="n">gte</span> <span class="err">大于等于</span><span class="p">.</span>
<span class="n">lt</span> <span class="err">小于</span><span class="p">.</span>
<span class="n">lte</span> <span class="err">小于等于</span><span class="p">.</span>
<span class="n">ne</span> <span class="err">不等于</span><span class="p">.</span>
<span class="n">in</span> <span class="err">位于给定列表中</span><span class="o">:</span> <span class="n">polls</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">id__in</span><span class="o">=</span><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">])</span> <span class="err">返回一个</span> <span class="n">polls</span> <span class="err">列表</span><span class="p">(</span><span class="n">ID</span> <span class="err">值分别是</span> <span class="mi">1</span><span class="err">或</span><span class="mi">3</span><span class="err">或</span><span class="mi">4</span><span class="p">).</span>
<span class="n">startswith</span> <span class="err">大小写敏感的</span> <span class="n">starts</span><span class="o">-</span><span class="n">with</span><span class="o">:</span> <span class="n">polls</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">question__startswith</span><span class="o">=</span><span class="s">"Would"</span><span class="p">).(</span><span class="err">仅</span><span class="n">PostgreSQL</span> <span class="err">和</span><span class="n">MySQL</span><span class="err">支持</span><span class="p">.</span> <span class="n">SQLite</span> <span class="err">的</span><span class="n">LIKE</span> <span class="err">语句不支持大小写敏感特性</span><span class="p">.</span> <span class="err">对</span><span class="n">Sqlite</span> <span class="err">来说</span><span class="p">,</span><span class="err">``</span><span class="n">startswith</span><span class="err">``</span> <span class="err">等于</span> <span class="n">istartswith</span><span class="p">)</span>
<span class="n">endswith</span> <span class="err">大小写敏感的</span> <span class="n">ends</span><span class="o">-</span><span class="n">with</span><span class="p">.</span> <span class="p">(</span><span class="err">仅</span><span class="n">PostgreSQL</span> <span class="err">和</span> <span class="n">MySQL</span><span class="p">)</span>
<span class="n">istartswith</span> <span class="err">大小写不敏感的</span> <span class="n">starts</span><span class="o">-</span><span class="n">with</span><span class="p">.</span>
<span class="n">iendswith</span> <span class="err">大小写不敏感的</span> <span class="n">ends</span><span class="o">-</span><span class="n">with</span><span class="p">.</span>
<span class="n">range</span> <span class="err">范围测试</span><span class="o">:</span> <span class="n">polls</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">pub_date__range</span><span class="o">=</span><span class="p">(</span><span class="n">start_date</span><span class="p">,</span> <span class="n">end_date</span><span class="p">))</span> <span class="err">返回</span> <span class="n">pub_date</span> <span class="err">位于</span> <span class="n">start_date</span> <span class="err">和</span> <span class="n">end_date</span> <span class="p">(</span><span class="err">包括</span><span class="p">)</span><span class="err">之间的所有民意测验</span>
<span class="n">year</span> <span class="err">对</span> <span class="n">date</span><span class="o">/</span><span class="n">datetime</span> <span class="err">字段</span><span class="p">,</span> <span class="err">进行精确的</span> <span class="err">年</span> <span class="err">匹配</span><span class="o">:</span> <span class="n">polls</span><span class="p">.</span><span class="n">get_count</span><span class="p">(</span><span class="n">pub_date__year</span><span class="o">=</span><span class="mi">2005</span><span class="p">).</span>
<span class="n">month</span> <span class="err">对</span> <span class="n">date</span><span class="o">/</span><span class="n">datetime</span> <span class="err">字段</span><span class="p">,</span> <span class="err">进行精确的</span> <span class="err">月</span> <span class="err">匹配</span><span class="o">:</span>
<span class="n">day</span> <span class="err">对</span> <span class="n">date</span><span class="o">/</span><span class="n">datetime</span> <span class="err">字段</span><span class="p">,</span> <span class="err">进行精确的</span> <span class="err">日</span> <span class="err">匹配</span><span class="o">:</span>
<span class="n">isnull</span> <span class="n">True</span><span class="o">/</span><span class="n">False</span><span class="p">;</span> <span class="err">做</span> <span class="n">IF</span> <span class="nb">NULL</span><span class="o">/</span><span class="n">IF</span> <span class="n">NOT</span> <span class="nb">NULL</span> <span class="err">查询</span><span class="o">:</span> <span class="n">polls</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">expire_date__isnull</span><span class="o">=</span><span class="n">True</span><span class="p">).</span>
</pre></div>
<h6 id="exact">如果未提供查找类型, 系统就认为查找类型是 exact . 下面两个语句是等价的:<a class="headerlink" href="#exact" title="Permanent link">¶</a></h6>
<div class="highlight"><pre><span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">get</span><span class="p">(</span><span class="n">id</span><span class="o">=</span><span class="mi">14</span><span class="p">)</span>
<span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">get</span><span class="p">(</span><span class="n">id__exact</span><span class="o">=</span><span class="mi">14</span><span class="p">)</span>
</pre></div>
<h5 id="and">查询允许多个条件参数, 逗号分隔的多个条件参数会被 "AND" 起来使用:<a class="headerlink" href="#and" title="Permanent link">¶</a></h5>
<div class="highlight"><pre><span class="n">polls</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span>
<span class="n">pub_date__year</span><span class="o">=</span><span class="mi">2005</span><span class="p">,</span>
<span class="n">pub_date__month</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span>
<span class="n">question__startswith</span><span class="o">=</span><span class="s">"Would"</span><span class="p">,</span>
<span class="p">)</span>
</pre></div>
<p>得到2005年1月公布的带有一个"Would"开头的问题的所有民意测验.</p>
<h5 id="pk-primary_key__exact">为了使用更加方便, 还提供有一个 pk 查找类型, 可以翻译成 (primary_key)__exact.<a class="headerlink" href="#pk-primary_key__exact" title="Permanent link">¶</a></h5>
<p>在这个民意测试的例子里, 下面两个语句是等价的.:</p>
<div class="highlight"><pre><span class="n">polls</span><span class="p">.</span><span class="n">get_object</span><span class="p">(</span><span class="n">id__exact</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>
<span class="n">polls</span><span class="p">.</span><span class="n">get_object</span><span class="p">(</span><span class="n">pk</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>
</pre></div>
<h5 id="pk">pk 也可以通过连接进行查询.<a class="headerlink" href="#pk" title="Permanent link">¶</a></h5>
<p>在这个民意测试的例子里, 下面两个语句是等价的:</p>
<div class="highlight"><pre><span class="n">choices</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">poll__id__exact</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>
<span class="n">choices</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">poll__pk</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>
</pre></div>
<p>如果传递的关键字参数非法, 将引发 TypeError 异常.</p>
<h4 id="or">OR 查询<a class="headerlink" href="#or" title="Permanent link">¶</a></h4>
<p>关键字参数查询的各个条件都是 "AND" 关系. 如果你需要一个复杂的查询(举例来说,你需要一个 OR 语句), 你需要使用 Q 对象. <br />
Q 对象是 django.core.meta.Q 的实例, 用来装载一系列关键字参数. 这些关键字参数就象指定给 get() 和 filter() 函数的关键字参数一样. 举例来说:</p>
<div class="highlight"><pre><span class="n">Q</span><span class="p">(</span><span class="n">question__startswith</span><span class="o">=</span><span class="err">'</span><span class="n">What</span><span class="err">'</span><span class="p">)</span>
</pre></div>
<p>Q 对象可以使用 & 和 | 运算符进行组合. 当两个Q对象进行 & 或 | 运算时,会生成一个新的Q对象.举例来说语句:</p>
<div class="highlight"><pre><span class="n">Q</span><span class="p">(</span><span class="n">question__startswith</span><span class="o">=</span><span class="err">'</span><span class="n">Who</span><span class="err">'</span><span class="p">)</span> <span class="o">|</span> <span class="n">Q</span><span class="p">(</span><span class="n">question__startswith</span><span class="o">=</span><span class="err">'</span><span class="n">What</span><span class="err">'</span><span class="p">)</span>
</pre></div>
<p>生成一个新的 Q 对象表示这两个 "question__startswith" 查询条件的 "OR" 关系. 等同于下面的 SQL WHERE 子句:</p>
<div class="highlight"><pre> <span class="n">WHERE</span> <span class="n">question</span> <span class="n">LIKE</span> <span class="err">'</span><span class="n">Who</span><span class="o">%</span><span class="err">'</span> <span class="n">OR</span> <span class="n">question</span> <span class="n">LIKE</span> <span class="err">'</span><span class="n">What</span><span class="o">%</span><span class="err">'</span>
</pre></div>
<p>查询函数可以接受一个或多个 Q 对象作为参数.如果提供有多个 Q 对象参数, 它们将被 "AND" 到一起. 举例来说:</p>
<div class="highlight"><pre><span class="n">polls</span><span class="p">.</span><span class="n">get_object</span><span class="p">(</span>
<span class="n">Q</span><span class="p">(</span><span class="n">question__startswith</span><span class="o">=</span><span class="err">'</span><span class="n">Who</span><span class="err">'</span><span class="p">),</span>
<span class="n">Q</span><span class="p">(</span><span class="n">pub_date__exact</span><span class="o">=</span><span class="n">date</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">2</span><span class="p">))</span> <span class="o">|</span> <span class="n">Q</span><span class="p">(</span><span class="n">pub_date__exact</span><span class="o">=</span><span class="n">date</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">))</span>
<span class="p">)</span>
</pre></div>
<p>翻译成 SQL 就是这样:</p>
<div class="highlight"><pre><span class="n">SELECT</span> <span class="o">*</span> <span class="n">from</span> <span class="n">polls</span> <span class="n">WHERE</span> <span class="n">question</span> <span class="n">LIKE</span> <span class="err">'</span><span class="n">Who</span><span class="o">%</span><span class="err">'</span>
<span class="n">AND</span> <span class="p">(</span><span class="n">pub_date</span> <span class="o">=</span> <span class="err">'</span><span class="mi">2005</span><span class="o">-</span><span class="mo">05</span><span class="o">-</span><span class="mo">02</span><span class="err">'</span> <span class="n">OR</span> <span class="n">pub_date</span> <span class="o">=</span> <span class="err">'</span><span class="mi">2005</span><span class="o">-</span><span class="mo">05</span><span class="o">-</span><span class="mo">06</span><span class="err">'</span><span class="p">)</span>
</pre></div>
<h3 id="_5">从结果集中取值<a class="headerlink" href="#_5" title="Permanent link">¶</a></h3>
<p>只有通过取值操作才能得到结果集包含的对象.取值操作可以通过迭代,切片,或其它专门的函数来实现. </p>
<h5 id="_6">一个结果集就是一个可迭代对象.<a class="headerlink" href="#_6" title="Permanent link">¶</a></h5>
<p>因此,可以通过一个循环来取出它的值:</p>
<div class="highlight"><pre><span class="k">for</span> <span class="n">p</span> <span class="n">in</span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">()</span><span class="o">:</span>
<span class="n">print</span> <span class="n">p</span>
</pre></div>
<p>将使用 Poll 对象的 <strong>repr</strong>() 方法打印出所有的 Poll 对象.</p>
<h5 id="_7">一个结果集也可以被切片, 使用数组符号操作:<a class="headerlink" href="#_7" title="Permanent link">¶</a></h5>
<div class="highlight"><pre><span class="n">fifth_poll</span> <span class="o">=</span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">()[</span><span class="mi">4</span><span class="p">]</span>
<span class="n">all_polls_but_the_first_two</span> <span class="o">=</span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">()[</span><span class="mi">2</span><span class="o">:</span><span class="p">]</span>
<span class="n">every_second_poll</span> <span class="o">=</span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">()[</span><span class="o">::</span><span class="mi">2</span><span class="p">]</span>
</pre></div>
<h6 id="-">结果集对象是惰性对象 - 也就是说,他们不是 真正的 包含他们表示对象的集合 (或列表).<a class="headerlink" href="#-" title="Permanent link">¶</a></h6>
<h6 id="python-django">Python 的协议魔法让结果集看起来是一个可迭代,可切片的对象. 事实上在幕后, Django 使用了缓存技术..<a class="headerlink" href="#python-django" title="Permanent link">¶</a></h6>
<p>如果你真的需要一个列表, 你可以强制对一个惰性对象取值:</p>
<div class="highlight"><pre><span class="n">querylist</span> <span class="o">=</span> <span class="n">list</span><span class="p">(</span><span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">())</span>
</pre></div>
<p>不过,最好不要这么做,尤其当一个结果集相当大时. 由于 Django 要创建每一个对象的内存表示,这将占用相当大的内存.</p>
<h3 id="_8">结果集及其缓存行为<a class="headerlink" href="#_8" title="Permanent link">¶</a></h3>
<p>每个结果集都包含一个 cache. 对一个新创建的结果集来说, 缓存区是空的.当一个结果集第一次被取值, Django 会进行一次数据库查询,并将查询结果放入缓存中, 之后返回用户需要的数据. 后面的取值操作会使用缓存中的数据而不用再次访问数据库.</p>
<p>必须时刻记住:结果集具有缓存行为. 下面两行语句生成了两个临时的结果集,并进行了取值,之后舍弃:</p>
<div class="highlight"><pre><span class="n">print</span> <span class="p">[</span><span class="n">p</span> <span class="k">for</span> <span class="n">p</span> <span class="n">in</span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">()]</span> <span class="err">#</span> <span class="n">Evaluate</span> <span class="n">the</span> <span class="n">Query</span> <span class="n">Set</span>
<span class="n">print</span> <span class="p">[</span><span class="n">p</span> <span class="k">for</span> <span class="n">p</span> <span class="n">in</span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">()]</span> <span class="err">#</span> <span class="n">Evaluate</span> <span class="n">the</span> <span class="n">Query</span> <span class="n">Set</span> <span class="n">again</span>
</pre></div>
<p>对一个小型的,低流量的站点来说,这不会造成严重问题. <br />
不过,对一个高访问量的站点来说,它双倍增加了数据库服务器的负担. <br />
另外,由于在两次操作之间可能有其它的用户增加或删除了投票,因此这两次操作得到结果可能并不相同.</p>
<h5 id="_9">要避免这个问题, 保存这个结果集并在后面重用该结果集:<a class="headerlink" href="#_9" title="Permanent link">¶</a></h5>
<div class="highlight"><pre><span class="n">queryset</span> <span class="o">=</span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">()</span>
<span class="n">print</span> <span class="p">[</span><span class="n">p</span> <span class="k">for</span> <span class="n">p</span> <span class="n">in</span> <span class="n">queryset</span><span class="p">]</span> <span class="err">#</span> <span class="n">Evaluate</span> <span class="n">the</span> <span class="n">query</span> <span class="n">set</span>
<span class="n">print</span> <span class="p">[</span><span class="n">p</span> <span class="k">for</span> <span class="n">p</span> <span class="n">in</span> <span class="n">queryset</span><span class="p">]</span> <span class="err">#</span> <span class="n">Re</span><span class="o">-</span><span class="n">use</span> <span class="n">the</span> <span class="n">cache</span> <span class="n">from</span> <span class="n">the</span> <span class="n">evaluation</span>
</pre></div>
<h3 id="_10">关系 (连接)<a class="headerlink" href="#_10" title="Permanent link">¶</a></h3>
<p>当你在 model 中定义了一个关系字段(也就是,一个ForeignKey, OneToOneField, 或 ManyToManyField).<br />
Django 使用关系字段的名字为 model 的每个实例添加一个 描述符. <br />
在访问对象或关联对象时, 这个描述符就象一个常规属性. <br />
举例来说, mychoice.poll 会返回 Choice 实例对象关联的 Poll 对象.</p>
<p>通过下面的关系,连接可以以非显式的方式进行: <br />
<code>choices.objects.filter(poll__slug="eggs")</code> <br />
得到一个 Choice 对象列表, 这些对象关联的 Poll 对象的 slug 字段值为 eggs. 允许多级连接. </p>
<p>通过一个对象实例的便利函数(convenience functions)就可直接查询该对象的关联对象. 举例来说, 如果 p 是一个 Poll 实例, p.choice_set() 将返回所有关联的 Choice 对象列表. 聪明的读者会注意到它等价于 choices.objects.filter(poll__id=p.id), 只是更加清晰.</p>
<h4 id="one-to-one-relations">One-to-one relations<a class="headerlink" href="#one-to-one-relations" title="Permanent link">¶</a></h4>
<h5 id="one-to-one-get_relatedobjectname">one-to-one 关系中的每个对象拥有一个 get_relatedobjectname() 方法.<a class="headerlink" href="#one-to-one-get_relatedobjectname" title="Permanent link">¶</a></h5>
<p>举例来说:</p>
<div class="highlight"><pre><span class="n">class</span> <span class="n">Place</span><span class="p">(</span><span class="n">meta</span><span class="p">.</span><span class="n">Model</span><span class="p">)</span><span class="o">:</span>
<span class="cp"># ...</span>
<span class="n">class</span> <span class="n">Restaurant</span><span class="p">(</span><span class="n">meta</span><span class="p">.</span><span class="n">Model</span><span class="p">)</span><span class="o">:</span>
<span class="cp"># ...</span>
<span class="n">the_place</span> <span class="o">=</span> <span class="n">meta</span><span class="p">.</span><span class="n">OneToOneField</span><span class="p">(</span><span class="n">places</span><span class="p">.</span><span class="n">Place</span><span class="p">)</span>
</pre></div>
<p>在上面的例子里, 每个 Place 会自动拥有一个 get_restaurant() 方法, <br />
且每个 Restaurant 会自动拥有一个 get_the_place() 方法.</p>
<h4 id="many-to-one-relations">Many-to-one relations<a class="headerlink" href="#many-to-one-relations" title="Permanent link">¶</a></h4>
<p>在 many-to-one 关系中, 关联对象(Many)会自动拥有一个 get_relatedobject() 方法. <br />
被关联的对象(one)会自动拥有 get_relatedobject(), get_relatedobject_list(), 和 get_relatedobject_count() 方法 (功能与模块级的 get_object(), filter(), 和 get_count() 相同).</p>
<p>在上面的民意测试例子里, 一个 Poll 对象 p 自动拥有下列方法:</p>
<div class="highlight"><pre><span class="n">p</span><span class="p">.</span><span class="n">get_choice</span><span class="p">()</span>
<span class="n">p</span><span class="p">.</span><span class="n">get_choice_list</span><span class="p">()</span>
<span class="n">p</span><span class="p">.</span><span class="n">get_choice_count</span><span class="p">()</span>
</pre></div>
<p>Choice 对象 c 则自动拥有下面的方法:
<code>c.get_poll()</code> </p>
<h4 id="many-to-many">Many-to-many 关系<a class="headerlink" href="#many-to-many" title="Permanent link">¶</a></h4>
<p>Many-to-many 关系类似<code>Many-to-one relations</code>_, 它生成同样的方法集.例外的是关联对象的 get_relatedobject_list() 方法返回一个实例的列表而不是一个仅一个实例.因此,若 Poll 和 Choice 是 many-to-many 关系, choice.get_poll_list() 将返回一个列表. </p>
<h5 id="_11">专门的结果集<a class="headerlink" href="#_11" title="Permanent link">¶</a></h5>
<p>除 filter 和 exclude() 之外, Django 提供了一系列结果集处理方法, 修改结果的类型, 或修改 sql 查询在数据库执行的方式.</p>
<div class="highlight"><pre><span class="n">order_by</span><span class="p">(</span><span class="o">*</span><span class="n">fields</span><span class="p">)</span>
</pre></div>
<p>根据 model 中提供 ordering tuple, 结果集会被自动排序. 不过, 排序也可以通过 order_by 方法显式的进行:</p>
<div class="highlight"><pre><span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">pub_date__year</span><span class="o">=</span><span class="mi">2005</span><span class="p">,</span>
<span class="n">pub_date__month</span><span class="o">=</span><span class="mi">1</span><span class="p">).</span><span class="n">order_by</span><span class="p">(</span><span class="err">'</span><span class="o">-</span><span class="n">pub_date</span><span class="err">'</span><span class="p">,</span> <span class="err">'</span><span class="n">question</span><span class="err">'</span><span class="p">)</span>
</pre></div>
<p>结果集将按降序排列 pub_date, 然后按升序排列 question."-pub_date" 中的负号表示降序(递减).要取随机序,使用"?", 象下面这样:</p>
<div class="highlight"><pre><span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">order_by</span><span class="o">=</span><span class="p">(</span><span class="sc">'?'</span><span class="p">)</span>
</pre></div>
<p>要按另一个表中的字段排序, 添加另一个表的名字和一个句点,象下面这样:</p>
<div class="highlight"><pre><span class="n">Choice</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">order_by</span><span class="o">=</span><span class="p">(</span><span class="err">'</span><span class="n">Poll</span><span class="p">.</span><span class="n">pub_date</span><span class="err">'</span><span class="p">,</span> <span class="err">'</span><span class="n">choice</span><span class="err">'</span><span class="p">)</span>
</pre></div>
<h3 id="valuesfields">values(*fields)<a class="headerlink" href="#valuesfields" title="Permanent link">¶</a></h3>
<p>类似 filter(), 不过它返回一个字典的列表而不是 model 实例对象的列表. </p>
<p>它接受一个可选参数: fields, 这是一个字段名列表或tuple.如果你没有指定 fields, 每个字段都会返回. <br />
否则就只返回你指定的字段名和值.这里有一个例子,使用上面定义的 Poll model</p>
<div class="highlight"><pre><span class="o">>>></span> <span class="n">from</span> <span class="n">datetime</span> <span class="n">import</span> <span class="n">datetime</span>
<span class="o">>>></span> <span class="n">p1</span> <span class="o">=</span> <span class="n">Poll</span><span class="p">(</span><span class="n">slug</span><span class="o">=</span><span class="err">'</span><span class="n">whatsup</span><span class="err">'</span><span class="p">,</span> <span class="n">question</span><span class="o">=</span><span class="s">"What's up?"</span><span class="p">,</span>
<span class="p">...</span> <span class="n">pub_date</span><span class="o">=</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">20</span><span class="p">),</span> <span class="n">expire_date</span><span class="o">=</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">20</span><span class="p">))</span>
<span class="o">>>></span> <span class="n">p1</span><span class="p">.</span><span class="n">save</span><span class="p">()</span>
<span class="o">>>></span> <span class="n">p2</span> <span class="o">=</span> <span class="n">Poll</span><span class="p">(</span><span class="n">slug</span><span class="o">=</span><span class="err">'</span><span class="n">name</span><span class="err">'</span><span class="p">,</span> <span class="n">question</span><span class="o">=</span><span class="s">"What's your name?"</span><span class="p">,</span>
<span class="p">...</span> <span class="n">pub_date</span><span class="o">=</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">20</span><span class="p">),</span> <span class="n">expire_date</span><span class="o">=</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">20</span><span class="p">))</span>
<span class="o">>>></span> <span class="n">p2</span><span class="p">.</span><span class="n">save</span><span class="p">()</span>
<span class="o">>>></span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">all</span><span class="p">()</span>
<span class="p">[</span><span class="n">What</span><span class="err">'</span><span class="n">s</span> <span class="n">up</span><span class="o">?</span><span class="p">,</span> <span class="n">What</span><span class="err">'</span><span class="n">s</span> <span class="n">your</span> <span class="n">name</span><span class="o">?</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">values</span><span class="p">()</span>
<span class="p">[{</span><span class="err">'</span><span class="n">id</span><span class="err">'</span><span class="o">:</span> <span class="mi">1</span><span class="p">,</span> <span class="err">'</span><span class="n">slug</span><span class="err">'</span><span class="o">:</span> <span class="err">'</span><span class="n">whatsup</span><span class="err">'</span><span class="p">,</span> <span class="err">'</span><span class="n">question</span><span class="err">'</span><span class="o">:</span> <span class="s">"What's up?"</span><span class="p">,</span> <span class="err">'</span><span class="n">pub_date</span><span class="err">'</span><span class="o">:</span> <span class="n">datetime</span><span class="p">.</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">20</span><span class="p">),</span> <span class="err">'</span><span class="n">expire_date</span><span class="err">'</span><span class="o">:</span> <span class="n">datetime</span><span class="p">.</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">20</span><span class="p">)},</span>
<span class="p">{</span><span class="err">'</span><span class="n">id</span><span class="err">'</span><span class="o">:</span> <span class="mi">2</span><span class="p">,</span> <span class="err">'</span><span class="n">slug</span><span class="err">'</span><span class="o">:</span> <span class="err">'</span><span class="n">name</span><span class="err">'</span><span class="p">,</span> <span class="err">'</span><span class="n">question</span><span class="err">'</span><span class="o">:</span> <span class="s">"What's your name?"</span><span class="p">,</span> <span class="err">'</span><span class="n">pub_date</span><span class="err">'</span><span class="o">:</span> <span class="n">datetime</span><span class="p">.</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">20</span><span class="p">),</span> <span class="err">'</span><span class="n">expire_date</span><span class="err">'</span><span class="o">:</span> <span class="n">datetime</span><span class="p">.</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">20</span><span class="p">)}]</span>
<span class="o">>>></span> <span class="n">Poll</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">values</span><span class="p">(</span><span class="n">fields</span><span class="o">=</span><span class="p">[</span><span class="err">'</span><span class="n">id</span><span class="err">'</span><span class="p">,</span> <span class="err">'</span><span class="n">slug</span><span class="err">'</span><span class="p">])</span>
<span class="p">[{</span><span class="err">'</span><span class="n">id</span><span class="err">'</span><span class="o">:</span> <span class="mi">1</span><span class="p">,</span> <span class="err">'</span><span class="n">slug</span><span class="err">'</span><span class="o">:</span> <span class="err">'</span><span class="n">whatsup</span><span class="err">'</span><span class="p">},</span> <span class="p">{</span><span class="err">'</span><span class="n">id</span><span class="err">'</span><span class="o">:</span> <span class="mi">2</span><span class="p">,</span> <span class="err">'</span><span class="n">slug</span><span class="err">'</span><span class="o">:</span> <span class="err">'</span><span class="n">name</span><span class="err">'</span><span class="p">}]</span>
</pre></div>
<h4 id="model-values">当你知道你要取得哪些字段的值时并且你不需要那些 model实例对象的功能时,使用 values() 函数.<a class="headerlink" href="#model-values" title="Permanent link">¶</a></h4>
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<h4>Published</h4>
<time pubdate="pubdate" datetime="2014-09-07T00:00:00+08:00">Sep 7, 2014</time>
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