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在Rails中使用 active_model_serializers 构建json response
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<p>Rails 本身提供很不错的json response功能，可以很方便的将model对象自动转换为json数据。如果我们期望返回的结果很简单也可以构建一个简单的hash,自行编辑你希望的key和value,由Rails 来帮你生成json。那如果是介于两者之间的情况呢？</p>
<ul>
<li>我希望将我的model以json的形式返回，但是我并不希望将所有的字段返回。</li>
<li>我希望将我的model中部分属性进行稍微的处理和计算后返回，这样客户端可以直接使用某些字段，而不需要将应用自身的逻辑暴露给客户端，如我有个model记录微信支付属性，有个字段是关于微信支付（”trade_status”），它的值有trade_success, “wait_for_payment”，显然我们可以返回给客户端 “支付成功”，”等待支付”更好。传统的做法，我们为了达到这个目的，我们不仅需要将trade_stauts进行构建，还需要将其他参数也重复写出来。</li>
</ul>
<p>ActiveModel::Serializers这个gem 包就可以更好的帮助我们解决上面的痛点，我们只需要对需要返回的属性列举出来，只对需要更改的属性进行计算，其他都不需要了。下面就介绍下它的一些常见用法。</p>
<p>首先安装Gem</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">gem <span class="string">&#x27;active_model_serializers&#x27;</span></span><br></pre></td></tr></table></figure>
<p>为了可以在我们的rails 项目中使用它，我们还需要在application controller中添加：</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">include</span> <span class="title class_">ActionController::Serialization</span></span><br></pre></td></tr></table></figure>
<p>active_model_serializers的使用就是在你是需要返回json的地方，为你的数据创建serializer,你的对象就能通过serializer变成希望的结果。这里假设我有一个User用户对象，User的的大致结构：</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">User</span></span><br><span class="line"> field <span class="symbol">:nick_name</span>, <span class="symbol">type:</span> <span class="title class_">String</span></span><br><span class="line"> field <span class="symbol">:email</span>, <span class="symbol">type:</span> <span class="title class_">String</span></span><br><span class="line"> field <span class="symbol">:age</span>, <span class="symbol">type:</span> <span class="title class_">Integer</span></span><br><span class="line"> field <span class="symbol">:password</span>, <span class="symbol">type:</span><span class="title class_">String</span></span><br><span class="line"> <span class="comment"># 等等更多属性。。。</span></span><br><span class="line"><span class="keyword">end</span></span><br></pre></td></tr></table></figure>
<p>我现在有个UserController,需要以JSON的形式返回用户的基本信息： 昵称，邮箱，是否成年（大于等于18岁算成年），接下来在app 目录下创建serializers 目录，并创建class UserSerializer对象，继承自 ActiveModel::Serializer, 然后我们的serializer就应该是这样：</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">UserSerializer</span> &lt; <span class="title class_ inherited__">ActiveModel::Serializer</span></span><br><span class="line"> attributes <span class="symbol">:nick_name</span>, <span class="symbol">:email</span>, <span class="symbol">:if_adult</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">def</span> <span class="title function_">if_adult</span></span><br><span class="line"> object.age &gt; <span class="number">18</span> <span class="comment">#object 就是需要被序列化的对象</span></span><br><span class="line"> <span class="keyword">end</span></span><br><span class="line"><span class="keyword">end</span></span><br></pre></td></tr></table></figure>
<p>在serializer中，我们只需要将我们需要的属性通过attributes 的形式列出来，如果model没有的属性，我们需要自行实现该方法。<br>我们也可以覆盖属性，如上的例子，我们也可以将if_adult 改成age,同样我们需要有：</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">def</span> <span class="title function_">age</span></span><br><span class="line"> object.age &gt; <span class="number">18</span></span><br><span class="line"><span class="keyword">end</span></span><br></pre></td></tr></table></figure>
<p>接下来完成我们的最后一步，在controller中使用serializer</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">UserController</span> &lt; <span class="title class_ inherited__">ApplicationController</span></span><br><span class="line"> <span class="keyword">def</span> <span class="title function_">show</span></span><br><span class="line"> <span class="comment">#假设 <span class="doctag">@user</span> 是我们经过查询后得到的对象</span></span><br><span class="line"> render <span class="symbol">json:</span> <span class="variable">@user</span>, <span class="symbol">serializer:</span> <span class="title class_">UserSerializer</span></span><br><span class="line"> <span class="keyword">end</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">def</span> <span class="title function_">index</span></span><br><span class="line"> <span class="comment">#假设 <span class="doctag">@users</span> 是我们经过查询后得到的对象</span></span><br><span class="line"> render <span class="symbol">json:</span> <span class="variable">@user</span>, <span class="symbol">each_serializer:</span> <span class="title class_">UserSerializer</span></span><br><span class="line"> <span class="keyword">end</span></span><br><span class="line"><span class="keyword">end</span></span><br></pre></td></tr></table></figure>
<p>我们我们希望对一个数组对象进行序列化，我们则需要使用each_serializer，使用该属性后，你的返回结果将变成对象，数组则包裹下对象之下。本例则类似于</p>
<figure class="highlight json"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">users<span class="punctuation">:</span> <span class="punctuation">[</span></span><br><span class="line"> <span class="punctuation">&#123;</span><span class="punctuation">&#125;</span><span class="punctuation">,</span></span><br><span class="line"> <span class="punctuation">&#123;</span><span class="punctuation">&#125;</span></span><br><span class="line"><span class="punctuation">]</span></span><br></pre></td></tr></table></figure>
<p>这里的”users”这是json的根属性， serializer支持用户自定义根属性，</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">render <span class="symbol">json:</span> <span class="variable">@user</span>, <span class="symbol">serializer:</span> <span class="title class_">UserSerializer</span>, <span class="symbol">root:</span> <span class="string">&#x27;customer&#x27;</span></span><br></pre></td></tr></table></figure>
<p>我非常喜欢active model serializers 的一个属性是，支持在serializer中访问你希望的属性，在rails 中，很多时候我们希望访问current_user对象，获取当前登录用户的相关信息。那serializer 则可以很好的做的这一点，在active model serializers 中有 serialization_scope 这样一个方法，在application controller 中声明此方法，传入你希望的参数，便可以在serializer class 中自由访问该对象。解决了serializer 与其他对象交互的问题。</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">serialization_scope <span class="symbol">:current_user</span></span><br></pre></td></tr></table></figure>
<p>在serializer中, scope 对象便是注入的 current_user对象</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">scope.email</span><br></pre></td></tr></table></figure>
<p>此外，active model serializers 支持在返回的json中额外注入meta属性，这样既保留了serializer的简单，模板化，也增添了自由度，使得用户可以自行添加一些额外的属性。</p>
<p>更详细的使用还请参考：<a target="_blank" rel="noopener" href="https://github.com/rails-api/active_model_serializers">Github</a></p>
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