电脑/python/数据库/sqlalchemy补充.html

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<header>
<h1 class="title">sqlalchemy补充</h1>
</header><nav id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#orgheadline1">1. 额外的属性支持</a></li>
<li><a href="#orgheadline2">2. 某一列的额外的别名</a></li>
<li><a href="#orgheadline3">3. 多列组合唯一性约束</a></li>
<li><a href="#orgheadline4">4. ORM层的内省</a></li>
</ul>
</div>
</nav>

<div class="outline-2">
<h2 id="orgheadline1">额外的属性支持</h2>
<div class="outline-text-2" id="text-1">
<p>
所谓的额外的属性并不是基于SQL表格的某一列的属性，而是在ORM之上建立起来的额外的属性，其一般是基于SQL表格的某一列或某些列的，是ORM封装之上提供的更加便利的属性接口。
</p>

<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">EmailAddress</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
    <span class="n">__tablename__</span> <span class="o">=</span> <span class="s1">&#39;email_address&#39;</span>

    <span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>

    <span class="n">_email</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s2">&quot;email&quot;</span><span class="p">,</span> <span class="n">String</span><span class="p">)</span>

    <span class="nd">@hybrid_property</span>
    <span class="k">def</span> <span class="nf">email</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="sd">&quot;&quot;&quot;Return the value of _email up until the last twelve</span>
<span class="sd">        characters.&quot;&quot;&quot;</span>

        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_email</span><span class="p">[:</span><span class="o">-</span><span class="mi">12</span><span class="p">]</span>

    <span class="nd">@email.setter</span>
    <span class="k">def</span> <span class="nf">email</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">email</span><span class="p">):</span>
        <span class="sd">&quot;&quot;&quot;Set the value of _email, tacking on the twelve character</span>
<span class="sd">        value @example.com.&quot;&quot;&quot;</span>

        <span class="bp">self</span><span class="o">.</span><span class="n">_email</span> <span class="o">=</span> <span class="n">email</span> <span class="o">+</span> <span class="s2">&quot;@example.com&quot;</span>
</pre></div>
</div>
</div>


<div class="outline-2">
<h2 id="orgheadline2">某一列的额外的别名</h2>
<div class="outline-text-2" id="text-2">
<p>
这里所谓的某一列额外的别名指并没有创建额外的列，而是在ORM层针对某一列可以用额外的别名来做类似的操作。
</p>
<div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.ext.declarative</span> <span class="kn">import</span> <span class="n">synonym_for</span>

<span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
    <span class="n">__tablename__</span> <span class="o">=</span> <span class="s1">&#39;my_table&#39;</span>

    <span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
    <span class="n">status</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>

    <span class="nd">@synonym_for</span><span class="p">(</span><span class="s2">&quot;status&quot;</span><span class="p">)</span>
    <span class="nd">@property</span>
    <span class="k">def</span> <span class="nf">job_status</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="s2">&quot;Status: &quot;</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">status</span>
</pre></div>

<p>
其等于:
</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
    <span class="n">__tablename__</span> <span class="o">=</span> <span class="s1">&#39;my_table&#39;</span>

    <span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
    <span class="n">status</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>

    <span class="nd">@property</span>
    <span class="k">def</span> <span class="nf">job_status</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="s2">&quot;Status: &quot;</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">status</span>

    <span class="n">job_status</span> <span class="o">=</span> <span class="n">synonym</span><span class="p">(</span><span class="s2">&quot;status&quot;</span><span class="p">,</span> <span class="n">descriptor</span><span class="o">=</span><span class="n">job_status</span><span class="p">)</span>
</pre></div>

<p>
也就是具体该列在ORM层可以通过 <code>status</code> 或者 <code>job_status</code> 来操作。具体参考 <a href="http://docs.sqlalchemy.org/en/latest/orm/mapped_attributes.html">这里</a> 。
</p>
</div>
</div>

<div class="outline-2">
<h2 id="orgheadline3">多列组合唯一性约束</h2>
<div class="outline-text-2" id="text-3">
<p>
请参看 <a href="http://stackoverflow.com/questions/10059345/sqlalchemy-unique-across-multiple-columns">这个网页</a> 。
</p>

<p>
如果是ORM层，则是:
</p>
<pre>
class Location(Base):
    __tablename__ = 'locations'
    id = Column(Integer, primary_key = True)
    customer_id = Column(Integer, ForeignKey('customers.customer_id'), nullable=False)
    location_code = Column(Unicode(10), nullable=False)
    __table_args__ = (UniqueConstraint('customer_id', 'location_code', name='_customer_location_uc'),
                     )
</pre>

<p>
上面flask_sqlalchemy的话可以使用 <code>db.UniqueConstraint</code> 。 比如:
</p>
<pre>
__table_args__ = (
    db.UniqueConstraint("main_directory", "sub_directory","filename",
        "filext"),
    )
</pre>

<p>
如果是Core层，则是:
</p>
<pre>
mytable = Table('mytable', meta,
    # ...
    Column('customer_id', Integer, ForeignKey('customers.customer_id')),
    Column('location_code', Unicode(10)),

    UniqueConstraint('customer_id', 'location_code', name='uix_1')
    )
</pre>
</div>
</div>


<div class="outline-2">
<h2 id="orgheadline4">ORM层的内省</h2>
<div class="outline-text-2" id="text-4">
<p>
Table层直接利用已经存在的数据库表格就是 <code>reflect</code> 的概念，因为ORM层多了很多额外的东西，其中最关键的是 relationship 的概念，而sqlalchemy的Automap这一章主要就是解决ORM层内省这个问题的。更详细的讨论请参看 <a href="http://docs.sqlalchemy.org/en/rel_1_0/orm/extensions/automap.html">官方文档</a> 。
</p>

<p>
最基本的应用就是 建立一个 <code>automap_base</code> 对象: 
</p>
<pre>
from sqlalchemy.ext.automap import automap_base

Base = automap_base()
</pre>

<p>
然后运行其 <code>prepare</code> 方法进行内省:
</p>
<pre>
Base.prepare(engine, reflect=True)
</pre>

<p>
然后对应的sqlalchemy ORM层的那些类就可以如下获得了:
</p>
<pre>
User = Base.classes.user
#或 Base.classes.get('user')
</pre>

<p>
对于一般的属性引用，这是没有问题的。就是relationship可能还是有问题，那么我们可以预先定义一些属性，在 <code>prepare</code> 之前，那么预先定义的那些东西也将覆盖后面的自动reflect那部分定义，这可以起到矫正的作用。然后预先定义的那个类就是后面要使用的类了，就不要用 <code>Base.classes.what</code> 这种风格再获取了。
</p>
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