Python-SQLAlchemyPython

Python SQLAlchemy ORM教程(2)

2018-12-18  本文已影响0人  brakchen

由于SQLAlchemy 中文资料比较少，所以根据官网给的tutorial外加其他大佬写的中文资料整合以后准备写一个SQLAlchemy 系列的基础入门教程。本系列可能会夹杂一些个人对于python 、SQLAlchemy 以及ORM的理解，如有出错部分，请指正我。

版本信息:

前序文章Python SQLAlchemy ORM教程(1)讲了怎么建立你数据库，插入数据，也讲了engine和session的相关概念，接下来我们开始看看怎么花式取数据

Querying

SQLAlchemy的官方教程说，你的任何查询都是基于session的，是session提供的方法，你需要通过session的query()方法生成一个query 对象。query()是必须传入一个参数的，这是参数可以是一个combination of class 或者 calss-instrumented descriptors

instrumented

关于 combination of class 可以简单理解为类，可能class-instructmented descriptors 比较容易让人发懵，

instrumentations是将属性附加到类的一个过程，也可以叫Python Descriptors

我们来看一些例子:

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n20" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">​

get

myobject.someattribut

set

myobject.someattribut = "foo"

del

del myobject.someattribut
​</pre>

以上的行为都可以称为 instrumentation

简而言之，你可以往query()内传入类名或者类的属性

继续

我们来看下传入类是怎么样的：

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n25" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">>>> for instance in session.query(User).order_by(User.id):
... print(instance.name, instance.fullname)
ed Ed Jones
wendy Wendy Williams
mary Mary Contrary
fred Fred Flinstone</pre>

我们再来看下传入类属性

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n27" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">>>> for name, fullname in session.query(User.name, User.fullname):
... print(name, fullname)
ed Ed Jones
wendy Wendy Williams
mary Mary Contrary
fred Fred Flinstone</pre>

SQLAlchemy 会根据你传入的参数个数，然后返回数量上与之匹配的tuple，

当然了你可以使用all()这个函数来生成named tuple，这个named tuple 是通过SQLAlchemy提供的keyedTuple类来完成的:

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n30" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">>>> for row in session.query(User, User.name).all():
... print(row.User, row.name)
<User(name='ed', fullname='Ed Jones', password='f8s7ccs')> ed
<User(name='wendy', fullname='Wendy Williams', password='foobar')> wendy
<User(name='mary', fullname='Mary Contrary', password='xxg527')> mary
<User(name='fred', fullname='Fred Flinstone', password='blah')> fred</pre>

你可以手动控制named tuple的属性名字，通过使用label()函数，我们来看下演示代码

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n32" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">>>> for row in session.query(User.name.label('name_label')).all():
... print(row.name_label)
ed
wendy
mary
fred</pre>

你可以给你的映射类取一个别名以方便区分，或者方便操作。aliased()函数可以帮你方便的给映射类取一个名字

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n34" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">>>> from sqlalchemy.orm import aliased

user_alias = aliased(User, name='user_alias')
​
SQL>>> for row in session.query(user_alias, user_alias.name).all():
... print(row.user_alias)
<User(name='ed', fullname='Ed Jones', password='f8s7ccs')>
<User(name='wendy', fullname='Wendy Williams', password='foobar')>
<User(name='mary', fullname='Mary Contrary', password='xxg527')>
<User(name='fred', fullname='Fred Flinstone', password='blah')></pre>

关于数据库LIMIT 和 OFFSET 的操作，我可以使用SQLAlchemy提供的order_by() filter() filter_by()配合Python的 slices 来进行操作

我们看一些实例

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n38" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">>>> for u in session.query(User).order_by(User.id)[1:3]:
... print(u)
<User(name='wendy', fullname='Wendy Williams', password='foobar')>
<User(name='mary', fullname='Mary Contrary', password='xxg527')></pre>

filter_by()

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n40" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">​

for name, in session.query(User.name).
... filter_by(fullname='Ed Jones'):
... print(name)
ed</pre>

filter() 可以让你可以更灵活的使用 regular python operators 来表达你想的结果。

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n42" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">>>> for name, in session.query(User.name).
... filter(User.fullname=='Ed Jones'):
... print(name)
ed</pre>

Query 对象是完全generative（你可以理解为，永动机调用），意味着你可以在Query对象后面使用N个filter()配合regular python operators 来完成数据筛选

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="python" contenteditable="false" cid="n45" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 1em 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; caret-color: rgb(51, 51, 51); color: rgb(51, 51, 51); font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; background-position: inherit inherit; background-repeat: inherit inherit;">>>> for user in session.query(User).
... filter(User.name=='ed').
... filter(User.fullname=='Ed Jones'):
... print(user)
<User(name='ed', fullname='Ed Jones', password='f8s7ccs')></pre>

Common Filter Operators

下面总结了一些常用的 filter operators，作为菜鸡，或者轻度使用数据库的使用者来说，已经可以满足日常使用的大部分需求了

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