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working on explaining the downsides of ORMs
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‎all.html‎

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@@ -3281,10 +3281,23 @@ <h1>Object-relational mappers (ORMs)</h1>
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<h2>Why are ORMs useful?</h2>
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<p>ORMs provide a high-level abstraction upon a
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<a href="/databases.html">relational database</a> that allows a developer to write
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Python code instead of SQL to create, read, update and delete data in
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their database. Developers can use the programming language they are
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comfortable with to work with a database instead of writing SQL statements or
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stored procedures.</p>
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Python code instead of SQL to create, read, update and delete data and
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schemas in their database. Developers can use the programming language they
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are comfortable with to work with a database instead of writing SQL
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statements or stored procedures. </p>
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<p>For example, in a developer would write the following SQL statement to
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retrieve every row in the USERS table where the <code>zip_code</code> column is 94107:</p>
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<div class="highlight"><pre>SELECT * FROM USERS WHERE zip_code=94107;
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</pre></div>
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<p>The equivalent Django ORM query would instead look like the following Python
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code:</p>
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<div class="highlight"><pre># obtain everyone in the 94107 zip code and assign to users variable
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users = Users.objects.filter(zip_code=94107)
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</pre></div>
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<p>The ability to write Python code instead of SQL can speed up web application
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development, especially at the beginning of a project. The potential
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development speed boost comes from not having to switch from Python code
@@ -3313,21 +3326,27 @@ <h2>Do I have to use an ORM for my web application?</h2>
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library called a <em>database connector</em>, such as
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<a href="http://initd.org/psycopg/">psycopg</a> (for PostgreSQL)
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or <a href="https://pypi.python.org/pypi/MySQL-python/1.2.5">MySQL-python</a> (for
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MySQL). Take a look at the table below which shows that ORMs can work with
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different web frameworks and database connectors.</p>
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MySQL). Take a look at the table below which shows how ORMs can work with
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different web frameworks and connectors and relational databases.</p>
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<p><img src="theme/img/orm-examples.png" width="100%" alt="Examples of how varying Python ORMs can work with different connectors and backends." class="technical-diagram" /></p>
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<p>For example, in the above table SQLAlchemy can work with or without an
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accompanying web framework as well as different database connectors.
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<p>The above table shows for example that SQLAlchemy can work with or without
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an accompanying web framework as well as varying database connectors.
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Developers can also use ORMs without a web framework, such as when creating
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a data analysis tool or a batch script without a user interface. </p>
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<h2>What are the downsides of using an ORM?</h2>
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<p>There are numerous downsides of ORMs, including</p>
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<ol>
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<li>impedance mismatch</li>
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<li>difficulty writing complex queries</li>
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<li>difficulty expressing complex queries</li>
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<li>potential for reduced performance</li>
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<li>shifting complexity into the application from the database layer</li>
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</ol>
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<h3>Impedance mismatch</h3>
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<p>The phrase "impedance mismatch" is commonly used in conjunction with ORMs.
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Impedance mismatch is a catch-all term for the difficulties that occur when
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moving data between relational tables and application objects. The gist
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is that the way a developer uses objects is different from how data is
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stored and joined in relational tables.</p>
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<h2>Django's ORM</h2>
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<p>The <a href="/django.html">Django</a> web framework comes with
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its own built-in object-relational mapping module, generally referred to

‎feeds/all.atom.xml‎

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<?xml version="1.0" encoding="utf-8"?>
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<feed xmlns="http://www.w3.org/2005/Atom"><title>Matt Makai</title><link href="http://www.fullstackpython.com/" rel="alternate"></link><link href="http://www.fullstackpython.com/feeds/all.atom.xml" rel="self"></link><id>http://www.fullstackpython.com/</id><updated>2015-06-14T17:20:54Z</updated></feed>
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<feed xmlns="http://www.w3.org/2005/Atom"><title>Matt Makai</title><link href="http://www.fullstackpython.com/" rel="alternate"></link><link href="http://www.fullstackpython.com/feeds/all.atom.xml" rel="self"></link><id>http://www.fullstackpython.com/</id><updated>2015-06-14T18:40:47Z</updated></feed>

‎object-relational-mappers-orms.html‎

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@@ -41,10 +41,23 @@ <h1>Object-relational mappers (ORMs)</h1>
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<h2>Why are ORMs useful?</h2>
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<p>ORMs provide a high-level abstraction upon a
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<a href="/databases.html">relational database</a> that allows a developer to write
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Python code instead of SQL to create, read, update and delete data in
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their database. Developers can use the programming language they are
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comfortable with to work with a database instead of writing SQL statements or
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stored procedures.</p>
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Python code instead of SQL to create, read, update and delete data and
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schemas in their database. Developers can use the programming language they
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are comfortable with to work with a database instead of writing SQL
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statements or stored procedures. </p>
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<p>For example, in a developer would write the following SQL statement to
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retrieve every row in the USERS table where the <code>zip_code</code> column is 94107:</p>
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<div class="highlight"><pre>SELECT * FROM USERS WHERE zip_code=94107;
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</pre></div>
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<p>The equivalent Django ORM query would instead look like the following Python
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code:</p>
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<div class="highlight"><pre># obtain everyone in the 94107 zip code and assign to users variable
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users = Users.objects.filter(zip_code=94107)
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</pre></div>
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<p>The ability to write Python code instead of SQL can speed up web application
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development, especially at the beginning of a project. The potential
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development speed boost comes from not having to switch from Python code
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library called a <em>database connector</em>, such as
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<a href="http://initd.org/psycopg/">psycopg</a> (for PostgreSQL)
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or <a href="https://pypi.python.org/pypi/MySQL-python/1.2.5">MySQL-python</a> (for
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MySQL). Take a look at the table below which shows that ORMs can work with
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different web frameworks and database connectors.</p>
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MySQL). Take a look at the table below which shows how ORMs can work with
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different web frameworks and connectors and relational databases.</p>
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<p><img src="theme/img/orm-examples.png" width="100%" alt="Examples of how varying Python ORMs can work with different connectors and backends." class="technical-diagram" /></p>
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<p>For example, in the above table SQLAlchemy can work with or without an
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accompanying web framework as well as different database connectors.
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<p>The above table shows for example that SQLAlchemy can work with or without
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an accompanying web framework as well as varying database connectors.
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Developers can also use ORMs without a web framework, such as when creating
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a data analysis tool or a batch script without a user interface. </p>
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<h2>What are the downsides of using an ORM?</h2>
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<p>There are numerous downsides of ORMs, including</p>
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<ol>
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<li>impedance mismatch</li>
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<li>difficulty writing complex queries</li>
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<li>difficulty expressing complex queries</li>
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<li>potential for reduced performance</li>
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<li>shifting complexity into the application from the database layer</li>
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</ol>
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<h3>Impedance mismatch</h3>
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<p>The phrase "impedance mismatch" is commonly used in conjunction with ORMs.
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Impedance mismatch is a catch-all term for the difficulties that occur when
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moving data between relational tables and application objects. The gist
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is that the way a developer uses objects is different from how data is
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stored and joined in relational tables.</p>
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<h2>Django's ORM</h2>
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<p>The <a href="/django.html">Django</a> web framework comes with
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its own built-in object-relational mapping module, generally referred to

‎source/content/pages/05-data/04-object-relational-mappers.markdown‎

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@@ -14,10 +14,23 @@ are more commonly used in application code.
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## Why are ORMs useful?
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ORMs provide a high-level abstraction upon a
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[relational database](/databases.html) that allows a developer to write
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Python code instead of SQL to create, read, update and delete data in
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their database. Developers can use the programming language they are
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comfortable with to work with a database instead of writing SQL statements or
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stored procedures.
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Python code instead of SQL to create, read, update and delete data and
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schemas in their database. Developers can use the programming language they
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are comfortable with to work with a database instead of writing SQL
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statements or stored procedures.
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For example, in a developer would write the following SQL statement to
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retrieve every row in the USERS table where the ``zip_code`` column is 94107:
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SELECT * FROM USERS WHERE zip_code=94107;
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27+
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The equivalent Django ORM query would instead look like the following Python
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code:
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# obtain everyone in the 94107 zip code and assign to users variable
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users = Users.objects.filter(zip_code=94107)
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2134

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The ability to write Python code instead of SQL can speed up web application
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development, especially at the beginning of a project. The potential
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library called a *database connector*, such as
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[psycopg](http://initd.org/psycopg/) (for PostgreSQL)
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or [MySQL-python](https://pypi.python.org/pypi/MySQL-python/1.2.5) (for
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MySQL). Take a look at the table below which shows that ORMs can work with
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different web frameworks and database connectors.
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MySQL). Take a look at the table below which shows how ORMs can work with
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different web frameworks and connectors and relational databases.
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<img src="theme/img/orm-examples.png" width="100%" alt="Examples of how varying Python ORMs can work with different connectors and backends." class="technical-diagram" />
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For example, in the above table SQLAlchemy can work with or without an
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accompanying web framework as well as different database connectors.
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The above table shows for example that SQLAlchemy can work with or without
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an accompanying web framework as well as varying database connectors.
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Developers can also use ORMs without a web framework, such as when creating
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a data analysis tool or a batch script without a user interface.
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There are numerous downsides of ORMs, including
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1. impedance mismatch
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1. difficulty writing complex queries
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1. difficulty expressing complex queries
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1. potential for reduced performance
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1. shifting complexity into the application from the database layer
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### Impedance mismatch
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The phrase "impedance mismatch" is commonly used in conjunction with ORMs.
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Impedance mismatch is a catch-all term for the difficulties that occur when
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moving data between relational tables and application objects. The gist
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is that the way a developer uses objects is different from how data is
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stored and joined in relational tables.
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## Django's ORM
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The [Django](/django.html) web framework comes with
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its own built-in object-relational mapping module, generally referred to
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and does not necessitate a recurring fee.
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## Schema migrations
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Schema migrations, for example when you need to add a new column to an
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existing table in your database, are not technically part of ORMs. However,

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