In this post I'll describe how to implement tagging with a relational database. What I mean by tagging are those little labels you see at the top of this blog post, which indicate how I've chosen to categorize the content. There are many ways to solve this problem, and I'll try to describe some of the more popular methods, as well as one unconventional approach using bitmaps. In each section I'll describe the database schema, try to list the benefits and drawbacks, and present example queries. I will use Peewee ORM for the example code, but hopefully these examples will easily translate to your tool-of-choice.
In my continuing adventures with SQLite, I had the idea of writing a RESTful search server utilizing SQLite's full-text search extension. You might think of it as a poor man's ElasticSearch – a very, very poor man.
So what is this project? Well, the idea I had was that instead of building out separate search implementations for my various projects, I would build a single lightweight search service I could use everywhere. I really like SQLite (and have previously blogged about using SQLite's full-text search with Python), and the full-text search extension is quite good, so it didn't require much imagination to take the next leap and expose it as a web-service.
Read on for more details.
For fun, I thought I'd write a post describing how to build a blog using Flask, a Python web-framework. Building a blog seems like, along with writing a Twitter-clone, a quintessential experience when learning a new web framework. I remember when I was attending a five-day Django tutorial presented by Jacob Kaplan-Moss, one of my favorite projects we did was creating a blog. After setting up the core of the site, I spent a ton of time adding features and little tweaks here-and-there. My hope is that this post will give you the tools to build a blog, and that you have fun customizing the site and adding cool new features.
In this post we'll cover the basics to get a functional site, but leave lots of room for personalization and improvements so you can make it your own. The actual Python source code for the blog will be a very manageable 200 lines.
This post is intended for beginner to intermediate-level Python developers, or experienced developers looking to learn a bit more about Python and Flask. For the mother of all Flask tutorials, check out Miguel Grinberg's 18 part Flask mega-tutorial.
Here are the features:
This post is a follow-up to my post about querying the top related item by group. In this post we'll go over ways to retrieve the top N related objects by group using the Peewee ORM. I've also presented the SQL and the underlying ideas behind the queries, so you can translate them to whatever ORM / query layer you are using.
Retrieving the top N per group is a pretty common task, for example:
In this post we'll discuss the following types of solutions:
In this post I'd like to share some techniques for querying the top item by group using the Peewee ORM. For example,
This is a common task, but one that can be a little tricky to implement in a single SQL query. To add a twist, we won't use window functions or other special SQL constructs, since they aren't supported by SQLite. If you're interested in finding the top N items per group, check out this follow-up post.
Recently I've been receiving a lot of questions about how to manage database connections with the peewee ORM. I thought I'd write a short post explaining the various ways peewee can help you manage your database connections, and outline some general best practices.
SQLite is an embedded database, which means that instead of running as a separate server process, the actual database engine resides within the application. This makes it possible for the database to call directly into the application when it would be beneficial to add some low-level, application-specific functionality. SQLite provides numerous hooks for inserting user code and callbacks, and, through virtual tables, it is even possible to construct a completely user-defined table. By extending the SQL language with Python, it is often possible to express things more elegantly than if we were to perform calculations after the fact.
In this post I'll describe how to extend SQLite with Python, adding functions and aggregates that will be callable directly from any SQL queries you execute. We'll wrap up by looking at SQLite's virtual table mechanism and seeing how to expose a SQL interface over external data sources.
I recently read a good write-up on tree structures in PostgreSQL. Hierarchical data is notoriously tricky to model in a relational database, and a variety of techniques have grown out of developers' attempts to optimize for certain types of queries.
In his post, Graeme describes several approaches to modeling trees, including:
In the comments, some users pointed out that the ltree extension could also be used to efficiently store and query materialized paths. LTrees support two powerful query languages (lquery and ltxtquery) for pattern-matching LTree labels and performing full-text searches on labels.
One technique that was not discussed in Graeme's post was the use of closure tables. A closure table is a many-to-many junction table storing all relationships between nodes in a tree. It is related to the adjacency model, in that each database row still stores a reference to its parent row. The closure table gets its name from the additional table, which stores each combination of ancestor/child nodes.
In my last post, I wrote about how to work with encrypted SQLite databases with Python. As an example application of these libraries, I showed some code fragments for a fictional diary program. Because I was thinking the examples directory of the peewee repo was looking a little thin, I decided to flesh out the diary program and include it as an example.
In this post, I'll go over the diary code in the hopes that you may find it interesting or useful. The code shows how to use the peewee SQLCipher extension. I've also implemented a simple command-line menu loop. All told, the code is less than 100 lines!
SQLCipher, created by Zetetic, is an open-source library that provides transparent 256-bit AES encryption for your SQLite databases. SQLCipher is used by a large number of organizations, including Nasa, SalesForce, Xerox and more. The project is open-source and BSD licensed. Best of all, there are open-source python bindings.
In this post, I'll show how to compile SQLCipher and the pysqlcipher bindings, then use peewee ORM to work with an encrypted SQLite database.