Tag Archives: database

Database Abstraction in Python

As I was recently working on trying out the Flask web framework for Python, I ended up wanting to access my MySQL database. Recently at work I have been using entity framework and I have gotten quite used to having a good database abstraction that allows programmatic creation of SQL. While such frameworks exist in Python, I thought it would interesting to try writing one. This is one great example of getting carried away with a seemingly simple task.

I aimed for these things:

  • Tables should be represented as objects which each instance of the object representing a row
  • These objects should be able to generate their own insert, select, and update queries
  • Querying the database should be accomplished by logical predicates, not by strings
  • Update queries should be optimized to only update those fields which have changed
  • The database objects should have support for “immutable” fields that are generated by the database

I also wanted to be able to do relations between tables with foreign keys, but I have decided to stop for now on that. I have a structure outlined, but it isn’t necessary enough at this point since all I wanted was a database abstraction for my simple Flask project. I will probably implement it later.

This can be found as a gist here: https://gist.github.com/kcuzner/5246020

Example

Before going into the code, here is an example of what this abstraction can do as it stands. It directly uses the DbObject and DbQuery-inheriting objects which are shown further down in this post.

This example first loads a user using a DbSelectQuery. The user is then modified and the DbObject-level function save() is used to save it. Next, a new user is created and saved using the same function. After saving, the primary key will have been populated and will be printed.

Change Tracking Columns

I started out with columns. I needed columns that track changes and have a mapping to an SQL column name. I came up with the following:

The Column class describes the column and is implemented as a descriptor. Each ColumnSet instance contains multiple columns and holds ColumnInstance objects which hold the individual column per-object properties, such as the value and whether it has been mutated or not. Each column type has a validation function to help screen invalid data from the columns. When a ColumnSet is initiated, it scans itself for columns and at that moment creates its ColumnInstances.

Generation of SQL using logical predicates

The next thing I had to create was the database querying structure. I decided that rather than actually using the ColumnInstance or Column objects, I would use a go-between object that can be assigned a “prefix”. A common thing to do in SQL queries is to rename the tables in the query so that you can reference the same table multiple times or use different tables with the same column names. So, for example if I had a table called posts and I also had a table called users and they both shared a column called ‘last_update’, I could assign a prefix ‘p’ to the post columns and a prefix ‘u’ to the user columns so that the final column name would be ‘p.last_update’ and ‘u.last_update’ for posts and users respectively.

Another thing I wanted to do was avoid the usage of SQL in constructing my queries. This is similar to the way that LINQ works for C#: A predicate is specified and later translated into an SQL query or a series of operations in memory depending on what is going on. So, in Python one of my queries looks like so:

This would print out a tuple (" WHERE x.column_1 = %s AND x.column_2 > %s", ["4", 5]). So, how does this work? I used operator overloading to create DbQueryExpression objects. The code is like so:

The __str__ function and arguments property return recursively generated expressions using the column prefixes (in the case of __str__) and the arguments (in the case of arguments). As can be seen, this supports parameterization of queries. To be honest, this part was the most fun since I was surprised it was so easy to make predicate expressions using a minimum of classes. One thing that I didn’t like, however, was the fact that the boolean and/or operators cannot be overloaded. For that reason I had to use the bitwise operators, so the expressions aren’t entirely correct when being read.

This DbQueryExpression is fed into my DbQuery object which actually does the translation to SQL. In the example above, we saw that I just passed a logical argument into my where function. This actually was a DbQueryExpression since my overloaded operators create DbQueryExpression objects when they are compared. The DbColumnSet object is an dynamically generated object containing the go-between column objects which is created from a DbObject. We will discuss the DbObject a little further down

The DbQuery objects are implemented as follows:

Each of the SELECT, INSERT, UPDATE, and DELETE query types inherits from a base DbQuery which does execution and such. I decided to make the DbQuery object take a PEP 249-style cursor object and execute the query itself. My hope is that this will make this a little more portable since, to my knowledge, I didn’t make the queries have any MySQL-specific constructions.

The different query types each implement a variety of statements corresponding to different parts of an SQL query: where(), limit(), orderby(), select(), from_table(), etc. These each take in either a DbQueryColumn (such as is the case with where(), orderby(), select(), etc) or a string to be appended to the query, such as is the case with limit(). I could easily have made limit take in two integers as well, but I was kind of rushing through because I wanted to see if this would even work. The query is built by creating the query object for the basic query type that is desired and then calling its member functions to add things on to the query.

Executing the queries can cause a callback “filter” function to be called which takes in the query and the cursor as arguments. I use this function to create new objects from the data or to update an object. It could probably be used for more clever things as well, but those two cases were my original intent in creating it. If no filter is specified, then the cursor is returned.

Table and row objects

At the highest level of this hierarchy is the DbObject. The DbObject definition actually represents a table in the database with a name and a single primary key column. Each instance represents a row. DbObjects also implement the methods for selecting records of their type and also updating themselves when they are changed. They inherit change tracking from the ColumnSet and use DbQueries to accomplish their querying goals. The code is as follows:

DbObjects require that the inheriting classes define two properties: dbo_tablename and primary_key. dbo_tablename is just a string giving the name of the table in the database and primary_key is a Column that will be used as the primary key.

To select records from the database, the select() function can be called from the class. This sets up a DbSelectQuery which will return an array of the DbObject that it is called for when the query is executed.

One fallacy of this structure is that at the moment it assumes that the primary key won’t be None if it has been set. In other words, the way I did it right now does not allow for null primary keys. The reason it does this is because it says that if the primary key hasn’t been set, it needs to generate a DbInsertQuery for the object when save() is called instead of a DbUpdateQuery. Both insert and update queries do not include every field. Immutable fields are always excluded and then later selected or inferred from the cursor object.