When you code in a Django web framework, we all know that you won’t directly work with databases. There is an ORM (Object-relational mapping) that will interact with the database using migration and SQL operation. So, I want this tutorial to show you how to implement an ORM manager from scratch.
Object–relational mapping (ORM) in computer science is a technique for converting data between incompatible type systems using object-oriented programming languages in order to create virtual database objects.
Python is a programming language that enables you to design freely anything that you want.
And honestly, I was able to implement a Mini ORM manager merely within 3 hours.
This project doesn’t contain a lot of features that a complete ORM contains (features like RUD). The purpose of this code is mainly to know how an ORM works, there is no intention to use it for further development.
Our Project is divided into 3 main components:
The Project files are also 3 python files (check this Github repository ):
We mentioned before that we will use SQLite database. Thanks to the python sqlite3 default library, we can connect directly with a database using a python script. If you check the documentation of sqlite3 library, you will notice that it is very easy to connect SQLite using python.
Just a few lines of python code will do the job. So, you need to instantiate an sqlite3 connection object with specifying the database file “
example.db”, then make a cursor to execute SQL commands without forgetting to commit the changes and to close the connection.
First, import the sqlite3 library and instantiate the connection:
import sqlite3 con = sqlite3.connect('example.db')
Then open a cursor, execute the commands you want and finally close the connection:
cursor= con.cursor() cursor.execute('''CREATE TABLE stocks (date text, trans text, symbol text, qty real, price real)''') cursor.execute("INSERT INTO stocks VALUES ('2006-01-05','BUY','RHAT',100,35.14)") connection.commit() connection.close()
However, this kind of implementation won’t be reliable in our project, because there is too much redundancy of this block of code. Imagine I will repeat this code with every SQL operation I use including CREATE, DELETE, UPDATE and RETRIEVE.
In fact, this problem has a solution, there is a thing in python called context manager. A context manager is a way to manage resources precisely without any cost in complexity and memory.
In order to make a context manager that can connect with a database, we make a class called
ConnectionSqliteManager that opens an SQL connection and instantiates the cursor object.
In this class, we need to write an
__exit__ magic class methods as a necessary ingredient for database resource management.
So the structure of the context manager class is as follows :
__enter__method connects the
example.dbfile (setup operation) and returns the
Connectionobject to variable
__exit__method takes care of closing the connection on exiting the
class ConnectionSqliteManager: def __init__(self,filename): self.filename = filename def __enter__(self): print("Connection started ...") self.connection = sql3.connect(self.filename) self.cursor = self.connection.cursor() return self def __exit__(self, type, value, traceback): print("Connection ended ...") self.connection.close() with ConnectionSqliteManager("example.db") as Connection: # Do what you want with Connection instance
In the scope
with , you manage the database within the scope, without taking the bother of opening and closing the connection manager.
You can make a decorator (Inside “ConnectionSqliteManager” class) on top of each SQL operation method, to commit the changes.
Never forget to add some sqlite3 error exception in each command execution.
#inside ConnectionSqliteManager class def commit(operation): def wrapper(self, *args,**kwargs): operation(self, *args, **kwargs) self.connection.commit() return wrapper @commit def Sql_operation(self): # execute an sql command here pass
A model is a datastore entity that has a key and a set of properties. . A model is a Python class that inherits from the Model class. The model class defines a new kind of datastore entity and the properties the kind is expected to take.
I won’t dive into too much implementation here. all you have to do is to define a model class like the following:
class Model(baseModel): base_model = base tablename = "Model" fields = ("field_1", "field_2")
In this code, you need to specify the name of the table of the database and its fields.
In order to prepare your model for migration, you have to add it in the
model_list = [Model, ]
Now let’s prepare the user for a good interface in our command prompt. To implement it, we used the
argparse default library to let the user input the arguments in the command prompt and execute the migration.
All details are in the
migrate_manager.py file, there is no need to explain each line of code. I made it as simple as possible.
So all you have to do is to execute this command.
python migrate_manager.py migrate
The output is as following:
Begin database Migration ... Model Migration Connection started ... Model: created successfully! 2022-01-04 02:29:53.402991: Commit is successful!! Connection ended ...
It’s good to implement a technique like ORM from scratch, it will help you understand and learn technologies quickly without any difficulties in grasping the concept behind it.