Django Internal: create()

A model instance is corresponds to a record in the database once saved. In this post i am going to
explore what happens behind the scene when you call  create(). It create a instance of model and save that instance which creates a record in the database.

let say we have a model defined as:

class Author(models.Model):
   name = models.CharField(max_length=100)
   age = models.IntegerField()


Lets go step by step to deconstruct Author.objects.create()

Creating instance of model

 instance = Author(name="roy", age=47)

1. signals invoked
Once starts it send signal pre_init and on successful completion of this method post_init is called.

2. check for given model fields not values.
check can fail if you more arguments than number of fields or attribute which does not present on model.
for example:
instance = Author(name="roy", age=47, code='python')
TypeError: 'code' is an invalid keyword argument for this function.

Each model has attribute called ._meta through which gives related details for models

As for information the models.Model  has an abstract base class called ModelBase which add attributes on a model itself like ._meta, errors (DoesNotExist, MultipleObjectsReturned) and validating model itself for its concrete definition.

Saving model instance
model method ._save_base() is called by save() to save model instance.  this where pre_save() and post_save() signals are invoked.

There are couple of important actors involved in here, I would like brief them.
 1. InsertQuery: instance of class is initiated for actual SQL' s raw query by passing model fields objects and values.
2. SQLInsertCompiler: This is SQL compiler for InsertQuery. It is a place where django models fields and values are parsed into SQL raw statement.
3. DatabaseWrapper: It is an wrapper for mysql-python client. It creates connection to SQL server and uses cursor to execute the raw SQL statement.

Queryset private method _insert() is place where things kick off at low level.  creating a record means INSERT SQL query. This happens inside a atomic transaction.
Basically django creates insert query gives that to compiler and compiler invoke database's wrapper method execute() by passing raw SQL statement as

     cursor.execute('INSERT INTO `author` (`name`, `age`) VALUES (%s, %s)', ['roy', 46])

Once above code is executed successfully record is being is created in database. And thereafter post_save signal is invoked. And that s how an model instance creates an record in database. 

Hope you found it interesting, keep reading.