scrapy-djangoitem 0.1.0

DjangoItem

DjangoItem is a class of item that gets its fields definition from a Django model, you simply create a DjangoItem and specify what Django model it relates to.

Besides of getting the model fields defined on your item, DjangoItem provides a method to create and populate a Django model instance with the item data.

Using DjangoItem

DjangoItem works much like ModelForms in Django, you create a subclass and define its django_model attribute to be a valid Django model. With this you will get an item with a field for each Django model field.

In addition, you can define fields that aren’t present in the model and even override fields that are present in the model defining them in the item.

Let’s see some examples:

Creating a Django model for the examples:

from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=255)
    age = models.IntegerField()

Defining a basic DjangoItem:

from scrapy_djangoitem import DjangoItem

class PersonItem(DjangoItem):
    django_model = Person

DjangoItem work just like Scrapy items:

>>> p = PersonItem()
>>> p['name'] = 'John'
>>> p['age'] = '22'

To obtain the Django model from the item, we call the extra method DjangoItem.save() of the DjangoItem:

>>> person = p.save()
>>> person.name
'John'
>>> person.age
'22'
>>> person.id
1

The model is already saved when we call DjangoItem.save(), we can prevent this by calling it with commit=False. We can use commit=False in DjangoItem.save() method to obtain an unsaved model:

>>> person = p.save(commit=False)
>>> person.name
'John'
>>> person.age
'22'
>>> person.id
None

As said before, we can add other fields to the item:

import scrapy
from scrapy_djangoitem import DjangoItem

class PersonItem(DjangoItem):
    django_model = Person
    sex = scrapy.Field()

>>> p = PersonItem()
>>> p['name'] = 'John'
>>> p['age'] = '22'
>>> p['sex'] = 'M'

And we can override the fields of the model with your own:

class PersonItem(DjangoItem):
    django_model = Person
    name = scrapy.Field(default='No Name')

This is useful to provide properties to the field, like a default or any other property that your project uses. Those additional fields won’t be taken into account when doing a DjangoItem.save().

DjangoItem caveats

DjangoItem is a rather convenient way to integrate Scrapy projects with Django models, but bear in mind that Django ORM may not scale well if you scrape a lot of items (ie. millions) with Scrapy. This is because a relational backend is often not a good choice for a write intensive application (such as a web crawler), specially if the database is highly normalized and with many indices.

Django settings set up

To use the Django models outside the Django application you need to set up the DJANGO_SETTINGS_MODULE environment variable and –in most cases– modify the PYTHONPATH environment variable to be able to import the settings module.

There are many ways to do this depending on your use case and preferences. Below is detailed one of the simplest ways to do it.

Suppose your Django project is named mysite, is located in the path /home/projects/mysite and you have created an app myapp with the model Person. That means your directory structure is something like this:

/home/projects/mysite
├── manage.py
├── myapp
│   ├── __init__.py
│   ├── models.py
│   ├── tests.py
│   └── views.py
└── mysite
    ├── __init__.py
    ├── settings.py
    ├── urls.py
    └── wsgi.py

Then you need to add /home/projects/mysite to the PYTHONPATH environment variable and set up the environment variable DJANGO_SETTINGS_MODULE to mysite.settings. That can be done in your Scrapy’s settings file by adding the lines below:

import sys
sys.path.append('/home/projects/mysite')

import os
os.environ['DJANGO_SETTINGS_MODULE'] = 'mysite.settings'

Notice that we modify the sys.path variable instead the PYTHONPATH environment variable as we are already within the python runtime. If everything is right, you should be able to start the scrapy shell command and import the model Person (i.e. from myapp.models import Person).