4.6 KiB
DjangoItem
:class:`DjangoItem` is a class of item that gets its fields definition from a Django model, you simply create a :class:`DjangoItem` and specify what Django model it relates to.
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Unknown interpreted text role "class".System Message: ERROR/3 (<stdin>, line 7); backlink
Unknown interpreted text role "class".Besides of getting the model fields defined on your item, :class:`DjangoItem` provides a method to create and populate a Django model instance with the item data.
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Unknown interpreted text role "class".Using DjangoItem
:class:`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.
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Unknown interpreted text role "class".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 :class:`DjangoItem`:
System Message: ERROR/3 (<stdin>, line 35); backlink
Unknown interpreted text role "class".from scrapy.contrib.djangoitem import DjangoItem
class PersonItem(DjangoItem):
django_model = Person
:class:`DjangoItem` work just like :class:`~scrapy.item.Item`:
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Unknown interpreted text role "class".System Message: ERROR/3 (<stdin>, line 42); backlink
Unknown interpreted text role "class".>>> p = PersonItem() >>> p['name'] = 'John' >>> p['age'] = '22'
To obtain the Django model from the item, we call the extra method :meth:`~DjangoItem.save` of the :class:`DjangoItem`:
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Unknown interpreted text role "class".>>> person = p.save() >>> person.name 'John' >>> person.age '22' >>> person.id 1
The model is already saved when we call :meth:`~DjangoItem.save`, we can prevent this by calling it with commit=False. We can use commit=False in :meth:`~DjangoItem.save` method to obtain an unsaved model:
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Unknown interpreted text role "meth".System Message: ERROR/3 (<stdin>, line 59); backlink
Unknown interpreted text role "meth".>>> 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.contrib.djangoitem import DjangoItem
class PersonItem(DjangoItem):
django_model = Person
sex = scrapy.Field()
>>> p = PersonItem() >>> p['name'] = 'John' >>> p['age'] = '22' >>> p['sex'] = 'M'
Note
fields added to the item won't be taken into account when doing a :meth:`~DjangoItem.save`
System Message: ERROR/3 (<stdin>, line 87); backlink
Unknown interpreted text role "meth".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.
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).