What Happens When You Save? ¶

When you save an object, Django performs the following steps:

1. Emit a pre_save signal. This provides a notification that an object is about to be saved. You can register a listener that will be invoked whenever this signal is emitted. Check the online documentation for more on signals.

2. Preprocess the data. Each field on the object is asked to perform any automated data modification that the field may need to perform.

   Most fields do no preprocessing – the field data is kept as is. Preprocessing is only used on fields that have special behavior, like file fields.

3. Prepare the data for the database. Each field is asked to provide its current value in a data type that can be written to the database.

   Most fields require no data preparation. Simple data types, such as integers and strings, are “ready to write” as a Python object. However, more complex data types often require some modification. For example, DateFields use a Python datetime object to store data. Databases don’t store datetime objects, so the field value must be converted into an ISO-compliant date string for insertion into the database.

4. Insert the data into the database. The preprocessed, prepared data is then composed into an SQL statement for insertion into the database.

5. Emit a post_save signal. As with the pre_save signal, this is used to provide notification that an object has been successfully saved.

Autoincrementing Primary Keys ¶

For convenience, each model is given an autoincrementing primary key field named id unless you explicitly specify primary_key=True on a field (see the section titled “AutoField” in Appendix A).

If your model has an AutoField , that autoincremented value will be calculated and saved as an attribute on your object the first time you call save() :

>>> b2 = Blog(name='Cheddar Talk', tagline='Thoughts on cheese.')
>>> b2.id     # Returns None, because b doesn't have an ID yet.
None

>>> b2.save()
>>> b2.id     # Returns the ID of your new object.
14

There’s no way to tell what the value of an ID will be before you call save() , because that value is calculated by your database, not by Django.

If a model has an AutoField but you want to define a new object’s ID explicitly when saving, just define it explicitly before saving, rather than relying on the autoassignment of the ID:

>>> b3 = Blog(id=3, name='Cheddar Talk', tagline='Thoughts on cheese.')
>>> b3.id
3
>>> b3.save()
>>> b3.id
3

If you assign auto-primary-key values manually, make sure not to use an already existing primary key value! If you create a new object with an explicit primary key value that already exists in the database, Django will assume you’re changing the existing record rather than creating a new one.

Given the preceding 'Cheddar Talk' blog example, this example would override the previous record in the database:

>>> b4 = Blog(id=3, name='Not Cheddar', tagline='Anything but cheese.')
>>> b4.save()  # Overrides the previous blog with ID=3!

Explicitly specifying auto-primary-key values is mostly useful for bulk-saving objects, when you’re confident you won’t have primary key collision.

Chaining Filters ¶

The result of refining a QuerySet is itself a QuerySet , so it’s possible to chain refinements together, for example:

>>> qs = Entry.objects.filter(headline__startswith='What')
>>> qs = qs.exclude(pub_date__gte=datetime.datetime.now())
>>> qs = qs.filter(pub_date__gte=datetime.datetime(2005, 1, 1))

This takes the initial QuerySet of all entries in the database, adds a filter, then an exclusion, and then another filter. The final result is a QuerySet containing all entries with a headline that starts with “What” that were published between January 1, 2005, and the current day.

It’s important to point out here that QuerySets are lazy – the act of creating a QuerySet doesn’t involve any database activity. In fact, the three preceding lines don’t make any database calls; you can chain filters together all day long and Django won’t actually run the query until the QuerySet is evaluated .

You can evaluate a QuerySet in any following ways:

• Iterating : A QuerySet is iterable, and it executes its database query the first time you iterate over it. For example, the following QuerySet isn’t evaluated until it’s iterated over in the for loop:

  qs = Entry.objects.filter(pub_date__year=2006)
  qs = qs.filter(headline__icontains="bill")
  for e in qs:
      print e.headline
  

  This prints all headlines from 2006 that contain “bill” but causes only one database hit.

• Printing it : A QuerySet is evaluated when you call repr() on it. This is for convenience in the Python interactive interpreter, so you can immediately see your results when using the API interactively.

• Slicing : As explained in the upcoming “Limiting QuerySets” section, a QuerySet can be sliced using Python’s array-slicing syntax. Usually slicing a QuerySet returns another (unevaluated)``QuerySet``, but Django will execute the database query if you use the “step” parameter of slice syntax.

• Converting to a list : You can force evaluation of a QuerySet by calling list() on it, for example:

  >>> entry_list = list(Entry.objects.all())
  

  Be warned, though, that this could have a large memory overhead, because Django will load each element of the list into memory. In contrast, iterating over a QuerySet will take advantage of your database to load data and instantiate objects only as you need them.

q1 = Entry.objects.filter(headline__startswith="What")
q2 = q1.exclude(pub_date__gte=datetime.now())
q3 = q1.filter(pub_date__gte=datetime.now())

Limiting QuerySets ¶

Use Python’s array-slicing syntax to limit your QuerySet to a certain number of results. This is the equivalent of SQL’s LIMIT and OFFSET clauses.

For example, this returns the first five entries ( LIMIT 5 ):

>>> Entry.objects.all()[:5]

This returns the sixth through tenth entries ( OFFSET 5 LIMIT 5 ):

>>> Entry.objects.all()[5:10]

Generally, slicing a QuerySet returns a new QuerySet – it doesn’t evaluate the query. An exception is if you use the “step” parameter of Python slice syntax. For example, this would actually execute the query in order to return a list of every second object of the first ten:

>>> Entry.objects.all()[:10:2]

To retrieve a single object rather than a list (e.g., SELECT foo FROM bar LIMIT 1 ), use a simple index instead of a slice. For example, this returns the first Entry in the database, after ordering entries alphabetically by headline:

>>> Entry.objects.order_by('headline')[0]

This is roughly equivalent to the following:

>>> Entry.objects.order_by('headline')[0:1].get()

Note, however, that the first of these will raise IndexError while the second will raise DoesNotExist if no objects match the given criteria.

Query Methods That Return New QuerySets ¶

Django provides a range of QuerySet refinement methods that modify either the types of results returned by the QuerySet or the way its SQL query is executed. These methods are described in the sections that follow. Some of the methods take field lookup arguments, which are discussed in detail a bit later on.

>> Entry.objects.filter(pub_date__year=2005).order_by('-pub_date', 'headline')

>>> Entry.objects.order_by('?')

# This list contains a Blog object.
>>> Blog.objects.filter(name__startswith='Beatles')
[Beatles Blog]

# This list contains a dictionary.
>>> Blog.objects.filter(name__startswith='Beatles').values()
[{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}]

>>> Blog.objects.values()
[{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}],
>>> Blog.objects.values('id', 'name')
[{'id': 1, 'name': 'Beatles Blog'}]

>>> Entry.objects.dates('pub_date', 'year')
[datetime.datetime(2005, 1, 1)]

>>> Entry.objects.dates('pub_date', 'month')
[datetime.datetime(2005, 2, 1), datetime.datetime(2005, 3, 1)]

>>> Entry.objects.dates('pub_date', 'day')
[datetime.datetime(2005, 2, 20), datetime.datetime(2005, 3, 20)]

>>> Entry.objects.dates('pub_date', 'day', order='DESC')
[datetime.datetime(2005, 3, 20), datetime.datetime(2005, 2, 20)]

>>> Entry.objects.filter(headline__contains='Lennon').dates('pub_date', 'day')
[datetime.datetime(2005, 3, 20)]

# Hits the database.
>>> e = Entry.objects.get(id=5)

# Hits the database again to get the related Blog object.
>>> b = e.blog

# Hits the database.
>>> e = Entry.objects.select_related().get(id=5)

# Doesn't hit the database, because e.blog has been prepopulated
# in the previous query.
>>> b = e.blog

class City(models.Model):
    # ...

class Person(models.Model):
    # ...
    hometown = models.ForeignKey(City)

class Book(models.Model):
    # ...
    author = models.ForeignKey(Person)

>>> b = Book.objects.select_related().get(id=4)
>>> p = b.author         # Doesn't hit the database.
>>> c = p.hometown       # Doesn't hit the database.

>>> b = Book.objects.get(id=4) # No select_related() in this example.
>>> p = b.author         # Hits the database.
>>> c = p.hometown       # Hits the database.

QuerySet Methods That Do Not Return QuerySets ¶

The following QuerySet methods evaluate the QuerySet and return something other than a QuerySet – a single object, value, and so forth.

>>> Entry.objects.get(id='foo') # raises Entry.DoesNotExist

>>> from django.core.exceptions import ObjectDoesNotExist
>>> try:
...     e = Entry.objects.get(id=3)
...     b = Blog.objects.get(id=1)
... except ObjectDoesNotExist:
...     print "Either the entry or blog doesn't exist."

>>> p = Person(first_name="Bruce", last_name="Springsteen")
>>> p.save()

>>> p = Person.objects.create(first_name="Bruce", last_name="Springsteen")

try:
    obj = Person.objects.get(first_name='John', last_name='Lennon')
except Person.DoesNotExist:
    obj = Person(first_name='John', last_name='Lennon', birthday=date(1940, 10, 9))
    obj.save()

obj, created = Person.objects.get_or_create(
    first_name = 'John',
    last_name  = 'Lennon',
    defaults   = {'birthday': date(1940, 10, 9)}
)

defaults = kwargs.pop('defaults', {})
params = dict([(k, v) for k, v in kwargs.items() if '__' not in k])
params.update(defaults)
obj = self.model(**params)
obj.save()

Foo.objects.get_or_create(
    defaults__exact = 'bar',
    defaults={'defaults': 'bar'}
)

# Returns the total number of entries in the database.
>>> Entry.objects.count()
4

# Returns the number of entries whose headline contains 'Lennon'
>>> Entry.objects.filter(headline__contains='Lennon').count()
1

>>> Blog.objects.in_bulk([1])
{1: Beatles Blog}
>>> Blog.objects.in_bulk([1, 2])
{1: Beatles Blog, 2: Cheddar Talk}
>>> Blog.objects.in_bulk([])
{}

>>> Entry.objects.latest('pub_date')

exact ¶

Performs an exact match:

>>> Entry.objects.get(headline__exact="Man bites dog")

This matches any object with the exact headline “Man bites dog”.

If you don’t provide a lookup type – that is, if your keyword argument doesn’t contain a double underscore – the lookup type is assumed to be exact .

For example, the following two statements are equivalent:

>>> Blog.objects.get(id__exact=14) # Explicit form
>>> Blog.objects.get(id=14) # __exact is implied

This is for convenience, because exact lookups are the common case.

iexact ¶

Performs a case-insensitive exact match:

>>> Blog.objects.get(name__iexact='beatles blog')

This will match 'Beatles Blog' , 'beatles blog' , 'BeAtLes BLoG' , and so forth.

contains ¶

Performs a case-sensitive containment test:

Entry.objects.get(headline__contains='Lennon')

This will match the headline 'Today Lennon honored' but not 'today lennon honored' .

SQLite doesn’t support case-sensitive LIKE statements; when using SQLite,``contains`` acts like icontains .

Entry.objects.filter(headline__contains='%')

SELECT ... WHERE headline LIKE '%\%%';

icontains ¶

Performs a case-insensitive containment test:

>>> Entry.objects.get(headline__icontains='Lennon')

Unlike contains , icontains will match 'today lennon honored' .

gt, gte, lt, and lte ¶

These represent greater than, greater than or equal to, less than, and less than or equal to:

>>> Entry.objects.filter(id__gt=4)
>>> Entry.objects.filter(id__lt=15)
>>> Entry.objects.filter(id__gte=0)

These queries return any object with an ID greater than 4, an ID less than 15, and an ID greater than or equal to 1, respectively.

You’ll usually use these on numeric fields. Be careful with character fields since character order isn’t always what you’d expect (i.e., the string “4” sorts after the string “10”).

in ¶

Filters where a value is on a given list:

Entry.objects.filter(id__in=[1, 3, 4])

This returns all objects with the ID 1, 3, or 4.

startswith ¶

Performs a case-sensitive starts-with:

>>> Entry.objects.filter(headline__startswith='Will')

This will return the headlines “Will he run?” and “Willbur named judge”, but not “Who is Will?” or “will found in crypt”.

istartswith ¶

Performs a case-insensitive starts-with:

>>> Entry.objects.filter(headline__istartswith='will')

This will return the headlines “Will he run?”, “Willbur named judge”, and “will found in crypt”, but not “Who is Will?”

endswith and iendswith ¶

Perform case-sensitive and case-insensitive ends-with:

>>> Entry.objects.filter(headline__endswith='cats')
>>> Entry.objects.filter(headline__iendswith='cats')

Similar to startswith and istartswith .

range ¶

Performs an inclusive range check:

>>> start_date = datetime.date(2005, 1, 1)
>>> end_date = datetime.date(2005, 3, 31)
>>> Entry.objects.filter(pub_date__range=(start_date, end_date))

You can use range anywhere you can use BETWEEN in SQL – for dates, numbers, and even characters.

year, month, and day ¶

For date/datetime fields, perform exact year, month, or day matches:

# Return all entries published in 2005
>>>Entry.objects.filter(pub_date__year=2005)

# Return all entries published in December
>>> Entry.objects.filter(pub_date__month=12)

# Return all entries published on the 3rd of the month
>>> Entry.objects.filter(pub_date__day=3)

# Combination: return all entries on Christmas of any year
>>> Entry.objects.filter(pub_date__month=12, pub_date_day=25)

isnull ¶

Takes either True or False , which correspond to SQL queries of IS NULL and IS NOT NULL , respectively:

>>> Entry.objects.filter(pub_date__isnull=True)

search ¶

A Boolean full-text search that takes advantage of full-text indexing. This is like contains but is significantly faster due to full-text indexing.

Note this is available only in MySQL and requires direct manipulation of the database to add the full-text index.

The pk Lookup Shortcut ¶

For convenience, Django provides a pk lookup type, which stands for “primary_key”.

In the example Blog model, the primary key is the id field, so these three statements are equivalent:

>>> Blog.objects.get(id__exact=14) # Explicit form
>>> Blog.objects.get(id=14) # __exact is implied
>>> Blog.objects.get(pk=14) # pk implies id__exact

The use of pk isn’t limited to __exact queries – any query term can be combined with pk to perform a query on the primary key of a model:

# Get blogs entries  with id 1, 4, and 7
>>> Blog.objects.filter(pk__in=[1,4,7])

# Get all blog entries with id > 14
>>> Blog.objects.filter(pk__gt=14)

pk lookups also work across joins. For example, these three statements are equivalent:

>>> Entry.objects.filter(blog__id__exact=3) # Explicit form
>>> Entry.objects.filter(blog__id=3) # __exact is implied
>>> Entry.objects.filter(blog__pk=3) # __pk implies __id__exact

The point of pk is to give you a generic way to refer to the primary key in cases where you’re not sure whether the model’s primary key is called id .

Lookups That Span Relationships ¶

Django offers a powerful and intuitive way to “follow” relationships in lookups, taking care of the SQL JOIN s for you automatically behind the scenes. To span a relationship, just use the field name of related fields across models, separated by double underscores, until you get to the field you want.

This example retrieves all Entry objects with a Blog whose name is 'Beatles Blog' :

>>> Entry.objects.filter(blog__name__exact='Beatles Blog')

This spanning can be as deep as you’d like.

It works backward, too. To refer to a “reverse” relationship, just use the lowercase name of the model.

This example retrieves all Blog objects that have at least one Entry whose headline contains 'Lennon' :

>>> Blog.objects.filter(entry__headline__contains='Lennon')

Foreign Key Relationships ¶

If a model has a ForeignKey , instances of that model will have access to the related (foreign) object via a simple attribute of the model, for example:

e = Entry.objects.get(id=2)
e.blog # Returns the related Blog object.

You can get and set via a foreign key attribute. As you may expect, changes to the foreign key aren’t saved to the database until you call save() , for example:

e = Entry.objects.get(id=2)
e.blog = some_blog
e.save()

If a ForeignKey field has null=True set (i.e., it allows NULL values), you can set it to NULL by assigning None to it and saving:

e = Entry.objects.get(id=2)
e.blog = None
e.save() # "UPDATE blog_entry SET blog_id = NULL ...;"

Forward access to one-to-many relationships is cached the first time the related object is accessed. Subsequent accesses to the foreign key on the same object instance are cached, for example:

e = Entry.objects.get(id=2)
print e.blog  # Hits the database to retrieve the associated Blog.
print e.blog  # Doesn't hit the database; uses cached version.

Note that the select_related() QuerySet method recursively prepopulates the cache of all one-to-many relationships ahead of time:

e = Entry.objects.select_related().get(id=2)
print e.blog  # Doesn't hit the database; uses cached version.
print e.blog  # Doesn't hit the database; uses cached version.

select_related() is documented in the “QuerySet Methods That Return New QuerySets” section.

“Reverse” Foreign Key Relationships ¶

Foreign key relationships are automatically symmetrical – a reverse relationship is inferred from the presence of a ForeignKey pointing to another model.

If a model has a ForeignKey , instances of the foreign key model will have access to a Manager that returns all instances of the first model that relate to that object. By default, this Manager is named FOO_set , where FOO is the source model name, lowercased. This Manager returns QuerySets , which can be filtered and manipulated as described in the “Retrieving Objects” section.

Here’s an example:

b = Blog.objects.get(id=1)
b.entry_set.all() # Returns all Entry objects related to Blog.

# b.entry_set is a Manager that returns QuerySets.
b.entry_set.filter(headline__contains='Lennon')
b.entry_set.count()

You can override the FOO_set name by setting the related_name parameter in the ForeignKey() definition. For example, if the Entry model was altered to blog = ForeignKey(Blog, related_name='entries') , the preceding example code would look like this:

b = Blog.objects.get(id=1)
b.entries.all() # Returns all Entry objects related to Blog.

# b.entries is a Manager that returns QuerySets.
b.entries.filter(headline__contains='Lennon')
b.entries.count()

related_name is particularly useful if a model has two foreign keys to the same second model.

You cannot access a reverse ForeignKey Manager from the class; it must be accessed from an instance:

Blog.entry_set # Raises AttributeError: "Manager must be accessed via instance".

In addition to the QuerySet methods defined in the “Retrieving Objects” section, the ForeignKey Manager has these additional methods:

• add(obj1, obj2, ...) : Adds the specified model objects to the related object set, for example:

  b = Blog.objects.get(id=1)
  e = Entry.objects.get(id=234)
  b.entry_set.add(e) # Associates Entry e with Blog b.
  

• create(**kwargs) : Creates a new object, saves it, and puts it in the related object set. It returns the newly created object:

  b = Blog.objects.get(id=1)
  e = b.entry_set.create(headline='Hello', body_text='Hi', pub_date=datetime.date(2005, 1, 1))
  # No need to call e.save() at this point -- it's already been saved.
  

  This is equivalent to (but much simpler than) the following:

  b = Blog.objects.get(id=1)
  e = Entry(blog=b, headline='Hello', body_text='Hi', pub_date=datetime.date(2005, 1, 1))
  e.save()
  

  Note that there’s no need to specify the keyword argument of the model that defines the relationship. In the preceding example, we don’t pass the parameter blog to create() . Django figures out that the new Entry object’s blog field should be set to b .

• remove(obj1, obj2, ...) : Removes the specified model objects from the related object set:

  b = Blog.objects.get(id=1)
  e = Entry.objects.get(id=234)
  b.entry_set.remove(e) # Disassociates Entry e from Blog b.
  

  In order to prevent database inconsistency, this method only exists on ForeignKey objects where null=True . If the related field can’t be set to None ( NULL ), then an object can’t be removed from a relation without being added to another. In the preceding example, removing e from b.entry_set() is equivalent to doing e.blog = None , and because the blog ForeignKey doesn’t have null=True , this is invalid.

• clear() : Removes all objects from the related object set:

  b = Blog.objects.get(id=1)
  b.entry_set.clear()
  

  Note this doesn’t delete the related objects – it just disassociates them.

  Just like remove() , clear() is only available on ForeignKey``s where ``null=True .

To assign the members of a related set in one fell swoop, just assign to it from any iterable object, for example:

b = Blog.objects.get(id=1)
b.entry_set = [e1, e2]

If the clear() method is available, any pre-existing objects will be removed from the entry_set before all objects in the iterable (in this case, a list) are added to the set. If the clear() method is not available, all objects in the iterable will be added without removing any existing elements.

Each “reverse” operation described in this section has an immediate effect on the database. Every addition, creation, and deletion is immediately and automatically saved to the database.

Many-to-Many Relationships ¶

Both ends of a many-to-many relationship get automatic API access to the other end. The API works just as a “reverse” one-to-many relationship (described in the previous section).

The only difference is in the attribute naming: the model that defines the ManyToManyField uses the attribute name of that field itself, whereas the “reverse” model uses the lowercased model name of the original model, plus '_set' (just like reverse one-to-many relationships).

An example makes this concept easier to understand:

e = Entry.objects.get(id=3)
e.authors.all() # Returns all Author objects for this Entry.
e.authors.count()
e.authors.filter(name__contains='John')

a = Author.objects.get(id=5)
a.entry_set.all() # Returns all Entry objects for this Author.

Like ForeignKey , ManyToManyField can specify related_name . In the preceding example, if the ManyToManyField in Entry had specified related_name='entries' , then each Author instance would have an entries attribute instead of entry_set .

Queries Over Related Objects ¶

Queries involving related objects follow the same rules as queries involving normal value fields. When specifying the value for a query to match, you may use either an object instance itself or the primary key value for the object.

For example, if you have a Blog object b with id=5 , the following three queries would be identical:

Entry.objects.filter(blog=b) # Query using object instance
Entry.objects.filter(blog=b.id) # Query using id from instance
Entry.objects.filter(blog=5) # Query using id directly

get_object_or_404() ¶

One common idiom to use get() and raise Http404 if the object doesn’t exist. This idiom is captured by get_object_or_404() . This function takes a Django model as its first argument and an arbitrary number of keyword arguments, which it passes to the default manager’s get() function. It raises Http404 if the object doesn’t exist, for example:

# Get the Entry with a primary key of 3
e = get_object_or_404(Entry, pk=3)

When you provide a model to this shortcut function, the default manager is used to execute the underlying get() query. If you don’t want to use the default manager, or if you want to search a list of related objects, you can provide get_object_or_404() with a Manager object instead:

# Get the author of blog instance e with a name of 'Fred'
a = get_object_or_404(e.authors, name='Fred')

# Use a custom manager 'recent_entries' in the search for an
# entry with a primary key of 3
e = get_object_or_404(Entry.recent_entries, pk=3)

get_list_or_404() ¶

get_list_or_404 behaves the same way as get_object_or_404() , except that it uses filter() instead of get() . It raises Http404 if the list is empty.