# Django 2.1.8.dev20190330215650 documentation

# 쿼리 만들기

:doc:`data models </topics/db/models>`을 만들면 Django는 자동으로 객체들을 만들고 읽고 수정하고 지울 수 있는 데이터베이스-추상화 API를 만들어냅니다. 이 문서는 이 API를 어떻게 사용하는지를 다룹니다. :doc:`data model reference </ref/models/index>`를 참조하면 다양한 모델 검색 옵션들에 대한 자세한 설명을 볼 수 있습니다.

이 가이드(와 데이터 모델 레퍼런스)에서는 블로그 어플리케이션을 구성하는 아래 모델을 이용할 것입니다.

    from django.db import models

    class Blog(models.Model):
        name = models.CharField(max_length=100)
        tagline = models.TextField()

        def __str__(self):
            return self.name

    class Author(models.Model):
        name = models.CharField(max_length=200)
        email = models.EmailField()

        def __str__(self):
            return self.name

    class Entry(models.Model):
        blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
        headline = models.CharField(max_length=255)
        body_text = models.TextField()
        pub_date = models.DateField()
        mod_date = models.DateField()
        authors = models.ManyToManyField(Author)
        n_comments = models.IntegerField()
        n_pingbacks = models.IntegerField()
        rating = models.IntegerField()

        def __str__(self):
            return self.headline

## 객체 만들기

데이터베이스-테이블 내용을 파이썬 객체로 표현하기 위해서 Django는 직관적인 시스템을 이용합니다. 모델 클래스가 데이터베이스의 테이블을 나타내고, 그 클래스의 인스턴스가 데이터베이스 테이블의 각 레코드를 표현하는 것입니다.

객체를 만들기 위해서 모델 클래스의 키워드 인수들을 이용해서 인스턴스화한 후에 :meth:`~django.db.models.Model.save`를 이용해서 데이터베이스에 저장합시다.

모델은 ``mysite/blog/models.py``에 있다고 가정합시다. 여기 예시가 있습니다:

    >>> from blog.models import Blog
    >>> b = Blog(name='Beatles Blog', tagline='All the latest Beatles news.')
    >>> b.save()

이 명령어는 SQL 명령어 중 `INSERT` 구문을 작동시킵니다. Django에서는 당신이 명시적으로 `save()`.를 실행할 때까지 데이터베이스를 수정하지 않습니다.

`save()` 명령어에 따른 결과값은 따로 없습니다.

더 보기

`save()` 에는 여기서 다루지 않은 다양한 옵션이 있습니다. 자세한 설명을 보려면 `save()` 문서를 참조하기 바랍니다.

객체를 만들고 저장하는 것을 한 번에 하려면 `create()` 명령어를 이용하세요.

## 객체 변경사항 저장하기

이미 데이터베이스에 있는 객체를 변경한 후 저장할 때에도 :meth:`~django.db.models.Model.save`를 쓰면 됩니다.

`b5``는 ``Blog` 테이블에 주어진 인스턴스로 데이터베이스에 저장되어 있습니다. 아래 예시를 통해서 이름을 바꾸고 데이터베이스를 갱신하겠습니다:

    >>> b5.name = 'New name'
    >>> b5.save()

이 명령어는 SQL의 `UPDATE` 구문을 실행시킵니다. Django에서는 당신이 명시적으로 `save()`.를 실행할 때까지 데이터베이스를 수정하지 않습니다.

### `ForeignKey``와 ``ManyToManyField` 필드 저장하기

`ForeignKey` 필드를 업데이트하는 것도 일반 필드를 저장하는 것과 정확하게 똑같이 작동합니다. 적합한 타입의 객체를 해당 필드에 배치하는 거죠. 아래 예시는 `Entry``와 ``Blog``의 인스턴스들이 데이터베이스에 이미 저장되어 있다는 가정 하에 ``Entry` 클래스의 인스턴스인 ``entry``의 속성인 ``blog``를 업데이트하는 내용입니다(가정에 따라 우리는 두 클래스의 인스턴스를 아래에서 부를 수 있죠):

    >>> from blog.models import Blog, Entry
    >>> entry = Entry.objects.get(pk=1)
    >>> cheese_blog = Blog.objects.get(name="Cheddar Talk")
    >>> entry.blog = cheese_blog
    >>> entry.save()

`add` 명령어를 필드에 사용해야 합니다. 이 예시는 `Author``의 인스턴스인 ``joe``를 ``entry` 객체다 추가하는 내용입니다.:

    >>> from blog.models import Author
    >>> joe = Author.objects.create(name="Joe")
    >>> entry.authors.add(joe)

:class:`~django.db.models.ManyToManyField`에 여러 레코드를 한 번에 추가하기 위해서, :meth:`~django.db.models.fields.related.RelatedManager.add`를 사용할 때 아래와 같이 여러 인수들을 이용할 수 있습니다:

    >>> john = Author.objects.create(name="John")
    >>> paul = Author.objects.create(name="Paul")
    >>> george = Author.objects.create(name="George")
    >>> ringo = Author.objects.create(name="Ringo")
    >>> entry.authors.add(john, paul, george, ringo)

잘못된 타입을 추가하거나 할당하면 Django가 에러 메시지를 띄울 것입니다.

## 객체 조회하기

데이터베이스에 있는 객체들을 조회하기 위해서는 모델의 :class:`~django.db.models.Manager`를 이용해서 :class:`~django.db.models.query.QuerySet`을 만들어야 합니다.

A `QuerySet` represents a collection of objects from your database. It can have zero, one or many filters. Filters narrow down the query results based on the given parameters. In SQL terms, a `QuerySet` equates to a `SELECT` statement, and a filter is a limiting clause such as `WHERE` or `LIMIT`.

You get a `QuerySet` by using your model's `Manager`. Each model has at least one `Manager`, and it's called `objects` by default. Access it directly via the model class, like so:

    >>> Blog.objects
    <django.db.models.manager.Manager object at ...>
    >>> b = Blog(name='Foo', tagline='Bar')
    >>> b.objects
    Traceback:
        ...
    AttributeError: "Manager isn't accessible via Blog instances."

주석

`Managers` are accessible only via model classes, rather than from model instances, to enforce a separation between "table-level" operations and "record-level" operations.

The `Manager` is the main source of `QuerySets` for a model. For example, `Blog.objects.all()` returns a `QuerySet` that contains all `Blog` objects in the database.

### Retrieving all objects

The simplest way to retrieve objects from a table is to get all of them. To do this, use the `all()` method on a `Manager`:

    >>> all_entries = Entry.objects.all()

The `all()` method returns a `QuerySet` of all the objects in the database.

### Retrieving specific objects with filters

The `QuerySet` returned by `all()` describes all objects in the database table. Usually, though, you'll need to select only a subset of the complete set of objects.

To create such a subset, you refine the initial `QuerySet`, adding filter conditions. The two most common ways to refine a `QuerySet` are:

`filter(**kwargs)`
  Returns a new `QuerySet` containing objects that match the given lookup parameters.
`exclude(**kwargs)`
  Returns a new `QuerySet` containing objects that do not match the given lookup parameters.

The lookup parameters (`**kwargs` in the above function definitions) should be in the format described in Field lookups below.

For example, to get a `QuerySet` of blog entries from the year 2006, use `filter()` like so:

    Entry.objects.filter(pub_date__year=2006)

With the default manager class, it is the same as:

    Entry.objects.all().filter(pub_date__year=2006)

#### Chaining filters

The result of refining a `QuerySet` is itself a `QuerySet`, so it's possible to chain refinements together. For example:

    >>> Entry.objects.filter(
    ...     headline__startswith='What'
    ... ).exclude(
    ...     pub_date__gte=datetime.date.today()
    ... ).filter(
    ...     pub_date__gte=datetime.date(2005, 1, 30)
    ... )

This takes the initial `QuerySet` of all entries in the database, adds a filter, then an exclusion, then another filter. The final result is a `QuerySet` containing all entries with a headline that starts with "What", that were published between January 30, 2005, and the current day.

#### Filtered `QuerySet`s are unique

Each time you refine a `QuerySet`, you get a brand-new `QuerySet` that is in no way bound to the previous `QuerySet`. Each refinement creates a separate and distinct `QuerySet` that can be stored, used and reused.

Example:

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

These three `QuerySets` are separate. The first is a base `QuerySet` containing all entries that contain a headline starting with "What". The second is a subset of the first, with an additional criteria that excludes records whose `pub_date` is today or in the future. The third is a subset of the first, with an additional criteria that selects only the records whose `pub_date` is today or in the future. The initial `QuerySet` (`q1`) is unaffected by the refinement process.

#### `QuerySet`s are lazy

`QuerySets` are lazy -- the act of creating a `QuerySet` doesn't involve any database activity. You can stack filters together all day long, and Django won't actually run the query until the `QuerySet` is evaluated. Take a look at this example:

    >>> q = Entry.objects.filter(headline__startswith="What")
    >>> q = q.filter(pub_date__lte=datetime.date.today())
    >>> q = q.exclude(body_text__icontains="food")
    >>> print(q)

Though this looks like three database hits, in fact it hits the database only once, at the last line (`print(q)`). In general, the results of a `QuerySet` aren't fetched from the database until you "ask" for them. When you do, the `QuerySet` is evaluated by accessing the database. For more details on exactly when evaluation takes place, see When QuerySets are evaluated.

### Retrieving a single object with `get()`

`filter()` will always give you a `QuerySet`, even if only a single object matches the query - in this case, it will be a `QuerySet` containing a single element.

If you know there is only one object that matches your query, you can use the `get()` method on a `Manager` which returns the object directly:

    >>> one_entry = Entry.objects.get(pk=1)

You can use any query expression with `get()`, just like with `filter()` - again, see Field lookups below.

Note that there is a difference between using `get()`, and using `filter()` with a slice of `[0]`. If there are no results that match the query, `get()` will raise a `DoesNotExist` exception. This exception is an attribute of the model class that the query is being performed on - so in the code above, if there is no `Entry` object with a primary key of 1, Django will raise `Entry.DoesNotExist`.

Similarly, Django will complain if more than one item matches the `get()` query. In this case, it will raise `MultipleObjectsReturned`, which again is an attribute of the model class itself.

### Other `QuerySet` methods

Most of the time you'll use `all()`, `get()`, `filter()` and `exclude()` when you need to look up objects from the database. However, that's far from all there is; see the QuerySet API Reference for a complete list of all the various `QuerySet` methods.

### Limiting `QuerySet`s

Use a subset of 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 5 objects (`LIMIT 5`):

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

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

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

Negative indexing (i.e. `Entry.objects.all()[-1]`) is not supported.

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 10:

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

Further filtering or ordering of a sliced queryset is prohibited due to the ambiguous nature of how that might work.

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:

    >>> 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. See `get()` for more details.

### Field lookups

Field lookups are how you specify the meat of an SQL `WHERE` clause. They're specified as keyword arguments to the `QuerySet` methods `filter()`, `exclude()` and `get()`.

Basic lookups keyword arguments take the form `field__lookuptype=value`. (That's a double-underscore). For example:

    >>> Entry.objects.filter(pub_date__lte='2006-01-01')

translates (roughly) into the following SQL:

    SELECT * FROM blog_entry WHERE pub_date <= '2006-01-01';

How this is possible

Python has the ability to define functions that accept arbitrary name-value arguments whose names and values are evaluated at runtime. For more information, see Keyword Arguments in the official Python tutorial.

The field specified in a lookup has to be the name of a model field. There's one exception though, in case of a `ForeignKey` you can specify the field name suffixed with `_id`. In this case, the value parameter is expected to contain the raw value of the foreign model's primary key. For example:

    >>> Entry.objects.filter(blog_id=4)

If you pass an invalid keyword argument, a lookup function will raise `TypeError`.

The database API supports about two dozen lookup types; a complete reference can be found in the field lookup reference. To give you a taste of what's available, here's some of the more common lookups you'll probably use:

`exact`

  An "exact" match. For example:

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

  Would generate SQL along these lines:

      SELECT ... WHERE headline = 'Cat 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`

  A case-insensitive match. So, the query:

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

  Would match a `Blog` titled `"Beatles Blog"`, `"beatles blog"`, or even `"BeAtlES blOG"`.

`contains`

  Case-sensitive containment test. For example:

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

  Roughly translates to this SQL:

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

  Note this will match the headline `'Today Lennon honored'` but not `'today lennon honored'`.

  There's also a case-insensitive version, `icontains`.

`startswith`, `endswith`
  Starts-with and ends-with search, respectively. There are also case-insensitive versions called `istartswith` and `iendswith`.

Again, this only scratches the surface. A complete reference can be found in the field lookup reference.

### 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='Beatles Blog')

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

It works backwards, too. To refer to a "reverse" relationship, just use the lowercase name of the model.

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

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

If you are filtering across multiple relationships and one of the intermediate models doesn't have a value that meets the filter condition, Django will treat it as if there is an empty (all values are `NULL`), but valid, object there. All this means is that no error will be raised. For example, in this filter:

    Blog.objects.filter(entry__authors__name='Lennon')

(if there was a related `Author` model), if there was no `author` associated with an entry, it would be treated as if there was also no `name` attached, rather than raising an error because of the missing `author`. Usually this is exactly what you want to have happen. The only case where it might be confusing is if you are using `isnull`. Thus:

    Blog.objects.filter(entry__authors__name__isnull=True)

will return `Blog` objects that have an empty `name` on the `author` and also those which have an empty `author` on the `entry`. If you don't want those latter objects, you could write:

    Blog.objects.filter(entry__authors__isnull=False, entry__authors__name__isnull=True)

#### Spanning multi-valued relationships

When you are filtering an object based on a `ManyToManyField` or a reverse `ForeignKey`, there are two different sorts of filter you may be interested in. Consider the `Blog`/`Entry` relationship (`Blog` to `Entry` is a one-to-many relation). We might be interested in finding blogs that have an entry which has both "Lennon" in the headline and was published in 2008. Or we might want to find blogs that have an entry with "Lennon" in the headline as well as an entry that was published in 2008. Since there are multiple entries associated with a single `Blog`, both of these queries are possible and make sense in some situations.

The same type of situation arises with a `ManyToManyField`. For example, if an `Entry` has a `ManyToManyField` called `tags`, we might want to find entries linked to tags called "music" and "bands" or we might want an entry that contains a tag with a name of "music" and a status of "public".

To handle both of these situations, Django has a consistent way of processing `filter()` calls. Everything inside a single `filter()` call is applied simultaneously to filter out items matching all those requirements. Successive `filter()` calls further restrict the set of objects, but for multi-valued relations, they apply to any object linked to the primary model, not necessarily those objects that were selected by an earlier `filter()` call.

That may sound a bit confusing, so hopefully an example will clarify. To select all blogs that contain entries with both "Lennon" in the headline and that were published in 2008 (the same entry satisfying both conditions), we would write:

    Blog.objects.filter(entry__headline__contains='Lennon', entry__pub_date__year=2008)

To select all blogs that contain an entry with "Lennon" in the headline as well as an entry that was published in 2008, we would write:

    Blog.objects.filter(entry__headline__contains='Lennon').filter(entry__pub_date__year=2008)

Suppose there is only one blog that had both entries containing "Lennon" and entries from 2008, but that none of the entries from 2008 contained "Lennon". The first query would not return any blogs, but the second query would return that one blog.

In the second example, the first filter restricts the queryset to all those blogs linked to entries with "Lennon" in the headline. The second filter restricts the set of blogs further to those that are also linked to entries that were published in 2008. The entries selected by the second filter may or may not be the same as the entries in the first filter. We are filtering the `Blog` items with each filter statement, not the `Entry` items.

주석

The behavior of `filter()` for queries that span multi-value relationships, as described above, is not implemented equivalently for `exclude()`. Instead, the conditions in a single `exclude()` call will not necessarily refer to the same item.

For example, the following query would exclude blogs that contain both entries with "Lennon" in the headline and entries published in 2008:

    Blog.objects.exclude(
        entry__headline__contains='Lennon',
        entry__pub_date__year=2008,
    )

However, unlike the behavior when using `filter()`, this will not limit blogs based on entries that satisfy both conditions. In order to do that, i.e. to select all blogs that do not contain entries published with "Lennon" that were published in 2008, you need to make two queries:

    Blog.objects.exclude(
        entry__in=Entry.objects.filter(
            headline__contains='Lennon',
            pub_date__year=2008,
        ),
    )

### Filters can reference fields on the model

In the examples given so far, we have constructed filters that compare the value of a model field with a constant. But what if you want to compare the value of a model field with another field on the same model?

Django provides `F expressions` to allow such comparisons. Instances of `F()` act as a reference to a model field within a query. These references can then be used in query filters to compare the values of two different fields on the same model instance.

For example, to find a list of all blog entries that have had more comments than pingbacks, we construct an `F()` object to reference the pingback count, and use that `F()` object in the query:

    >>> from django.db.models import F
    >>> Entry.objects.filter(n_comments__gt=F('n_pingbacks'))

Django supports the use of addition, subtraction, multiplication, division, modulo, and power arithmetic with `F()` objects, both with constants and with other `F()` objects. To find all the blog entries with more than twice as many comments as pingbacks, we modify the query:

    >>> Entry.objects.filter(n_comments__gt=F('n_pingbacks') * 2)

To find all the entries where the rating of the entry is less than the sum of the pingback count and comment count, we would issue the query:

    >>> Entry.objects.filter(rating__lt=F('n_comments') + F('n_pingbacks'))

You can also use the double underscore notation to span relationships in an `F()` object. An `F()` object with a double underscore will introduce any joins needed to access the related object. For example, to retrieve all the entries where the author's name is the same as the blog name, we could issue the query:

    >>> Entry.objects.filter(authors__name=F('blog__name'))

For date and date/time fields, you can add or subtract a `timedelta` object. The following would return all entries that were modified more than 3 days after they were published:

    >>> from datetime import timedelta
    >>> Entry.objects.filter(mod_date__gt=F('pub_date') + timedelta(days=3))

The `F()` objects support bitwise operations by `.bitand()`, `.bitor()`, `.bitrightshift()`, and `.bitleftshift()`. For example:

    >>> F('somefield').bitand(16)

### The `pk` lookup shortcut

For convenience, Django provides a `pk` lookup shortcut, 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

### Escaping percent signs and underscores in `LIKE` statements

The field lookups that equate to `LIKE` SQL statements (`iexact`, `contains`, `icontains`, `startswith`, `istartswith`, `endswith` and `iendswith`) will automatically escape the two special characters used in `LIKE` statements -- the percent sign and the underscore. (In a `LIKE` statement, the percent sign signifies a multiple-character wildcard and the underscore signifies a single-character wildcard.)

This means things should work intuitively, so the abstraction doesn't leak. For example, to retrieve all the entries that contain a percent sign, just use the percent sign as any other character:

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

Django takes care of the quoting for you; the resulting SQL will look something like this:

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

Same goes for underscores. Both percentage signs and underscores are handled for you transparently.

### Caching and `QuerySet`s

Each `QuerySet` contains a cache to minimize database access. Understanding how it works will allow you to write the most efficient code.

In a newly created `QuerySet`, the cache is empty. The first time a `QuerySet` is evaluated -- and, hence, a database query happens -- Django saves the query results in the `QuerySet`’s cache and returns the results that have been explicitly requested (e.g., the next element, if the `QuerySet` is being iterated over). Subsequent evaluations of the `QuerySet` reuse the cached results.

Keep this caching behavior in mind, because it may bite you if you don't use your `QuerySet`s correctly. For example, the following will create two `QuerySet`s, evaluate them, and throw them away:

    >>> print([e.headline for e in Entry.objects.all()])
    >>> print([e.pub_date for e in Entry.objects.all()])

That means the same database query will be executed twice, effectively doubling your database load. Also, there's a possibility the two lists may not include the same database records, because an `Entry` may have been added or deleted in the split second between the two requests.

To avoid this problem, simply save the `QuerySet` and reuse it:

    >>> queryset = Entry.objects.all()
    >>> print([p.headline for p in queryset]) # Evaluate the query set.
    >>> print([p.pub_date for p in queryset]) # Re-use the cache from the evaluation.

#### When `QuerySet`s are not cached

Querysets do not always cache their results. When evaluating only part of the queryset, the cache is checked, but if it is not populated then the items returned by the subsequent query are not cached. Specifically, this means that limiting the queryset using an array slice or an index will not populate the cache.

For example, repeatedly getting a certain index in a queryset object will query the database each time:

    >>> queryset = Entry.objects.all()
    >>> print(queryset[5]) # Queries the database
    >>> print(queryset[5]) # Queries the database again

However, if the entire queryset has already been evaluated, the cache will be checked instead:

    >>> queryset = Entry.objects.all()
    >>> [entry for entry in queryset] # Queries the database
    >>> print(queryset[5]) # Uses cache
    >>> print(queryset[5]) # Uses cache

Here are some examples of other actions that will result in the entire queryset being evaluated and therefore populate the cache:

    >>> [entry for entry in queryset]
    >>> bool(queryset)
    >>> entry in queryset
    >>> list(queryset)

주석

Simply printing the queryset will not populate the cache. This is because the call to `__repr__()` only returns a slice of the entire queryset.

## Complex lookups with `Q` objects

Keyword argument queries -- in `filter()`, etc. -- are "AND"ed together. If you need to execute more complex queries (for example, queries with `OR` statements), you can use `Q objects`.

A `Q object` (`django.db.models.Q`) is an object used to encapsulate a collection of keyword arguments. These keyword arguments are specified as in "Field lookups" above.

For example, this `Q` object encapsulates a single `LIKE` query:

    from django.db.models import Q
    Q(question__startswith='What')

`Q` objects can be combined using the `&` and `|` operators. When an operator is used on two `Q` objects, it yields a new `Q` object.

For example, this statement yields a single `Q` object that represents the "OR" of two `"question__startswith"` queries:

    Q(question__startswith='Who') | Q(question__startswith='What')

This is equivalent to the following SQL `WHERE` clause:

    WHERE question LIKE 'Who%' OR question LIKE 'What%'

You can compose statements of arbitrary complexity by combining `Q` objects with the `&` and `|` operators and use parenthetical grouping. Also, `Q` objects can be negated using the `~` operator, allowing for combined lookups that combine both a normal query and a negated (`NOT`) query:

    Q(question__startswith='Who') | ~Q(pub_date__year=2005)

Each lookup function that takes keyword-arguments (e.g. `filter()`, `exclude()`, `get()`) can also be passed one or more `Q` objects as positional (not-named) arguments. If you provide multiple `Q` object arguments to a lookup function, the arguments will be "AND"ed together. For example:

    Poll.objects.get(
        Q(question__startswith='Who'),
        Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6))
    )

... roughly translates into the SQL:

    SELECT * from polls WHERE question LIKE 'Who%'
        AND (pub_date = '2005-05-02' OR pub_date = '2005-05-06')

Lookup functions can mix the use of `Q` objects and keyword arguments. All arguments provided to a lookup function (be they keyword arguments or `Q` objects) are "AND"ed together. However, if a `Q` object is provided, it must precede the definition of any keyword arguments. For example:

    Poll.objects.get(
        Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6)),
        question__startswith='Who',
    )

... would be a valid query, equivalent to the previous example; but:

    # INVALID QUERY
    Poll.objects.get(
        question__startswith='Who',
        Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6))
    )

... would not be valid.

더 보기

The OR lookups examples in the Django unit tests show some possible uses of `Q`.

## Comparing objects

To compare two model instances, just use the standard Python comparison operator, the double equals sign: `==`. Behind the scenes, that compares the primary key values of two models.

Using the `Entry` example above, the following two statements are equivalent:

    >>> some_entry == other_entry
    >>> some_entry.id == other_entry.id

If a model's primary key isn't called `id`, no problem. Comparisons will always use the primary key, whatever it's called. For example, if a model's primary key field is called `name`, these two statements are equivalent:

    >>> some_obj == other_obj
    >>> some_obj.name == other_obj.name

## Deleting objects

The delete method, conveniently, is named `delete()`. This method immediately deletes the object and returns the number of objects deleted and a dictionary with the number of deletions per object type. Example:

    >>> e.delete()
    (1, {'weblog.Entry': 1})

You can also delete objects in bulk. Every `QuerySet` has a `delete()` method, which deletes all members of that `QuerySet`.

For example, this deletes all `Entry` objects with a `pub_date` year of 2005:

    >>> Entry.objects.filter(pub_date__year=2005).delete()
    (5, {'webapp.Entry': 5})

Keep in mind that this will, whenever possible, be executed purely in SQL, and so the `delete()` methods of individual object instances will not necessarily be called during the process. If you've provided a custom `delete()` method on a model class and want to ensure that it is called, you will need to "manually" delete instances of that model (e.g., by iterating over a `QuerySet` and calling `delete()` on each object individually) rather than using the bulk `delete()` method of a `QuerySet`.

When Django deletes an object, by default it emulates the behavior of the SQL constraint `ON DELETE CASCADE` -- in other words, any objects which had foreign keys pointing at the object to be deleted will be deleted along with it. For example:

    b = Blog.objects.get(pk=1)
    # This will delete the Blog and all of its Entry objects.
    b.delete()

This cascade behavior is customizable via the `on_delete` argument to the `ForeignKey`.

Note that `delete()` is the only `QuerySet` method that is not exposed on a `Manager` itself. This is a safety mechanism to prevent you from accidentally requesting `Entry.objects.delete()`, and deleting all the entries. If you do want to delete all the objects, then you have to explicitly request a complete query set:

    Entry.objects.all().delete()

## Copying model instances

Although there is no built-in method for copying model instances, it is possible to easily create new instance with all fields' values copied. In the simplest case, you can just set `pk` to `None`. Using our blog example:

    blog = Blog(name='My blog', tagline='Blogging is easy')
    blog.save() # blog.pk == 1

    blog.pk = None
    blog.save() # blog.pk == 2

Things get more complicated if you use inheritance. Consider a subclass of `Blog`:

    class ThemeBlog(Blog):
        theme = models.CharField(max_length=200)

    django_blog = ThemeBlog(name='Django', tagline='Django is easy', theme='python')
    django_blog.save() # django_blog.pk == 3

Due to how inheritance works, you have to set both `pk` and `id` to None:

    django_blog.pk = None
    django_blog.id = None
    django_blog.save() # django_blog.pk == 4

This process doesn't copy relations that aren't part of the model's database table. For example, `Entry` has a `ManyToManyField` to `Author`. After duplicating an entry, you must set the many-to-many relations for the new entry:

    entry = Entry.objects.all()[0] # some previous entry
    old_authors = entry.authors.all()
    entry.pk = None
    entry.save()
    entry.authors.set(old_authors)

For a `OneToOneField`, you must duplicate the related object and assign it to the new object's field to avoid violating the one-to-one unique constraint. For example, assuming `entry` is already duplicated as above:

    detail = EntryDetail.objects.all()[0]
    detail.pk = None
    detail.entry = entry
    detail.save()

## Updating multiple objects at once

Sometimes you want to set a field to a particular value for all the objects in a `QuerySet`. You can do this with the `update()` method. For example:

    # Update all the headlines with pub_date in 2007.
    Entry.objects.filter(pub_date__year=2007).update(headline='Everything is the same')

You can only set non-relation fields and `ForeignKey` fields using this method. To update a non-relation field, provide the new value as a constant. To update `ForeignKey` fields, set the new value to be the new model instance you want to point to. For example:

    >>> b = Blog.objects.get(pk=1)

    # Change every Entry so that it belongs to this Blog.
    >>> Entry.objects.all().update(blog=b)

The `update()` method is applied instantly and returns the number of rows matched by the query (which may not be equal to the number of rows updated if some rows already have the new value). The only restriction on the `QuerySet` being updated is that it can only access one database table: the model's main table. You can filter based on related fields, but you can only update columns in the model's main table. Example:

    >>> b = Blog.objects.get(pk=1)

    # Update all the headlines belonging to this Blog.
    >>> Entry.objects.select_related().filter(blog=b).update(headline='Everything is the same')

Be aware that the `update()` method is converted directly to an SQL statement. It is a bulk operation for direct updates. It doesn't run any `save()` methods on your models, or emit the `pre_save` or `post_save` signals (which are a consequence of calling `save()`), or honor the `auto_now` field option. If you want to save every item in a `QuerySet` and make sure that the `save()` method is called on each instance, you don't need any special function to handle that. Just loop over them and call `save()`:

    for item in my_queryset:
        item.save()

Calls to update can also use `F expressions` to update one field based on the value of another field in the model. This is especially useful for incrementing counters based upon their current value. For example, to increment the pingback count for every entry in the blog:

    >>> Entry.objects.all().update(n_pingbacks=F('n_pingbacks') + 1)

However, unlike `F()` objects in filter and exclude clauses, you can't introduce joins when you use `F()` objects in an update -- you can only reference fields local to the model being updated. If you attempt to introduce a join with an `F()` object, a `FieldError` will be raised:

    # This will raise a FieldError
    >>> Entry.objects.update(headline=F('blog__name'))

## Related objects

When you define a relationship in a model (i.e., a `ForeignKey`, `OneToOneField`, or `ManyToManyField`), instances of that model will have a convenient API to access the related object(s).

Using the models at the top of this page, for example, an `Entry` object `e` can get its associated `Blog` object by accessing the `blog` attribute: `e.blog`.

(Behind the scenes, this functionality is implemented by Python descriptors. This shouldn't really matter to you, but we point it out here for the curious.)

Django also creates API accessors for the "other" side of the relationship -- the link from the related model to the model that defines the relationship. For example, a `Blog` object `b` has access to a list of all related `Entry` objects via the `entry_set` attribute: `b.entry_set.all()`.

All examples in this section use the sample `Blog`, `Author` and `Entry` models defined at the top of this page.

### One-to-many relationships

#### Forward

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.

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()`. 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 assign `None` to remove the relation. Example:

    >>> 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. 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. Example:

    >>> 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.

#### Following relationships "backward"

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. 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 above.

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, on_delete=models.CASCADE, related_name='entries')`, the above 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()

#### Using a custom reverse manager

By default the `RelatedManager` used for reverse relations is a subclass of the default manager for that model. If you would like to specify a different manager for a given query you can use the following syntax:

    from django.db import models

    class Entry(models.Model):
        #...
        objects = models.Manager()  # Default Manager
        entries = EntryManager()    # Custom Manager

    b = Blog.objects.get(id=1)
    b.entry_set(manager='entries').all()

If `EntryManager` performed default filtering in its `get_queryset()` method, that filtering would apply to the `all()` call.

Of course, specifying a custom reverse manager also enables you to call its custom methods:

    b.entry_set(manager='entries').is_published()

#### Additional methods to handle related objects

In addition to the `QuerySet` methods defined in "Retrieving objects" above, the `ForeignKey` `Manager` has additional methods used to handle the set of related objects. A synopsis of each is below, and complete details can be found in the related objects reference.

`add(obj1, obj2, ...)`
  Adds the specified model objects to the related object set.
`create(**kwargs)`
  Creates a new object, saves it and puts it in the related object set. Returns the newly created object.
`remove(obj1, obj2, ...)`
  Removes the specified model objects from the related object set.
`clear()`
  Removes all objects from the related object set.
`set(objs)`
  Replace the set of related objects.

To assign the members of a related set, use the `set()` method with an iterable of object instances. For example, if `e1` and `e2` are `Entry` instances:

    b = Blog.objects.get(id=1)
    b.entry_set.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 similar to a "backward" one-to-many relationship, above.

One 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 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 above example, if the `ManyToManyField` in `Entry` had specified `related_name='entries'`, then each `Author` instance would have an `entries` attribute instead of `entry_set`.

Another difference from one-to-many relationships is that in addition to model instances, the `add()`, `set()`, and `remove()` methods on many-to-many relationships accept primary key values. For example, if `e1` and `e2` are `Entry` instances, then these `set()` calls work identically:

    a = Author.objects.get(id=5)
    a.entry_set.set([e1, e2])
    a.entry_set.set([e1.pk, e2.pk])

### One-to-one relationships

One-to-one relationships are very similar to many-to-one relationships. If you define a `OneToOneField` on your model, instances of that model will have access to the related object via a simple attribute of the model.

For example:

    class EntryDetail(models.Model):
        entry = models.OneToOneField(Entry, on_delete=models.CASCADE)
        details = models.TextField()

    ed = EntryDetail.objects.get(id=2)
    ed.entry # Returns the related Entry object.

The difference comes in "reverse" queries. The related model in a one-to-one relationship also has access to a `Manager` object, but that `Manager` represents a single object, rather than a collection of objects:

    e = Entry.objects.get(id=2)
    e.entrydetail # returns the related EntryDetail object

If no object has been assigned to this relationship, Django will raise a `DoesNotExist` exception.

Instances can be assigned to the reverse relationship in the same way as you would assign the forward relationship:

    e.entrydetail = ed

### How are the backward relationships possible?

Other object-relational mappers require you to define relationships on both sides. The Django developers believe this is a violation of the DRY (Don't Repeat Yourself) principle, so Django only requires you to define the relationship on one end.

But how is this possible, given that a model class doesn't know which other model classes are related to it until those other model classes are loaded?

The answer lies in the `app registry`. When Django starts, it imports each application listed in `INSTALLED_APPS`, and then the `models` module inside each application. Whenever a new model class is created, Django adds backward-relationships to any related models. If the related models haven't been imported yet, Django keeps tracks of the relationships and adds them when the related models eventually are imported.

For this reason, it's particularly important that all the models you're using be defined in applications listed in `INSTALLED_APPS`. Otherwise, backwards relations may not work properly.

### 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

## Falling back to raw SQL

If you find yourself needing to write an SQL query that is too complex for Django's database-mapper to handle, you can fall back on writing SQL by hand. Django has a couple of options for writing raw SQL queries; see Performing raw SQL queries.

Finally, it's important to note that the Django database layer is merely an interface to your database. You can access your database via other tools, programming languages or database frameworks; there's nothing Django-specific about your database.