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Filtering

If you want to allow the user to filter your querysets by a number of different attributes, it makes sense to encapsulate your filters into a FilterSchema class. FilterSchema is a regular Schema, so it's using all the necessary features of Pydantic, but it also adds some bells and whistles that ease the translation of the user-facing filtering parameters into database queries.

Start off with defining a subclass of FilterSchema:

from ninja import FilterSchema, Field
from typing import Optional


class BookFilterSchema(FilterSchema):
    name: Optional[str] = None
    author: Optional[str] = None
    created_after: Optional[datetime] = None

Next, use this schema in conjunction with Query in your API handler: 

@api.get("/books")
def list_books(request, filters: BookFilterSchema = Query(...)):
    books = Book.objects.all()
    books = filters.filter(books)
    return books

Just like described in defining query params using schema, Django Ninja converts the fields defined in BookFilterSchema into query parameters.

You can use a shorthand one-liner .filter() to apply those filters to your queryset: 

@api.get("/books")
def list_books(request, filters: BookFilterSchema = Query(...)):
    books = Book.objects.all()
    books = filters.filter(books)
    return books

Under the hood, FilterSchema converts its fields into Q expressions which it then combines and uses to filter your queryset.

Alternatively to using the .filter method, you can get the prepared Q-expression and perform the filtering yourself. That can be useful, when you have some additional queryset filtering on top of what you expose to the user through the API: 

@api.get("/books")
def list_books(request, filters: BookFilterSchema = Query(...)):

    # Never serve books from inactive publishers and authors
    q = Q(author__is_active=True) | Q(publisher__is_active=True)

    # But allow filtering the rest of the books
    q &= filters.get_filter_expression()
    return Book.objects.filter(q)

By default, the filters will behave the following way:

  • None values will be ignored and not filtered against;
  • Every non-None field will be converted into a Q-expression based on the Field definition of each field;
  • All Q-expressions will be merged into one using AND logical operator;
  • The resulting Q-expression is used to filter the queryset and return you a queryset with a .filter clause applied.

Customizing Fields

By default, FilterSet will use the field names to generate Q expressions: 

class BookFilterSchema(FilterSchema):
    name: Optional[str] = None
The name field will be converted into Q(name=...) expression.

When your database lookups are more complicated than that, you can explicitly specify them in the field definition using a "q" kwarg: 

class BookFilterSchema(FilterSchema):
    name: Optional[str] = Field(None, q='name__icontains')
You can even specify multiple lookup keyword argument names as a list: 
class BookFilterSchema(FilterSchema):
    search: Optional[str] = Field(None, q=['name__icontains',
                                     'author__name__icontains',
                                     'publisher__name__icontains'])
By default, field-level expressions are combined using "OR" connector, so with the above setup, a query parameter ?search=foobar will search for books that have "foobar" in either of their name, author or publisher.

Combining expressions

By default,

  • Field-level expressions are joined together using OR operator.
  • The fields themselves are joined together using AND operator.

So, with the following FilterSchema... 

class BookFilterSchema(FilterSchema):
    search: Optional[str] = Field(None, q=['name__icontains', 'author__name__icontains'])
    popular: Optional[bool] = None
...and the following query parameters from the user 
http://localhost:8000/api/books?search=harry&popular=true
the FilterSchema instance will look for popular books that have harry in the book's or author's name.

You can customize this behavior using an expression_connector argument in field-level and class-level definition: 

class BookFilterSchema(FilterSchema):
    active: Optional[bool] = Field(None, q=['is_active', 'publisher__is_active'],
                                   expression_connector='AND')
    name: Optional[str] = Field(None, q='name__icontains')

    class Config:
        expression_connector = 'OR'

An expression connector can take the values of "OR", "AND" and "XOR", but the latter is only supported in Django starting with 4.1.

Now, a request with these query parameters 

http://localhost:8000/api/books?name=harry&active=true
...shall search for books that have harry in their name or are active themselves and are published by active publishers.

Filtering by Nones

You can make the FilterSchema treat None as a valid value that should be filtered against.

This can be done on a field level with a ignore_none kwarg: 

class BookFilterSchema(FilterSchema):
    name: Optional[str] = Field(None, q='name__icontains')
    tag: Optional[str] = Field(None, q='tag', ignore_none=False)

This way when no other value for "tag" is provided by the user, the filtering will always include a condition tag=None.

You can also specify this settings for all fields at the same time in the Config: 

class BookFilterSchema(FilterSchema):
    name: Optional[str] = Field(None, q='name__icontains')
    tag: Optional[str] = Field(None, q='tag', ignore_none=False)

    class Config:
        ignore_none = False

Custom expressions

Sometimes you might want to have complex filtering scenarios that cannot be handled by individual Field annotations. For such cases you can implement your field filtering logic as a custom method. Simply define a method called filter_<fieldname> which takes a filter value and returns a Q expression:

class BookFilterSchema(FilterSchema):
    tag: Optional[str] = None
    popular: Optional[bool] = None

    def filter_popular(self, value: bool) -> Q:
        return Q(view_count__gt=1000) | Q(download_count__gt=100) if value else Q()
Such field methods take precedence over what is specified in the Field() definition of the corresponding fields.

If that is not enough, you can implement your own custom filtering logic for the entire FilterSet class in a custom_expression method:

class BookFilterSchema(FilterSchema):
    name: Optional[str] = None
    popular: Optional[bool] = None

    def custom_expression(self) -> Q:
        q = Q()
        if self.name:
            q &= Q(name__icontains=self.name)
        if self.popular:
            q &= (
                Q(view_count__gt=1000) |
                Q(downloads__gt=100) |
                Q(tag='popular')
            )
        return q
The custom_expression method takes precedence over any other definitions described earlier, including filter_<fieldname> methods.