drf - 偏移分页组件 LimitOffsetPagination

Posted 2020-11-13 863652104kai

目录

• 数据准备
• 分页组件部分源码分析一
• 自定义分页组件
• 分页组件部分源码分析二

数据准备

model.py文件

定义两个表Car表和Brand表，其中Car中的brand字段外键关联Brand表

from django.db import models
class BaseModel(models.Model):
    is_delete = models.BooleanField(default=False)
    create_time = models.DateTimeField(auto_now_add=True)
    class Meta:
        abstract = True


class Car(BaseModel):
    name = models.CharField(max_length=32)
    price = models.DecimalField(max_digits=5, decimal_places=2)
    brand = models.ForeignKey('Brand', db_constraint=False, on_delete=models.DO_NOTHING, related_name='cars')
    @property
    def brand_name(self):
        return self.brand.name
    class Meta:
        db_table = 'old_boy_car'
        verbose_name = '汽车'
        verbose_name_plural = verbose_name
    def __str__(self):
        return self.name
    
class Brand(BaseModel):
    name = models.CharField(max_length=32)
    class Meta:
        db_table = 'old_boy_brand'
        verbose_name = '品牌'
        verbose_name_plural = verbose_name
    def __str__(self):
        return self.name

新建的serializer.py文件

brand字段只参与反序列化，brand_name只参与序列化

from rest_framework.serializers import ModelSerializer
from . import models
class CarModelSerializer(ModelSerializer):
    class Meta:
        model = models.Car
        fields = ('name','price','brand','brand_name')
        extra_kwargs = {
            "brand":{
                'write_only':True
            },
            'brand_name':{
                'read_only':True
            },
        }

分页组件部分源码分析一

通常是在获取多条数据list方法中，进行分页显示。所以viewsets.py文件中list方法是分页入口

class ListModelMixin:
    def list(self, request, *args, **kwargs):
        queryset = self.filter_queryset(self.get_queryset())  # 先筛选

        page = self.paginate_queryset(queryset)  # 后分页，再序列化显示
        if page is not None:
            serializer = self.get_serializer(page, many=True)
            return self.get_paginated_response(serializer.data)

        serializer = self.get_serializer(queryset, many=True)
        return Response(serializer.data)

1. 先调用paginate_queryset方法，此方法继承至generics中的GenericAPIView类

def paginate_queryset(self, queryset):
    """
    Return a single page of results, or `None` if pagination is disabled.
    """
    if self.paginator is None:  # self.paginator分页器，如果为None表示不分页
        return None
    return self.paginator.paginate_queryset(queryset, self.request, view=self)

2. 调用paginator方法

@property
def paginator(self):
    """
    The paginator instance associated with the view, or `None`.
    """
    if not hasattr(self, '_paginator'):
        # 如果配置中的pagination_class为空就不分页，所以要分页就需要配置pagination_class类，else中实例化分页类
        if self.pagination_class is None:
            self._paginator = None
        else:
            self._paginator = self.pagination_class()
    return self._paginator

3. 自定义pagination_class

4. 调用pagination_class类中的paginate_queryset方法

?

自定义分页组件

新建的paginations.py文件，CarPageNumberPagination继承了drf分页组件中的PageNumberPagination类

from rest_framework.pagination import LimitOffsetPagination
class CarLimitOffsetPagination(LimitOffsetPagination):
    default_limit = 3  # 设置每页显示的数量为3，此时url中下一页的offset就会往后偏移3条
    
    # 优先使用limit_query_param来设置显示条数
    limit_query_param = 'limit'
    offset_query_param = 'offset'  # 偏移数，就是多少条数据后显示
    max_limit = 5
    
# url链接：car/?limit=2  一页显示2条，默认显示第一页
# url链接：car/?limit=2&offset=4  往后偏移4条数据后，再显示自定义的2条数据，limit输入的数不能大于max_limit设置的数，大于则按max_limit来显示

注意：car/?limit=2&offset=4，若数据总条数只有5条，偏移offset=4条后显示就只显示1条数据

view.py文件：需要自定义pagination_class

from rest_framework.viewsets import ModelViewSet
from . import models, serializer
from .paginations import CarLimitOffsetPagination

class CarModelViewSet(ModelViewSet):
    queryset = models.Car.objects.filter(is_delete=False)
    serializer_class = serializer.CarModelSerializer
    # 自定义pagination_class
    parser_classes = CarLimitOffsetPagination

分页组件部分源码分析二

1. 调用CarLimitOffsetPagination的paginate_queryset方法(继承至LimitOffsetPagination)

def paginate_queryset(self, queryset, request, view=None):
    self.count = self.get_count(queryset)  # 获取数据的总数量
    self.limit = self.get_limit(request)
    if self.limit is None:
        return None

    self.offset = self.get_offset(request)
    self.request = request
    if self.count > self.limit and self.template is not None:
        self.display_page_controls = True

    if self.count == 0 or self.offset > self.count:
        return []
    return list(queryset[self.offset:self.offset + self.limit])

2. 调用 get_limit 方法获取 limit

def get_limit(self, request):
    if self.limit_query_param:  # limit_query_param 为'limit'
        try:
            return _positive_int(
                request.query_params[self.limit_query_param],
                strict=True,
                cutoff=self.max_limit  # 最大limit
            )
        except (KeyError, ValueError):
            pass

    return self.default_limit  # drf中默认设置为None，可以在类中配置

3. 设置了limit_query_param时，self.limit 为调用_positive_int方法后的结果

def _positive_int(integer_string, strict=False, cutoff=None):
    """
    Cast a string to a strictly positive integer.
    """
    ret = int(integer_string)  # url传入的值,在url中：car/?limit=4.则ret=4
    if ret < 0 or (ret == 0 and strict):
        raise ValueError()
    if cutoff:
        return min(ret, cutoff)
    return ret

4. 调用 get_offset方法获取 offset

def get_offset(self, request):
    try:
        return _positive_int(
            request.query_params[self.offset_query_param],  # offset_query_param = 'offset'
        )
    except (KeyError, ValueError):
        return 0