Django F表達式使用

需求說明：按產品、價格分組統計數據，採用Django ORM語句寫法爲：

res = CtProductPrice.objects.filter(umark=0, mdate__contains=yearmonth ).values_list(

        'product_id',

        'price'

    ).annotate(

        productnum = Count('product_id') ,

        countprice = Count('price') ,

        sumnum = Sum('num') ,

        sumprice = Sum( F('price') * F('num'), output_field=DecimalField() )

).values(

    'productnum',

    'countprice',

     'sumnum',

    'sumprice',

    'product_id'

)

打印查詢語句 print( res.query )查看語句是否符合SQL語法

其中遇到錯誤：Expression contains mixed types. You must set output_field

解決辦法：引入from django.db.models import Count, Sum, F, DecimalField，F表達式運算增長output_field類型 Sum( F('price') * F('num'), output_field=DecimalField() )

 

 

參考資料：https://docs.djangoproject.com/en/2.1/ref/models/expressions/