外连接模型化在django [英] outer join modelisation in django

问题描述

我有很多关系表，在联合基础中有一些数据

I have a many to many relationship table whith some datas in the jointing base

我的模型的基本版本看起来像：

a basic version of my model look like:

class FooLine(models.Model):
    name = models.CharField(max_length=255)

class FooCol(models.Model):
    name = models.CharField(max_length=255)

class FooVal(models.Model):
    value = models.CharField(max_length=255)
    line = models.ForeignKey(FooLine)
    col = models.ForeignKey(FooCol)

如果值不存在，我试图搜索某个行的某个行的每个值（基本上我正在尝试显示没有填充的值的空值的fooval表）
a典型值sql将是

I'm trying to search every values for a certain line with a null if the value is not present (basically i'm trying to display the fooval table with null values for values that haven't been filled) a typical sql would be

SELECT value FROM FooCol LEFT OUTER JOIN 
  (FooVal JOIN FooLine 
  ON FooVal.line_id == FooLine.id AND FooLine.name = "FIXME") 
ON FooCol.id = col_id;

有没有办法使用django模型来模拟上述查询

Is there any way to modelise above query using django model

谢谢

推荐答案

外连接可以被视为黑客，因为SQL缺少导航。

Outer joins can be viewed as a hack because SQL lacks "navigation".

你有一个简单的if-statement情况。

What you have is a simple if-statement situation.

for line in someRangeOfLines:
    for col in someRangeOfCols:
        try:
            cell= FooVal.objects().get( col = col, line = line )
        except FooVal.DoesNotExist:
            cell= None

这是外连接真的是 - 一次尝试查找一个NULL替换。

That's what an outer join really is -- an attempted lookup with a NULL replacement.

唯一的优化是如下。

matrix = {}
for f in FooVal.objects().all():
    matrix[(f.line,f.col)] = f

for line in someRangeOfLines:
    for col in someRangeOfCols:
        cell= matrix.get((line,col),None)

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