Django查询每次迭代都命中数据库 [英] Django query hitting the db for every iteration

问题描述

我有一些像这样的 model.py ：

class Muestraonline(models.Model):
    accessionnumber = models.ForeignKey(Muestra, related_name='online_accessionnumber')
    muestraid = models.ForeignKey(Muestra)
    taxonid = models.ForeignKey(Taxon, null=True, blank=True)
    ...

class Muestra(models.Model):
    muestraid = models.IntegerField(primary_key=True, db_column='MuestraID') # Field name made lowercase.
    latitudedecimal = models.DecimalField(decimal_places=6, null=True, max_digits=20, db_column='LatitudeDecimal', blank=True) # Field name made lowercase.
    longitudedecimal = models.DecimalField(decimal_places=6, null=True, max_digits=20, db_column='LongitudeDecimal', blank=True) # Field name made lowercase.
    ...

我想要的 view.py 以获得唯一的标本，并找到所有共享该分类单元的标本。对于相关的标本，我只需要纬度/经度信息：

And my view.py I want to get the unique specimen and find all specimens which share that taxonid. For the related specimens I just need the lat/long info:

def specimen_detail(request, accession_number=None, specimen_id=None):
    specimen = Muestraonline.objects.get(accessionnumber=accession_number, muestraid=specimen_id)
    related_specimens = Muestraonline.objects.filter(taxonid=specimen.taxonid).exclude(id=specimen.id)

    # create array for the related specimen points 
    related_coords = []
    # loop through results and populate array
    for relative in related_specimens:
        latlon = (format(relative.muestraid.latitudedecimal), format(relative.muestraid.longitudedecimal))
        related_coords.append(latlon) 
    related_coords = simplejson.dumps(related_coords)

但是当我循环遍历 related_specimens 时，它最终会为每个<$ c $一次命中数据库c>相对。我是否应该仅通过一个额外的数据库查询就可以获取所需格式的 latitudedecimal 和 longitudedecimal 值？我知道我在这里的方法中缺少一些非常基本的东西，只是不确定能做到这一点的最佳方法。

But when I loop through related_specimens it ends up hitting the db once for each relative. Shouldn't I be able to get the latitudedecimal and longitudedecimal values in the format I need with only one extra db query? I know I missing something very basic in my approach here, just not sure of the best way to get this done.

任何帮助将不胜感激。

推荐答案

只需使用选择相关：

related_specimens = Muestraonline.objects.filter(taxonid=specimen.taxonid).exclude(id=specimen.id).select_related('muestraid')

这将把您的 Muestraonline 模型加入到 Muestra 的幕后，以及每个 Muestraonline 实例还将包含 Muestra 的缓存实例。

That will join your Muestraonline model to Muestra behind the scenes and each Muestraonline instance returned by the QuerySet will also contain a cached instance of Muestra.

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