Question

我在我的Django应用程序中实现了搜索，允许通过多个字段进行搜索。这导致Django总是使用LEFT OUTER JOIN，在我的情况下会产生错误的结果。但是，当我将从LEFT OUTER JOIN生成的SQL更改为INNER JOIN时，它会返回正确的结果。

我认为这与我下面代码中Q object的方式有关。

from django.db import models, transaction
...
def construct_search(field_name):
    if field_name.startswith('^'):
        return "%s__istartswith" % field_name[1:]
    elif field_name.startswith('='):
        return "%s__iexact" % field_name[1:]
    elif field_name.startswith('@'):
        return "%s__search" % field_name[1:]
    else:
        return "%s__icontains" % field_name


class CoreSearchMixin(object):
    """Subclasses must define search_fields = [field_1, ...field_n]
    where the field is a string, the name of a field, and can contain the following prefix characters:

    '^': the search field must start with the search term, case insensitive
    '=': the search field must exactly equal the search term, case insensitive
    '@': full-text search

    If no prefix is given, any string that contains the search field will match.
    """
    search_fields = None
    search_form_class = SearchForm

    @cachedproperty
    def search_form(self):
        return self.search_form_class(getattr(self.request, self.request.method))

    def get_query_help_message(self):
        """Returns a comma separated list of fields that are used in the search, to help the user
        create a search.
        """
        fields = []
        if self.search_fields:
            for search_field in self.search_fields:
                field = get_field_from_path(self.model, search_field)
                fields.append(field.verbose_name.title())
        return ",".join(fields)

    def get_filtered_queryset(self, queryset):
        if self.search_form.is_valid():
            self.query = self.search_form.cleaned_data['q']
        else:
            self.query = None
        if self.search_fields and self.query:
            orm_lookups = (construct_search(str(search_field).replace('.', '__'))
                           for search_field in self.search_fields)
            chained_or_queries = None
            for bit in self.query.split():
                or_queries = (models.Q(**{orm_lookup: bit})
                              for orm_lookup in orm_lookups)
                if chained_or_queries:
                    chained_or_queries = itertools.chain(chained_or_queries, or_queries)
                else:
                    chained_or_queries = or_queries
            return queryset.filter(reduce(operator.or_, chained_or_queries))
        else:
            return queryset

    def get_context_data(self, **kwargs):
        return super(CoreSearchMixin, self).get_context_data(
            search_form=self.search_form,
            query_help_message=self.get_query_help_message(),
            search_fields=self.search_fields,
            **kwargs
        )

在上面的代码中，如何确保使用INNER JOIN而不是LEFT OUTER JOIN？

Answer 1

根据您的问题，您想要搜索多个字段。但是，遵循您的逻辑，返回在OR序列中找到的第一个结果，而不返回OR序列中可能的后续匹配;请记住，OR运算符会停止对结果进行评估。

要将OUTER LEFT JOIN转换为INNER JOIN，您需要AND / OR Q个搜索字段组合的对象排列（最佳）？），或单独查询它们并在结果上进行交集（次优），或自己编写SQL（次优）。

PS：在编写用于Django的Datatables API包装器之前，我遇到过这个问题。 PS：我会考虑重构，并进一步评论你的代码get_filtered_queryset;我花了几分钟的时间来绕过这里发生的事情。

强制Django使用INNER JOIN而不是LEFT OUTER JOIN

1 个答案: