在非主键关系上加入和查询Django模型？ [英] Join and query Django models on non-primary-key relationship?

问题描述

我有两个通过不是主键的字段在逻辑上相关的模型。可以在不引入 ForeignKey 列的情况下查询它们（例如， select_related（…））吗？

I've got two models that are logically related through a field that is not the primary key. Is it possible to query them (ex, select_related(…)) without introducing a ForeignKey column?

例如，考虑人为模型：

class LogEntry(Model):
    source_name = CharField(…)
    log_message = CharField(…)

class LogSource(Model):
    name = CharField(…)
    domain = CharField(…)

我希望能够查询 LogEntry ，加入并过滤相关的 LogSource （例如，这样我就可以访问 log_entry.source 而不用其他查询）：

I would like to be able to query LogEntry, joining in and filtering on the related LogSource (ex, so I can access log_entry.source without additional queries):

LogEntry.objects
    .select_related(
        source=Join(LogSource, on="logsource.name = logentry.source_name")),
    )
    .filter(source__domain="example.com")

是否可能在不引入ForeignKey的情况下？

Is this possible without introducing a ForeignKey?

推荐答案

您应该能够o通过使用 extra（） 和 tables 选项。

You should be able to do this by using extra() with the tables option.

LogEntry.objects.extra(
    tables=['logsource'],
    where=['logsource.name=logentry.source_name',
           'logsource_domain="example.com"',
           ]
)

另一种选择是更改 source_name 到外键，但指定 db_column 和 to_field 参数以使用现有列。我知道您说过您不想添加外键，但这是可以接受的，因为它仅更改模型，而不更改数据库表中的列。但是，请注意，Django可能要创建外键约束。一种破解方法是伪造该迁移，以便不在数据库中创建约束。

Another option is to change source_name to a foreign key, but specify the db_column and to_field arguments to use the existing columns. I know that you said that you didn't want to add a foreign key, but it might be acceptable because it only changes the models, not the columns in the database tables. However, be aware that Django might want to create a foreign key constraint. One hack would be to fake that migration so that the constraint isn't created in the db.

class LogEntry(Model):
    source_name = models.ForeignKey(db_column=source_name', to_field='name')

这样， log_entry.source_name 将是 LogSource 实例，以及 log_entry.source_name_id 将是存储在 source_name 列中的值。在转换为外键后，将字段从 source_name 重命名为 source 可能很有意义，但这不是必需的。

log_entry.source_name would then be the LogSource instance, and log_entry.source_name_id would be the value stored in the source_name column. It might make sense to rename the field from source_name to source after converting to a foreign key, but that's not necessary.

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