Django模型和初级 - 外键关系 [英] Django models and primary-foreign key relationships

问题描述

我正在编写一个Django应用程序，该应用程序使用现有的旧数据以及重新映射表和关系。我已经给了一个包含表之间的主外键关系的表，但是我很难理解如何使它适应Django模型。

例如，表 country_metrics 被定义为

  + ------- ------ + --------------- + ------ + ------ + --------- + ---- --- + 
 | Field |类型|空|关键|默认|额外| 
 + ------------- + --------------- + ------ + ----- +  - ------- + ------- + 
 | ContractNum | varchar（10）| NO | | NULL | | 
 | fips | varchar（5）| NO | | NULL | | 
 | metric_id | int（10）| NO | | NULL | | 
 | time_id | int（10）| NO | | NULL | | 
 |值|十进制（12,3）| NO | | NULL | | 
 | eff_date |日期| NO | | NULL | | 
 | exp_date |日期|是| | NULL | | 
 + ------------- + --------------- + ------ + ----- +  - ------- + ------- + 
  

我一直给出关系映射

  ContractNum主键，外键引用表合约
 fips主键，外键引用表国家
 metric_id主键，外键引用表指标
 time_id主键，外键引用表时间
  

它被建模在类

  class ContractCountyMetrics（models.Model）：
 contractnum = models.CharField（max_length = 10L，db_column ='ContractNum'，primary_key = true）＃字段名称小写。 
 fips = models.CharField（max_length = 5L）
 metric_id = models.IntegerField（）
 time_id = models.IntegerField（）
 value = models.DecimalField（max_digits = decimal_places = 3）
 eff_date = models.DateField（）
 exp_date = models.DateField（null = True，blank = True）
 contractKey = models.ForeignKey（Contracts）
 fipsKey = models.ForeignKey（县）
 metricKey = models.ForeignKey（Metrics）
 timeKey = models.ForeignKey（Time）
 class Meta：
 db_table ='txn_contract_county_metrics'
 unique_together（contractKey，fipsKey，metricKey，timeKey）
  

我读到， unique_together 基本上模拟一个复合键，并允许多个字段具有主键功能。我在正确的轨道上吗？

解决方案

从我读的 unique_together 基本上模拟复合
键，并允许多个字段具有主键功能。
我在正确的轨道上？

不完全。它只是将一个复合 UNIQUE KEY 添加到指定的字段中，并且在创建表时只有真正的效果，如果您使用Django访问传统表。在Django中仍然不支持复合 PRIMARY KEY （请参阅错误＃373 ）。

不幸的是，这可能意味着您将无法将Django与遗留表一起使用， code> PRIMARY KEY ，而不修改表以包含一个Django兼容的 PRIMARY KEY ，即单个唯一，

另请参见这个问题。

I'm writing a Django app that uses existing legacy data combined with a remapping of tables and relationships. I've been given a table containing the primary-foreign key relationships between the tables, but am having trouble understanding how to adapt it to Django models.

For example, a table country_metrics is defined as

+-------------+---------------+------+-----+---------+-------+
| Field       | Type          | Null | Key | Default | Extra |
+-------------+---------------+------+-----+---------+-------+
| ContractNum | varchar(10)   | NO   |     | NULL    |       |
| fips        | varchar(5)    | NO   |     | NULL    |       |
| metric_id   | int(10)       | NO   |     | NULL    |       |
| time_id     | int(10)       | NO   |     | NULL    |       |
| value       | decimal(12,3) | NO   |     | NULL    |       |
| eff_date    | date          | NO   |     | NULL    |       |
| exp_date    | date          | YES  |     | NULL    |       |
+-------------+---------------+------+-----+---------+-------+

and I've been given a relationship mapping of

ContractNum primary key, foreign key references table contracts
fips        primary key, foreign key references table country
metric_id   primary key, foreign key references table metrics
time_id     primary key, foreign key references table time

Its been modeled in the class

class ContractCountyMetrics(models.Model):
  contractnum = models.CharField(max_length=10L, db_column='ContractNum', primary_key = true) # Field name made lowercase.
  fips = models.CharField(max_length=5L)
  metric_id = models.IntegerField()
  time_id = models.IntegerField()
  value = models.DecimalField(max_digits=14, decimal_places=3)
  eff_date = models.DateField()
  exp_date = models.DateField(null=True, blank=True)
  contractKey = models.ForeignKey(Contracts)
  fipsKey = models.ForeignKey(Counties)
  metricKey = models.ForeignKey(Metrics)
  timeKey = models.ForeignKey(Time)
  class Meta:
      db_table = 'txn_contract_county_metrics'
      unique_together("contractKey", "fipsKey", "metricKey", "timeKey")

From what I read, unique_togetherbasically simulates a composite key and allows more than one field to have primary key functionality. Am I on the right track?

解决方案

From what I read, unique_together basically simulates a composite key and allows more than one field to have primary key functionality. Am I on the right track?

Not quite. It just adds a composite UNIQUE KEY to the specified fields, and only really has any effect during the creation of the table, which won't apply if you're using Django to access a legacy table. There's still no support for composite PRIMARY KEYs in Django (see bug #373).

Unfortunately, this probably means that you won't be able to use Django with a legacy table which has a composite PRIMARY KEY, without modifying the table to include a Django-compatible PRIMARY KEY, i.e. a key on a single, unique, field.

See also this question.

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