title: Django rest framework legacy DB foreign keys appearing as Null on API
body: <p>I am using django connected to a legacy mysql DB. So i used inspectdb to get the DB tables. There are 2 models that I am using:</p>
  <pre><code>class ScCustomerTypes(models.Model):
      customer_type_id = models.AutoField(primary_key=True)
      customer_type = models.CharField(max_length=255, blank=True, null=True)
  
      class Meta:
          managed = False
          db_table = 'sc_customer_types'
  
  
  class ScCustomers(models.Model):
      customer_id = models.CharField(primary_key=True, max_length=100)
      identification_number = models.CharField(unique=True, max_length=100)
      customer_type = models.ForeignKey('ScCustomerTypes', models.DO_NOTHING, related_name='customer_types2customers', to_field='customer_type_id')
      customer_names = models.CharField(max_length=100)
      phone_number = models.CharField(max_length=100, blank=True, null=True)
      location = models.CharField(max_length=100, blank=True, null=True)
      created_by = models.CharField(max_length=100, blank=True, null=True)
      created_at = models.DateTimeField(blank=True, null=True)
      updated_at = models.DateTimeField(blank=True, null=True)
      updated_by = models.CharField(max_length=100, blank=True, null=True)
  
      class Meta:
          managed = False
          db_table = 'sc_customers'
  </code></pre>
  <p>Using the above I made the below serializer:</p>
  <pre><code>from rest_framework import serializers
  from allDBModels.models import ScCustomers, ScCustomerTypes
  from employees.serializers import AllEmployeeSerializer
  
  class AllCustomerTypesSerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = ScCustomerTypes
          fields = ('customer_type',)
  
  class AllCustomerSerializer(serializers.HyperlinkedModelSerializer):
      customer_type_id = AllCustomerTypesSerializer()
  
      class Meta:
          model = ScCustomers
          fields = ('customer_id', 'customer_type_id', 'customer_names', 'phone_number', 'identification_number', 'location', 'created_by', 'created_at', 'updated_by', 'updated_at',)
  </code></pre>
  <p>Issue now is when i pull the api it returns as such:</p>
  <pre><code>{
          &quot;customer_id&quot;: &quot;ICXY38ZP&quot;,
          &quot;customer_type_id&quot;: {
              &quot;customer_type&quot;: null
          },
          &quot;customer_names&quot;: &quot;Test xxxxxxxxxxx&quot;,
          &quot;phone_number&quot;: &quot;2547xxxxxxx&quot;,
          &quot;identification_number&quot;: &quot;yyyyyyyyyy&quot;,
          &quot;location&quot;: null,
          &quot;created_by&quot;: &quot;3b1e3f79&quot;,
          &quot;created_at&quot;: &quot;2020-10-20T13:58:21Z&quot;,
          &quot;updated_by&quot;: null,
          &quot;updated_at&quot;: null
      },
  </code></pre>
  <p>The issue is that its not returning the customer_type. What could be the issue here? I tested this with a db in sqlite with tables created by models and it works perfectly. So what is the issue</p>