title: Django convert FK field from INT to CHAR in order to use FIXTURES
body: <p><strong>MODEL DEPENDENCIA</strong></p>
  <pre><code>class DependenciaManager(models.Manager):
      def get_by_natural_key(self, descripcion, departamento):
          return self.get(descripcion=descripcion, departamento=departamento)
  
  
  class Dependencia(models.Model):
      descripcion = models.CharField(max_length=300, blank=False, null=False)
      cod_institucion = models.PositiveIntegerField(
          blank=True,
          null=True,
      )
      departamento = models.ForeignKey(
          Departamento,
          on_delete=models.PROTECT,
          blank=False,
          null=True,
          to_field=&quot;descripcion&quot;,
      )
  
      objects = DependenciaManager()
  
      class Meta:
          constraints = [
              models.UniqueConstraint(
                  fields=[&quot;descripcion&quot;, &quot;departamento&quot;], name=&quot;unico_nombre_pordpto&quot;
              )
          ]
  
      def natural_key(self):
          return (self.descripcion, self.departamento)
  
      def __str__(self):
          return f&quot;{self.descripcion}&quot;
  </code></pre>
  <p><strong>MODEL PUESTO</strong></p>
  <pre><code>class Puesto(models.Model):
      numero = models.PositiveIntegerField(blank=False, null=False, primary_key=True)
      cargo = models.ForeignKey(
          Cargo,
          on_delete=models.PROTECT,
          blank=False,
          null=True,
          to_field=&quot;descripcion_cargo&quot;,
      )
      dependencia = models.ForeignKey(
          Dependencia,
          on_delete=models.PROTECT,
          blank=False,
          null=True,
          
      )
  
      def __str__(self):
          return f&quot;{self.numero}&quot;
  </code></pre>
  <p>What I'm trying to accomplish is to have the &quot;dependencia&quot; field on Puesto model as Charfield, in order to pass for example this &quot;[NTRA. SRA. DE LA ASUNCION,Capital]&quot; and that can be mapped to the Dependencia Model, in the fields &quot;Descripcion&quot; and &quot;Departamento&quot;.</p>
  <p>The &quot;descripcion&quot; field in &quot;Dependencia&quot; can't have the argument unique= True, that's why I use the constraint of unique together.</p>
  <p>Right now dependencia field in the puesto model is of type INT, because is pointing to ID (pk) in Dependencia model.</p>