title: Properly using Foreign Key references in search_fields, Django admin
body: <p>I've got a weird conundrum that I need some help with in Django 1.8.4 using python 3.4 in a virtual-env.</p>
  
  <p>I've got 2 models in 2 different apps... as follows with multiple Foreign Key references.</p>
  
  <p><strong>Inventory App</strong></p>
  
  <pre><code>class InventoryItem(models.Model):
      item_unique_code = models.CharField(max_length=256, blank=False, null=False)
      category = models.CharField(max_length=256, blank=False, null=False,choices=[('RAW','Raw Material'),('FG','Finished Good'),('PKG','Packaging')])
      name = models.CharField(max_length=64, blank=False, null=False)
      supplier = models.CharField(max_length=96, blank=False,null=False)
      approved_by = models.CharField(max_length=64, editable=False)
      date_approved = models.DateTimeField(auto_now_add=True, editable=False)
      comments = models.TextField(blank=True, null=True)
  
      def __str__(self):
          return "%s | %s | %s" % (self.item_unique_code,self.name,self.supplier)
  
      class Meta:
          managed = True
          unique_together = (('item_unique_code', 'category', 'name', 'supplier'),)
  </code></pre>
  
  <p><strong>Recipe App</strong></p>
  
  <pre><code>class RecipeControl(models.Model):
      #recipe_name choice field needs to be a query set of all records containing "FG-Finished Goods"
      recipe_name = models.ForeignKey(items.InventoryItem, related_name='recipe_name', limit_choices_to={'category': 'FG'})
      customer = models.ForeignKey(customers.CustomerProfile, related_name='customer')
      ingredient = models.ForeignKey(items.InventoryItem, related_name='ingredient')
      min_weight = models.DecimalField(max_digits=16, decimal_places=2, blank=True, null=True)
      max_weight = models.DecimalField(max_digits=16, decimal_places=2, blank=True, null=True)
      active_recipe = models.BooleanField(default=False)
      active_by = models.CharField(max_length=64, editable=False)
      revision = models.IntegerField(default=0)
      last_updated = models.DateTimeField(auto_now_add=True, editable=False)
  
      def __str__(self):
         return "%s" % (self.recipe_name)
  
      class Meta:
          managed = True
          unique_together = (('recipe_name', 'customer', 'ingredient'),)
  </code></pre>
  
  <p>I've been getting some weird results in my Recipe's Admin class...</p>
  
  <pre><code>from django.contrib import admin
  from django.contrib.auth.models import User
  from .models import RecipeControl
  from Inventory import models
  
  class RecipeView(admin.ModelAdmin):
      def save_model(self, request, obj, form, change): 
          obj.active_by = request.user.username
          obj.save()
  
      fieldsets = [
          ('Recipe Information',               {'fields': ['recipe_name', 'customer']}),
          ('Ingredients', {'fields': ['ingredient','min_weight','max_weight','active_recipe']}),
          ('Audit Trail', {'fields': ['active_by','revision','last_updated'],'classes':['collaspe']}),
      ]
  
      list_select_related = ['recipe_name','customer','ingredient']
      search_fields = ['recipe_name','customer','ingredient','active_by']
      readonly_fields = ('last_updated','active_by')
      list_display = ['recipe_name','customer','ingredient','min_weight','max_weight','last_updated','active_by', 'active_recipe']
  
  admin.site.register(RecipeControl, RecipeView)
  </code></pre>
  
  <p>The issue I've come across is if I try to do a search on any ForeignKey field, Django throws this error...</p>
  
  <pre><code>Exception Type: TypeError at /admin/Recipe/recipecontrol/
  Exception Value: Related Field got invalid lookup: icontains
  </code></pre>
  
  <p>According to the <a href="https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.search_fields" rel="noreferrer">Django Admin Doc's</a> and other older questions on stackoverflow on the subject it says I should be doing something along the lines of <em>search_fields = ['inventoryitem__name']</em> but I think this is in reference to FK's in the same app model.py. 
  Is there a more correct way of referencing/importing other models from other apps that I'm missing or do I have to use some kind of callable method magic to get the search function to look up correctly? I've tried a multitude of different combinations and nothing seems to work. I'm relatively new to Django so I'm confident it's something simple. </p>