title: Django migrations - AddField is actually removing it
body: <p>I have a django blog project, and recently made changes to my local environment and so am now manually migrating to my production server. I've managed to nearly complete this, however when in psql / postgres I use the command:</p>
  
  <blockquote>
    <p>\d posts_post</p>
  </blockquote>
  
  <p>The foreign key field:</p>
  
  <blockquote>
    <p>"Category" ( models.ForeignKey('Category', null=True, blank=True)) </p>
  </blockquote>
  
  <p>Is nowhere to be seen as per the terminal. I've made another migration file as such:</p>
  
  <pre><code>from __future__ import unicode_literals
  from django.db import migrations, models
  import mptt.fields
  import django.utils.timezone
  import mptt
  from django.db import migrations
  
  class Migration(migrations.Migration):
      dependencies = [
          ('posts', '0016_remove_post_category'),
              ]
      operations = [
          migrations.AddField(
                  model_name="Post",
                  name = "Category",
                  field =  models.ForeignKey('Category', null=True, blank=True)
              ),
             ]
  </code></pre>
  
  <p>However when I run the migrate command it actually removes the field.</p>
  
  <p>a) Why does the manager remove my field instead of adding it?</p>
  
  <p>b) Why is it doing this despite apparently not existing in the first place?</p>