title: django.db.utils.IntegrityError: 1452 'Cannot add or update a child row: a foreign key constraint fails
body: <p>I'm trying to link a foreignKey on provider and a M2Mfield on bestbuy_type - however each time I try saving anything to either of these fields I get the error:</p>
  
  <pre><code>(1452, 'Cannot add or update a child row: a foreign key constraint fails (`savingschampion`.`products_masterproduct`, CONSTRAINT `provider_id_refs_id_2ea9c584` FOREIGN KEY (`provider_id`) REFERENCES `products_provider` (`id`))')
  </code></pre>
  
  <p>The fields are specified in my model as:</p>
  
  <pre><code>class MasterProduct(BaseModel):
      provider = models.ForeignKey('products.Provider', related_name = 'master_products', blank=True, null=True)
      bestbuy_type = models.ManyToManyField('products.BestBuy',blank=True, null=True)
      ...other (no relationship) fields which work fine
  </code></pre>
  
  <p>Using this does actually populate the correct values in django admin for the fields however the error is produced on the save.</p>
  
  <p>Using MySQL and the engine specified is:</p>
  
  <pre><code>'ENGINE': 'django.db.backends.mysql'
  </code></pre>
  
  <p>Does anyone have any idea why this would be happening?</p>