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POMany to many field not shown in object while using signals to detect save operation in django
 

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  1. POMany to many field not shown in object while using signals to detect save operation in django
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    <p>This is a follow up question to : <a href="https://stackoverflow.com/questions/17257910/cant-get-post-save-to-work-in-django/17259611?noredirect=1#17259611">Cant get post_save to work in Django</a></p> <p><strong>My models are :</strong> </p> <pre><code>class Car(models.Model): name = models.CharField(max_length=50) ... some other attributes of Car ... class Person(models.Model): car = models.ManyToManyField(Car) name = models.CharField(max_lenght=100) ... Some other attributes of Person ... class License(models.Model): person = models.ForeignKey(Person) ... Other attributes of License ... </code></pre> <p><strong>Signal handler:</strong></p> <pre><code>def signal_handler(sender, **kwargs): print 'Person saved!' generate_license() post_save.connect(signal_handler, sender=Person, dispatch_uid="Unique person") </code></pre> <p><strong>The intent:</strong> When an instance of Person is created, I want to generate a License object. So I filter out the last tuple that was added into License and then use its contents to generate a License instance.</p> <pre><code>def generate_license(): import pdb pdb.set_trace() man = Person.objects.filter().order_by('-time_added')[:1][0] # Restricting the filter to one query and then taking the value at the 0th index. order_by '-time_added' gives the latest tuple at the top. license = License.objects.create(...Info about car, person...) </code></pre> <p><strong>The Error:</strong></p> <p>An example: Say <code>Car</code> has 3 instances: </p> <ol> <li>BMW</li> <li>FERRARI</li> <li>LAMBORGHINI</li> </ol> <p>Now when I add an instance of Person from the admin, example:</p> <p><code>per</code> be an instance with <code>car = BMW, FERRARI</code> and <code>name = Bob</code> When I click <code>save</code> in admin, the <code>set_trace()</code> starts. So after the query in <code>generate_license</code> :</p> <p>In <code>pdb</code>, when the query executes, I try printing out <code>per.car.all()</code> but it gives me <code>[]</code> and when I try printing out <code>per.name</code>, it does print out <code>Bob</code>. So I am not really getting as to how <code>per.name</code> is saved and <code>per.car</code> isnt.</p> <p>Also, when the request is completed, that is I have pressed <code>c</code> in <code>pdb</code>, I again click on save for the same instance, this time it reads the <code>per.car.all()</code> perfectly while if before saving, I added <code>LAMBORGHINI</code>, it only shows up <code>BMW</code> and <code>FERRARI</code>. So what I guess is happening is that the <code>many-to-many</code> field is coming a request late. Although I can't point out the reason for this. Need some help. Am I missing something?</p> <p><strong>Question:</strong> Is there a specific way to identify an <code>update signal</code> from a <code>create signal</code>? I mean I do not want to generate a new <code>License</code> every time a data is updated. A new <code>License</code> will only generate when the data has been created. So, how to distinguish between <code>update</code> and <code>save</code> signals?</p>
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