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Logical or of Django many to many queries returns duplicate results

Posted by: admin December 31, 2017 Leave a comment

Questions:

I have models with many to many relationships like this:

class Contact(models.Model):
    name = models.TextField()
    address = models.TextField()

class Mail(models.Model):
    to = models.ManyToManyField(Contact, related_name='received_mails')
    cc = models.ManyToManyField(Contact, related_name='cced_mails')

I want to obtain the set of contacts that are in either the to field or the cc field for a given email. Let’s try:

>>> Contact.objects.filter(received_mails__id=111)
[<Contact: [email protected]>]
>>> Contact.objects.filter(cced_mails__id=111)
[<Contact: [email protected]>]

So far so good. We have one contact for each relationship. But it would be nice to get them both into the same QuerySet.

>>> Contact.objects.filter(Q(received_mails__id=111) | Q(cced_mails__id=111))
[<Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, <Contact: [email protected]>, '...(remaining elements truncated)...']

What happened? I have a feeling it is something to do with joining tables in SQL, but I don’t really understand what is happening under the hood with many to many relationships. It could be that what I am trying to do is stupid, or that there is an easy way to do it. Either way, I’m happy to be set on the right path.

Edit: this is the query of the QuerySet:

SELECT `mailshareapp_contact`.`id`, `mailshareapp_contact`.`name`,
`mailshareapp_contact`.`address` FROM `mailshareapp_contact`
LEFT OUTER JOIN `mailshareapp_mail_to`
ON (`mailshareapp_contact`.`id` = `mailshareapp_mail_to`.`contact_id`)
LEFT OUTER JOIN `mailshareapp_mail_cc`
ON (`mailshareapp_contact`.`id` = `mailshareapp_mail_cc`.`contact_id`)
WHERE (`mailshareapp_mail_to`.`mail_id` = 111
OR `mailshareapp_mail_cc`.`mail_id` = 111 )
Answers:

As SQL returns all matching records, Django dutifully maps them to objects. What you’re looking for is the .distinct() queryset method that makes SQL collapse all duplicate rows into one.