Concatenate a specific field from child objects into parent object?

Question

I have two models:

class Chapter(models.Model):
    chapter = models.IntegerField()
    verses_amount = models.IntegerField()
    chapter_content = models.TextField()
    

class Verse(models.Model):
    chapter = models.ForeignKey(Chapter, on_delete=CASCADE)
    verse = models.TextField()
    verse_number = models.IntegerField()

On Verse, I store a paragraph in the verse field. How can I query all children (Verse), get their verse paragraph, concatenate all those and put that in Chapter.chapter_content?

Eg

verses = Verse.objects.filter()[0:2]
for verse in verses:
    print(verse.verse)

>> 'Lorem Ipsum.'
>> 'Second Lorem Ipsum Iteration 2.'

In the parent Chapter, I want chapter_content to hold the two values as one long string:

'Lorem Ipsum. Second Lorem Ipsum Iteration 2.'

Wouter · Accepted Answer · 2021-10-11T13:42:21.650

You could do something like this:

chapter = Chapter.objects.get(chapter=1)  # or whatever chapter you want to get

content = ""

for verse in chapter.verse_set.all():
   content += f"{verse.verse} "

content = content.rstrip()

chapter.chapter_content = content
chapter.save()

But it is bad practice to store the verse both in Verse objects as in Chapter objects, because what if you want to update just one verse?

You could try to make the chapter_content a method property in the Chapter model.

class Chapter(models.Model):
    chapter = models.IntegerField()
    verses_amount = models.IntegerField()

    @property
    def chapter_content(self):
        content = ""
        for verse in self.verse_set.all():
            content += f"{verse.verse} "
        return content.rstrip()
    

class Verse(models.Model):
    chapter = models.ForeignKey(Chapter, on_delete=CASCADE)
    verse = models.TextField()
    verse_number = models.IntegerField()

Note that you still have to sort the verses by their order number.

I like the `property` approach - will it work with django rest as well? Can it be used in the serialized response? — erikvm, Oct 11 '21 at 12:47
Yes, it will. See https://stackoverflow.com/questions/17066074/modelserializer-using-model-property — Wouter, Oct 11 '21 at 12:51
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/238025/discussion-between-erikvm-and-wouter). — erikvm, Oct 11 '21 at 13:15

Concatenate a specific field from child objects into parent object?

1 Answers1