JSON field increment with F expressions

Author: TanyaAng

requirements/local.txt

@@ -19,5 +19,5 @@ boto3-stubs==1.26.81
 
 flake8==6.0.0
 isort==5.12.0
-black==23.1.0
+black==23.3.0
 pre-commit==3.2.2

styleguide_example/blog_examples/f_expressions/__init__.py

@@ -0,0 +1,30 @@
+from django.db.models import CharField, F, IntegerField, JSONField, Value
+from django.db.models.expressions import Func
+from django.db.models.functions import Cast
+
+"""
+An F() object represents the value of a model field, transformed value of a model field, or annotated column.
+It makes it possible to refer to model field values and perform database operations using them without actually
+having to pull them out of the database into Python memory.
+"""
+
+
+class JSONIncrement(Func):
+    function = "jsonb_set"
+
+    def __init__(self, full_path, value=1, **extra):
+        field_name, *key_path_parts = full_path.split("__")
+
+        if not field_name:
+            raise ValueError("`full_path` can not be blank.")
+
+        if len(key_path_parts) < 1:
+            raise ValueError("`full_path` must contain at least one key.")
+
+        key_path = ",".join(key_path_parts)
+
+        new_value_expr = Cast(Cast(F(full_path), IntegerField()) + value, CharField())
+
+        expressions = [F(field_name), Value(f"{{{key_path}}}"), Cast(new_value_expr, JSONField())]
+
+        super().__init__(*expressions, output_field=JSONField(), **extra)

styleguide_example/blog_examples/f_expressions/service.py

@@ -0,0 +1,21 @@
+# Generated by Django 4.1.6 on 2023-05-09 11:10
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('blog_examples', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='SomeDataModel',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(blank=True, max_length=255)),
+                ('stored_field', models.JSONField(blank=True)),
+            ],
+        ),
+    ]

styleguide_example/blog_examples/migrations/0002_somedatamodel.py

@@ -29,3 +29,13 @@ class TimestampsOpinionated(models.Model):
 
     created_at = models.DateTimeField(default=timezone.now)
     updated_at = models.DateTimeField(blank=True, null=True)
+
+
+class SomeDataModel(models.Model):
+    name = models.CharField(
+        max_length=255,
+        blank=True,
+    )
+    stored_field = models.JSONField(
+        blank=True,
+    )

styleguide_example/blog_examples/models.py

@@ -0,0 +1,35 @@
+from django.test import TestCase
+
+from styleguide_example.blog_examples.f_expressions.service import JSONIncrement
+from styleguide_example.blog_examples.models import SomeDataModel
+
+
+class JSONIncrementTests(TestCase):
+    def setUp(self) -> None:
+        self.first_entity_stored_field = {"first_key": 1, "second_key": 2, "third_key": 3}
+        self.second_entity_stored_field = {"first_key": 4, "second_key": 5, "third_key": 6}
+        self.increment_value = 10
+
+    def save_model_entities(self):
+        entities = SomeDataModel.objects.bulk_create(
+            [
+                SomeDataModel(name="First name", stored_field=self.first_entity_stored_field),
+                SomeDataModel(name="Second name", stored_field=self.second_entity_stored_field),
+            ]
+        )
+        return entities
+
+    def test_json_increment_in_model(self):
+        self.save_model_entities()
+
+        SomeDataModel.objects.filter(name="First name").update(
+            stored_field=JSONIncrement("stored_field__first_key", self.increment_value)
+        )
+
+        changed_entity = SomeDataModel.objects.first()
+        second_entity = SomeDataModel.objects.last()
+
+        self.assertEqual(
+            self.first_entity_stored_field["first_key"] + self.increment_value, changed_entity.stored_field["first_key"]
+        )
+        self.assertEqual(self.second_entity_stored_field["first_key"], second_entity.stored_field["first_key"])