Postgres allows merging/concatenating two JSONB fields into one. Quoting the relevant Postgres docs:
jsonb || jsonb → jsonb - Concatenates two jsonb values.
As far as I can see, the Django ORM does not provide an operator for this type of concatenation. django.db.models.expressions.Combinable does not have a || operator.
django.contrib.postgres.search.SearchQueryCombinable has a || operator, but this class does not seem to apply here.
How can I merge two JSONFields using Django functions, but without evaluating the queryset?
Since expressions usually are of the mixin class Combinable, we can use the _combine method on an F field and customize the operator to be ||. ExpressionWrapper is necessary because Django cannot infer the output_field of our customized combination.
from django.db import models
from django.db.models import ExpressionWrapper, F
class MyModel(models.Model):
jsonfield1 = models.JSONField()
jsonfield2 = models.JSONField()
MyModel.objects.annotate(
concatenated_jsonfields=ExpressionWrapper(
F("jsonfield1")._combine(F("jsonfield2"), "||", reversed=False),
output_field=models.JSONField(),
)
)