Django bulk update with string replace

Question

I am trying to update and modify a string field Django's ORM. The equivalent SQL to do this is:

UPDATE example_table SET string_field = REPLACE(string_field, 'old text', 'new text');

With that query, I expect old text and old text more text to be replaced with new text and new text more text respectively, for all the entries in the string_field column.

Bulk update() seems promising, but doesn't allow me to modify only part of the field, and F() expressions only implement numeric changes, not string replace. I also looked at using raw queries to run the above SQL, but that seems like a sideways hack (especially since F() exists to do the same functionality on numbers), and I couldn't get them to actually execute.

I ended up with this, but it seems a shame to execute all the extra queries when I know there's a one line SQL statement to do it.

for entry in ExampleModel.objects.all():
    entry.string_field = entry.string_field.replace('old text', 'new text', 1)
    entry.save()

Does this feature not exist in Django's ORM for strings yet? Is there something I overlooked in the docs?

Related SO questions:

• generate update query using django orm
• SQL statement to remove part of a string

Answer 1

Tested with django 1.9

from django.db.models import F, Func, Value

ExampleModel.objects.filter(<condition>).update(
    string_field=Func(
        F('string_field'),
        Value('old text'), Value('new text'),
        function='replace',
    )
)

Answer 2

You could make your own F-like object to represent the string replacing in SQL. Here is a proof of concept:

from django.db.models.expressions import ExpressionNode

class StringReplaceF(ExpressionNode):
    def __init__(self, field, replace_from, replace_to):
        self.field = field
        self.replace_from = replace_from
        self.replace_to = replace_to
        super(StringReplaceF, self).__init__()

    def evaluate(self, evaluator, qn, connection):
        return (
            "REPLACE({}, %s, %s)".format(self.field),
            (self.replace_from, self.replace_to)
        )

 >>> f = StringReplaceF('string_field', 'old text', 'new text')
 >>> ExampleModel.objects.update(string_field=f)

You'd need to do a bit more work with the class if you need it to behave nicely with other F objects, but then again, the existing F objects don't seem to work with strings anyway.