Django startswith on fields - django

Question

let's say that I have an Address model with a postcode field. I can lookup addresses with postcode starting with "123" with this line:
Address.objects.filter(postcode__startswith="123")
Now, I need to do this search the "other way around". I have an Address model with a postcode_prefix field, and I need to retrieve all the addresses for which postcode_prefix is a prefix of a given code, like "12345". So if in my db I had 2 addresses with postcode_prefix = "123" and "234", only the first one would be returned.
Something like:
Address.objects.filter("12345".startswith(postcode_prefix))
The problem is that this doesn't work.
The only solution I can come up with is to perform a filter on the first char, like:
Address.objects.filter(postcode_prefix__startswith="12345"[0])
and then, when I get the results, make a list comprehension that filters them properly, like this:
results = [r for r in results if "12345".startswith(r.postcode_prefix)]
Is there a better way to do it in django?

Answer 1

Edit: This does not answer the original question but how to word a query the other way around.
I think what you are trying to do with your "something like" line is properly written as this:
Address.objects.filter(postcode__startswith=postcode_prefix)

Answer 2

In SQL terms, what you want to achieve reads like ('12345' is the postcode you are searching for):
SELECT *
FROM address
WHERE '12345' LIKE postcode_prefix||'%'
This is not really a standard query and I do not see any possibility to achieve this in Django using only get()/filter().
However, Django offers a way to provide additional SQL clauses with extra():
postcode = '12345'
Address.objects.extra(where=["%s LIKE postcode_prefix||'%%'"], params=[postcode])
Please see the Django documentation on extra() for further reference. Also note that the extra contains pure SQL, so you need to make sure that the clause is valid for your database.
Hope this works for you.

Answer 3

Bit of a mouthful but you can do this by annotating your search value and then filtering against it. All happens pretty quickly in-database.
from django.db.models import Value as V, F, CharField
Address.objects.exclude(
postcode_prefix=''
).annotate(
postcode=Value('12345', output_field=CharField())
).filter(
postcode__startswith=F('postcode_prefix')
)
The exclude is only necessary if postcode_prefix can be empty. This would result in an SQL like '%', which would match every postcode.
I'm sure you could do this via a nice templated function these days too... But this is clean enough for me.

Answer 4

A possible alternative. (Have no idea how it compares to the accepted solution with a column as the second param to like, in execution time)
q=reduce(lambda a,b:a|b, [Q(postcode__startswith=postcode[:i+1]) for i in range(len(postcode))])
Thus, you generate all prefixes, and or them together...

Answer 5

The raw SQL query that would do that you need looks something like this:
select * from postal_code_table where '1234567' like postal_code||'%'
This query will select any postal_code from your table that is a substring of '1234567' and also must start from begining, ie: '123', '1234', etc.
Now to implement this in Django, the preferred method is using a custom look up:
from django.db.models.fields import Field
from django.db.models import Lookup
#Field.register_lookup
class LowerStartswithContainedBy(Lookup):
'''Postgres LIKE query statement'''
lookup_name = 'istartswithcontainedby'
def as_sql(self, compiler, connection):
lhs, lhs_params = self.process_lhs(compiler, connection)
rhs, rhs_params = self.process_rhs(compiler, connection)
params = lhs_params + rhs_params
return f"LOWER({rhs}) LIKE LOWER({lhs}) || '%%'", params
Now you can write a django query such as the following:
PostCode.objects.filter(code__istartswithcontainedby='1234567')
Similarly, if you are just looking for substring and do not require the startswith condition, simply modify the return line of as_sql method to the following:
return f"LOWER({rhs}) LIKE '%%' || LOWER({lhs}) || '%%'", params
For more detailed explanation, see my git gist Django custom lookup

Answer 6

A. If not the issue https://code.djangoproject.com/ticket/13363,
you could do this:
queryset.extra(select={'myconst': "'this superstring is myconst value'"}).filter(myconst__contains=F('myfield'))
Maybe, they will fix an issue and it can work.
B. If not the issue 16731 (sorry not providing full url, not enough rep, see another ticket above) you could filter by fields that added with '.annotate', with creation of custom aggreation function, like here:
http://coder.cl/2011/09/custom-aggregates-on-django/
C. Last and successful. I have managed to do this using monkeypatching of the following:
django.db.models.sql.Query.query_terms
django.db.models.fields.Field.get_prep_lookup
django.db.models.fields.Field.get_db_prep_lookup
django.db.models.sql.where.WhereNode.make_atom
Just defined custom lookup '_starts', which has reverse logic of '_startswith'