Django full text search: TrigramDistance not working - django

Question

I am using Django 2.0 and postgresql 9.6.
I have installed pg_trgm extension in the postgresql
I am trying to understand how triagram works: I am trying the following. I want words nearer to "sridam" to be matched using triagram
from django.contrib.postgres.search import TrigramDistance
from django.contrib.postgres.search import SearchQuery, SearchRank, SearchVector
vector = SearchVector('title',config='english_unaccent', weight='A') + SearchVector('description',config='english_unaccent', weight='B')
query = SearchQuery('sridam',config='english_unaccent')
Article.objects.annotate(distance=TrigramDistance(vector, query)).filter(distance__lte=0.7).order_by('distance')
ProgrammingError: operator does not exist: tsvector <-> tsquery
LINE 1: ...SCE("articles_article"."description", '')), 'B')) <-> plaint...
^
HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts.

Answer 1

You can't calculate the Trigram distance between a SearchVector and a SearchQuery because "A trigram is a group of three consecutive characters.".
As specified in the Django documentation TrigramDistance "accepts a field name or expression, and a string or expression. Returns the Trigram distance between the two arguments.".

Answer 2

Before using postgres extensions you have to apply extension migrations to your model.
SOLUTION 1
Make empty migration file:
python3 manage.py makemigrations --empty yourapp
then go to the new migration file and add following
from django.db import migrations
from django.contrib.postgres.operations import TrigramExtension
class Migration(migrations.Migration):
operations = [
TrigramExtension()
]
Finally run python3 manage.py migrate to apply migrations to your database and you are ready to use the extension.
SOLUTION 2
You can go to psql console and apply migrations directly:
CREATE EXTENSION pg_trgm;