from django.contrib.postgres.search import TrigramSimilarity
from django.db.models import Case, When, Q, IntegerField

def filter_by_trigram_similarity(qs, field_name, search_term, similarity=0.15):
    if not search_term:
        return qs

    similarity_name = field_name + '_similarity'
    strict_matching_name = field_name + '_strict_matching'

    qs = qs.annotate(
        **{
            strict_matching_name: Case(
                When(**{field_name + '__iexact': search_term, 'then': 3}),
                When(**{field_name + '__istartswith': search_term, 'then': 2}),
                When(**{field_name + '__icontains': search_term, 'then': 1}),
                output_field=IntegerField(),
                default=0
            )
        }
    )

    qs = qs.annotate(**{similarity_name: TrigramSimilarity(field_name, search_term)})

    qs = qs.filter(
        Q(**{similarity_name + '__gt': similarity}) |
        # following line is required if column's max length is big,
        # because in this case similarity can be less
        # than minimum similarity, but strict match exists
        Q(**{strict_matching_name + '__gt': 0})
    )

    return qs.order_by(
        '-' + strict_matching_name,
        '-' + similarity_name)