django-expression-index

django-expression-index provides implementation of subclass of django.db.models.Index, which enables indexing tables using expressions.

In Django 3.2 this solution is obsoleted by built-in support of expression index.

What problem does this solve?

Currently django.db.models.Index only accepts field names in fields parameter. There is no way to add expression index other than using raw SQL.

This project implements ExpressionIndex class, which accepts list of any django.db.models.expressions.Expression in its expressions parameter.

How to use it?

Here is an example of adding index based on lowercased models.CharField value.

from django.db import models
from django.db.models.functions import Lower
from django_expression_index import ExpressionIndex

class Profile(models.Model):
    name = models.CharField(max_length=255)
    
    class Meta:
        indexes = [
            ExpressionIndex(expressions=[Lower('name')])
        ]

After adding ExpressionIndex to your indexes, run makemigrations and migrate commands. The following SQL code will be generated and executed on your database:

CREATE TABLE "myapp_profile" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "name" varchar(255) NOT NULL);
CREATE INDEX "myapp_profile_9a3539_idx" ON "myapp_profile" (LOWER("name"));

ExpressionIndex constructor replaces fields parameter with expressions parameter. All remaining parameters are relayed to django.db.models.Index constructor.

How does it work?

ExpressionIndex overrides create_sql method and uses django's default query compiler to render the expression.

There is a monkey-patch implemented on django.db.models.sql.query.Query instance, which replaces resolve_ref. The patch forces using SimpleCol instead of Col class to render bare field names referred by the expression, without prefixing them with table name.

    def compile_expression(self, expression, compiler):
        def resolve_ref(original, name, allow_joins=True, reuse=None, summarize=False, simple_col=False):
            return original(name, allow_joins, reuse, summarize, True)
        
        query=compiler.query
        query.resolve_ref=partial(resolve_ref, query.resolve_ref)
        expression=expression.resolve_expression(query, allow_joins=False)
        sql, params=expression.as_sql(compiler, compiler.connection)
        return sql % params

If you know a better solution, please let me know!

Compatibility

It was tested with Django 2.2.13 and 3.x. In release 0.2.1 code was updated to make it compatible with Django 3.2