funcindex.py

# Copyright 2017, Markus Holtermann
# 
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# modification, are permitted provided that the following conditions are
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from django.db import connection
from django.db.models.indexes import Index

class FuncIndex(Index):
    suffix = 'func'

    def __init__(self, expression, name=None):
        self.expression, self.name = expression, name

    def create_sql(self, model, schema_editor):
        compiler = connection.ops.compiler('SQLCompiler')(None, connection, 'default')
        func_sql, params = compiler.compile(self.expression)
        params = tuple(map(schema_editor.quote_value, params))
        columns = func_sql % params
        quote_name = schema_editor.quote_name
        return schema_editor.sql_create_index % {
            'table': quote_name(model._meta.db_table),
            'name': quote_name(self.name),
            'columns': columns,
            'using': '',
            'extra': '',
        }


from django.db import models
from django.db.models.expressions import Ref, Value
from django.db.models.functions import Concat, Lower

from myapp.models import MyModel
      
with connection.schema_editor() as schema_editor:
    func_index = FuncIndex(
        Concat(
            Lower(Ref('some_field', None), output_field=models.CharField()),
            Value('blub'),
            Lower(Ref('other_field', None), output_field=models.CharField()),
        ),
        name='some_func_index'
    )
    print(func_index.create_sql(MyModel, schema_editor))
    # CREATE INDEX "some_func_index" ON "testapp_mymodel" (COALESCE(LOWER("some_field"), '') || COALESCE(COALESCE('blub', '') || COALESCE(LOWER("other_field"), ''), ''))