Dear Team,

In 

In the following function `django_mongodb_backend.indexes.get_pymongo_index_model` we are adding the partial filter expression on type of the field if the index to be added is declared unique in the models file.

if unique:
    kwargs["unique"] = True
    # Indexing on $type matches the value of most SQL databases by
    # allowing multiple null values for the unique constraint.
    if field:
        column = column_prefix + field.column
        filter_expression[column].update({"$type": field.db_type(schema_editor.connection)})
    else:
        for field_name, _ in self.fields_orders:
            field_ = model._meta.get_field(field_name)
            filter_expression[field_.column].update(
                {"$type": field_.db_type(schema_editor.connection)}
            )

This results in making the index partial on Atlas and simple queries like `Book.objects.get(name='Mongo DB')` does not use the created index. I verified this on Atlas showing index usages as 0 and through query plans too. Remove the index and creating it manually on Atlas solved the problem but then I will have to do it everytime new migration is added with unique index or unique constraint.

Am I doing something wrong while writing the queries or is it an expected issue?