djorm-ext-pgtrgm

Django pgtrgm is a Django application with some addons regarding PostgreSQL trigram (or trigraph) text comparison

A trigram is a group of three consecutive characters taken from a string. We can measure the similarity of two strings by counting the number of trigrams they share. This simple idea turns out to be very effective for measuring the similarity of words in many natural languages.

For example, the set of trigrams in the string "cat" is " c", " ca", "cat", and "at ".

With this extension, we could use this feature inside Django ORM with a simple QuerySet filter keyword

It is distributed under the terms of the GNU Lesser General Public License

PostgreSQL syntax

text % text -> boolean Returns true if its arguments have a similarity that is greater than the current similarity threshold set by set_limit.

How to install and use pg_trgm extension in PostgreSQL 9.2

Requeriments

• Django 1.5 (or higher). It's possible that works with other versions lower 1.5
• Postgres pg_trgm extension. In debian/ubuntu: sudo apt-get install postgres-contrib

Installation

• In your settings.py

    INSTALLED_APPS = (

        'djorm_pgtrgm',
    )

• You need to register the extension in your database. Run ./manage.py dbshell and then execute:

    CREATE EXTENSION pg_trgm;

• Optionally, you can create an index over a text column for the purpose of very fast similarity searches. For example, supose you will filter a lot over the field description of the model myapp.Product:

    CREATE INDEX desctiption_trgm_idx ON myapp_product USING gist (description gist_trgm_ops);

Usage

Adds a new query set filter keyword to allow text searching.

    MyModel.objects.filter(field_name__similar='whatever')

To ensure results ordered by similarity, you could do this:

    MyModel.objects.filter(field_name__similar='whatever').
                      extra(select={'distance': "similarity(name, 'whatever')"}).
                      order_by('-distance'))

In a sake of brevity, you could use a provided SimilarManager that has a filter_o method.

    from djorm_pgtrgm import SimilarManager

    class MyModel(models.Model):
        objects = SimilarManager()

        # your fields
        ...

filter_o is a shortcut for the filter + extra + order_by in the snippet above.

    MyModel.objects.filter_o(field_name__similar='whatever')

So, this will return every similar MyModel instance with a field_name similar to 'whatever' and sorted by the distance ot each intance's field_name value to target 'whatever'. In addition, an extra field field_name_distance is added to each item in the queryset.

Development

You can get the last version of djorm-ext-pgtrgm by doing a clone of its repository:

git clone git://github.com/jleivaizq/djorm-ext-pgtrgm.git