OSMAlchemy 0.1.post1

Non-goals

OSMAlchemy does not aim to replace large-scale OSM data frameworks like PostGIS, Osmosis or whatever. In fact, in terms of performance and otherwise, it cannot keep up with them.

If you are running a huge project that handles massive amounts of map data, has millions of requests or users, then OSMAlchemy is not for you (YMMV).

OSMAlchemy fills a niche for projects that have limited resources and cannot handle a full copy of planet.osm and an own API backend and expect to handle limited amounts of map data.

It might, however, be cool to use OSMAlchemy as ORM proxy with an own API backend. Who knows?

It might, as well, turn out that OSMAlchemy is an incredibly silly idea under all circumstances.

Installation

OSMAlchemy can be installed just like any other standard Python package by one of…

# pip3 install OSMAlchemy
# python3 setup.py install

…or what ever kind of distribution and install system you prefer.

Using plain SQLAlchemy

Make sure to get at least an engine from SQLAlchemy. Even better, get a declarative base and a scoped session:

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, scoped_session

engine = create_engine("sqlite:////tmp/foo.db")
base = declarative_base(bind=engine)
session = scoped_session(sessionmaker(bind=engine))

You can then initialise OSMAlchemy like so:

osmalchemy = OSMAlchemy((engine, base, session), overpass=True)

And probably install the databases:

base.metadata.create_all()

Using Flask-SQLAlchemy and Flask-Restless

Imagine you have an SQLAlchemy object from Flask-SQLAlchemy bound to your Flask application. called db, and a Flask-Restless API manager as manager:

from osmalchemy import OSMAlchemy
osm = OSMAlchemy(db, overpass=True)
db.create_all()
osm.create_api(manager)

You should now magically be able to query OSM via the REST API. Keep in mind that, with no filter provided, OSMAlchemy refuses to do automatic updates from Overpass. However, providing a query in the default JSON query way in Flask-Restless will give you live data and cache it in the database.

Limitations

Only some basic SQL queries are supported by the online update code. This is because compiling SQLAlchemy’s queries to OverpassQL is very complex. If you are very good at algorithms and building compilers, feel free to help us out!

The following kinds of queries are fully supported:

# A node with a specific id
session.query(osmalchemy.node).filter_by(id=12345).one()

# All nodes within a bounding box
session.query(osmalchemy.node).filter(
    and_(latitude>51.0, latitude<51.1, longitude>7.0, longitude<7.1)
).all()

# All nodes having a specific tag
session.query(osmalchemy.node).filter(
    osmalchemy.node.tags.any(key="name", value="Schwarzrheindorf Kirche")
).all()

You can go mad combining the two with and_() and or_(). You can also query for tags of ways and relations and for ways and relations by id.

Not supported (yet) are queries for ways or relations by coordinates. You also cannot query for nodes related to a way or anything related to a relation - having a way or a relation, accessing it will, however, magically pull and update the nodes and members and add them to the database:

# Get all nodes that are members of a (unique) named way
session.query(osmalchemy.way).filter(
    osmalchemy.way.tags.any(key="name", value="My Unique Way")
).one().nodes

This should, in reality, cover most use cases. If you encounter a use case that is not supported, please open an issue asking whether it can be supported (if you have an idea how it can be, please add it or even implement it and open a pull request).

Projects using OSMAlchemy

OSMAlchemy was designed for use in the Veripeditus Augmented Reality framework.

Code status