mapmeld / graphpostgisql

GraphQL for PostGIS

GraphQL + PostGIS

Experiment with GraphQL, PostGIS, and OSM data

Very much based on Jason Dusek's Graphpostgresql

Install Prerequisites

Install PostGIS and Osm2pgsql. On my machine, I had to install osm2pgsql with a special parameter to include protocol buffers.

Download an OSM PBF file (find some at Metro Extracts)

Set up Database

Import an OSM PBF extract file into PostGIS database named "osm":

initdb pg_data
postgres -D pg_data &
createdb osm
psql -d osm -c "CREATE EXTENSION postgis;"
psql -d osm -c "CREATE EXTENSION postgis_topology;"
osm2pgsql -s -d osm import.osm.pbf

You need to have primary keys in your data to use GraphQL. OSM2PGSQL doesn't do this automatically, because a few items will have repeated invalid, negative osm_ids. Remove them before setting the primary key.

psql -d osm
DELETE FROM planet_osm_point WHERE osm_id < 0;
ALTER TABLE planet_osm_point ADD PRIMARY KEY (osm_id);
DELETE FROM planet_osm_line WHERE osm_id < 0;
ALTER TABLE planet_osm_line ADD PRIMARY KEY (osm_id);
DELETE FROM planet_osm_polygon WHERE osm_id < 0;
ALTER TABLE planet_osm_polygon ADD PRIMARY KEY (osm_id);
DELETE FROM planet_osm_roads WHERE osm_id < 0;
ALTER TABLE planet_osm_roads ADD PRIMARY KEY (osm_id);

Load the graphql schema file:

psql -d osm -c "\i graphql.sql"

Make queries

Here's a sample query looking up the id of all points:

SELECT graphql.run($$
  planet_osm_point { id }
$$);

Now to show the usefulness of GraphQL:

When the user clicks on a restaurant and I know the OSM id is 560983277, then I can query the database for the tags which are relevant to a restaurant in OSM data:

SELECT graphql.run($$
  planet_osm_point("560983277") {
    amenity,
    cuisine,
    internet_access,
    website,
    opening_hours
  }
$$);

If your OSM data extract didn't have some of these tags, it might not have the columns and fail. Just remove them from the query!

Thanks to PostGIS, you should be able to return the GeoJSON of a field:

SELECT graphql.run($$
  planet_osm_point("560983277") {
    name,
    ST_AsGeoJSON(way)
  }
$$);

This returns one JSON record, with the name, and the GeoJSON escaped in the "st_asgeojson" field:

{"name":"Bukkateen","st_asgeojson":"{\"type\":\"Point\",\"coordinates\":[831049.01,1010592.89]}"}

Most PostGIS functions with two arguments are accepted in the modified GraphQL set.

Not all of them make sense in the current state - this ST_Buffer query returns PostGIS geometry and not GeoJSON:

SELECT graphql.run($$
  planet_osm_polygon("213537579") {
    operator,
    ST_Buffer(way, 10.11)
  }
$$);

If you are using non-OSM PostGIS data, you could compare multiple fields in this record, for example you could return a calculated distance:

SELECT graphql.run($$
  geos("geo_id") {
    field,
    ST_Distance(startpt, endpt)
  }
$$);

I'm trying to get this example working, but the parser is concerned about POLYGON(())

SELECT graphql.run($$
  planet_osm_polygon("213537579") {
    operator,
    ST_Within(way, ST_GeomFromText('POLYGON((806850.5 1004992.93, 809346.74 1006557.97, 807892.59 1005245.2, 806850.5 1004992.93))'))
  }
$$);

Debug queries

For any query, use to_sql to see the SQL which you would be running:

SELECT graphql.to_sql($$
  planet_osm_point {
    name,
    amenity
  }
$$);

Responds with:

to_sql | SELECT json_agg("sub/1") AS planet_osm_point
       |   FROM planet_osm_point,
       |        LATERAL (
       |          SELECT planet_osm_point.id
       |        ) AS "sub/1"

License

GraphpostgresQL and this repo use the open source PostgresQL license.