PostGIS ActiveRecord Adapter
The activerecord-postgis-adapter provides access to features of the PostGIS geospatial database from ActiveRecord. It extends the standard postgresql adapter to provide support for the spatial data types and features added by the PostGIS extension. It uses the RGeo library to represent spatial data in Ruby.
The adapter provides three basic capabilities:
First, it provides spatial migrations. It extends the ActiveRecord migration syntax to support creating spatially-typed columns and spatial indexes. You can control the various PostGIS-provided attributes such as srid, dimension, and geographic vs geometric math.
Second, it recognizes spatial types and casts them properly to RGeo geometry objects. The adapter can configure these objects automatically based on the srid and dimension in the database table, or you can tell it to convert the data to a different form. You can also set attribute data using WKT format.
Third, it lets you include simple spatial data in queries. WKT format data and RGeo objects can be embedded in where clauses.
The adapter requires PostgreSQL 9.0+.
Version 3.x supports ActiveRecord 4.2+
JRuby is not supported due to AR-JDBC not supporting 4.2 yet.
ActiveRecord 4.2+ Ruby 1.9.3+ PostGIS 2.0+
Version 2.x supports ActiveRecord 4.0.x and 4.1.x
If you are using version 2.x, you should read the version 2.x README
ActiveRecord 4.0.0 - 4.1.x Ruby 1.9.3+, JRuby PostGIS 2.0+
Gemfile for JRuby:
gem 'activerecord-postgis-adapter' gem 'activerecord-jdbcpostgresql-adapter', '~> 1.3.9' gem 'ffi-geos'
JRuby support for Rails 4.0 and 4.1 was added in version 2.2.0
Version 0.6.x supports ActiveRecord 3.x
If you are using version 0.6.x, you should read the version 0.6.x / 2.x README
ActiveRecord 3.x only Ruby 1.8.7+, JRuby, Rubinius PostGIS 1.5+
gem 'activerecord-postgis-adapter', '~> 0.6.6'
Please read PostGIS 1 Notes if you would like to use the adapter with an older version of PostGIS.
You must modify your
config/database.yml file to use the postgis
adapter. At minimum, you will need to change the
adapter field from
postgis. Recommended configuration:
development: username: your_username adapter: postgis host: localhost schema_search_path: public
If you have installed your PostGIS extension in a schema other than
is the default, add that schema to your
development: schema_search_path: public, postgis
Here are some other options that are supported:
development: adapter: postgis encoding: unicode postgis_extension: postgis # default is postgis schema_search_path: public,postgis pool: 5 database: my_app_development # your database name username: my_app_user # the username your app will use to connect password: my_app_password # the user's password su_username: my_global_user # a superuser for the database su_password: my_global_pasword # the superuser's password
This adapter uses the
rgeo gem, which has additional dependencies.
Please see the README documentation for
rgeo for more information: https://github.com/rgeo/rgeo
Creating a Spatial Rails App
This section covers starting a new Rails application from scratch. If you need to add geospatial capabilities to an existing Rails application (i.e. you need to convert a non-spatial database to a spatial database), see the section on "Upgrading a Database With Spatial Features" below.
To create a new Rails application using
activerecord-postgis-adapter, start by
using the postgresql adapter.
rails new my_app --database=postgresql
Add the adapter gem to the Gemfile:
Once you have set up your database config, run:
to create your development database. The adapter will add the PostGIS extension to your database.
Once you have installed the adapter, edit your
config/database.yml as described above.
Upgrading an Existing Database
If you have an existing Rails app that uses Postgres, and you want to add geospatial features, follow these steps.
First, add the
activerecord-postgis-adapter gem to the Gemfile, and update
your bundle by running
Next, modify your
config/database.yml file to invoke the postgis adapter, as
Once you have set up your database configs, run:
This rake task adds the PostGIS extension to your existing database.
Creating Spatial Tables
To store spatial data, you must create a column with a spatial type. PostGIS provides a variety of spatial types, including point, linestring, polygon, and different kinds of collections. These types are defined in a standard produced by the Open Geospatial Consortium. You can specify options indicating the coordinate system and number of coordinates for the values you are storing.
The activerecord-postgis-adapter extends ActiveRecord's migration syntax to support these spatial types. The following example creates five spatial columns in a table:
create_table :my_spatial_table do |t| t.column :shape1, :geometry t.geometry :shape2 t.line_string :path, srid: 3785 t.st_point :lonlat, geographic: true t.st_point :lonlatheight, geographic: true, has_z: true end
The first column, "shape1", is created with type "geometry". This is a general "base class" for spatial types; the column declares that it can contain values of any spatial type.
The second column, "shape2", uses a shorthand syntax for the same type as the shape1 column.
You can create a column either by invoking
column or invoking the name of the type directly.
The third column, "path", has a specific geometric type,
also specifies an SRID (spatial reference ID) that indicates which coordinate
system it expects the data to be in. The column now has a "constraint" on it;
it will accept only LineString data, and only data whose SRID is 3785.
The fourth column, "lonlat", has the
st_point type, and accepts only Point
data. Furthermore, it declares the column as "geographic", which means it
accepts longitude/latitude data, and performs calculations such as distances
using a spheroidal domain.
The fifth column, "lonlatheight", is a geographic (longitude/latitude) point that also includes a third "z" coordinate that can be used to store height information.
The following are the data types understood by PostGIS and exposed by activerecord-postgis-adapter:
:geometry-- Any geometric type
:st_point-- Point data
:line_string-- LineString data
:st_polygon-- Polygon data
:geometry_collection-- Any collection type
:multi_point-- A collection of Points
:multi_line_string-- A collection of LineStrings
:multi_polygon-- A collection of Polygons
Following are the options understood by the adapter:
:geographic-- If set to true, create a PostGIS geography column for longitude/latitude data over a spheroidal domain; otherwise create a geometry column in a flat coordinate system. Default is false. Also implies :srid set to 4326.
:srid-- Set a SRID constraint for the column. Default is 4326 for a geography column, or -1 for a geometry column. Note that PostGIS currently (as of version 2.0) requires geography columns to have SRID 4326, so this constraint is of limited use for geography columns.
:has_z-- Specify that objects in this column include a Z coordinate. Default is false.
:has_m-- Specify that objects in this column include an M coordinate. Default is false.
To create a PostGIS spatial index, add
using: :gist to your index:
add_index :my_table, :lonlat, using: :gist # or change_table :my_table do |t| t.index :lonlat, using: :gist end
Point and Polygon Types with ActiveRecord 4.2+
Prior to version 3, the
polygon types were supported. In ActiveRecord 4.2, the Postgresql
adapter added support for the native Postgresql
polygon types, which conflict with this
adapter's types of the same names. The PostGIS point type must be referenced as
st_point, and the
PostGIS polygon type must be referenced as
ActiveRecord's usefulness stems from the way it automatically configures classes based on the database structure and schema. If a column in the database has an integer type, ActiveRecord automatically casts the data to a Ruby Integer. In the same way, the activerecord-postgis-adapter automatically casts spatial data to a corresponding RGeo data type.
RGeo offers more flexibility in its type system than can be interpreted solely from analyzing the database column. For example, you can configure RGeo objects to exhibit certain behaviors related to their serialization, validation, coordinate system, or computation. These settings are embodied in the RGeo factory associated with the object.
You can configure the adapter to use a particular factory (i.e. a particular combination of settings) for data associated with each type in the database.
Here is an example:
RGeo::ActiveRecord::SpatialFactoryStore.instance.tap do |config| # By default, use the GEOS implementation for spatial columns. config.default = RGeo::Geos.factory_generator # But use a geographic implementation for point columns. config.register(RGeo::Geographic.spherical_factory(srid: 4326), geo_type: "point") end
For more explanation of
SpatialFactoryStore, see the rgeo-activerecord README
Working With Spatial Data
Of course, you're using this adapter because you want to work with geospatial data in your ActiveRecord models. Once you've installed the adapter, set up your database, and run your migrations, you can interact directly with spatial data in your models as RGeo objects.
RGeo is a Ruby implementation of the industry standard OGC Simple Features specification. It's a set of data types that can represent a variety of geospatial objects such as points, lines, polygons, and collections. It also provides the standard set of spatial analysis operations such as computing intersections or bounding boxes, calculating length or area, and so forth. We recommend browsing the RGeo documentation for a clearer understanding of its capabilities. For now, just note that the data values you will be working with are all RGeo geometry objects.
Reading and Writing Spatial Columns
When you access a spatial attribute on your ActiveRecord model, it is given to you as an RGeo geometry object (or nil, for attributes that allow null values). You can then call the RGeo api on the object. For example, consider the MySpatialTable class we worked with above:
record = MySpatialTable.find(1) p = record.lonlat # Returns an RGeo::Feature::Point puts p.x # displays the x coordinate puts p.geometry_type.type_name # displays "Point"
The RGeo factory for the value is determined by how you configured the
ActiveRecord class, as described above. In this case, we explicitly set a
spherical factory for the
factory = p.factory # returns a spherical factory
You can set a spatial attribute by providing an RGeo geometry object, or by providing the WKT string representation of the geometry. If a string is provided, the activerecord-postgis-adapter will attempt to parse it as WKT and set the value accordingly.
record.lonlat = 'POINT(-122 47)' # sets the value to the given point
If the WKT parsing fails, the value currently will be silently set to nil. In the future, however, this will raise an exception.
record.lonlat = 'POINT(x)' # sets the value to nil
If you set the value to an RGeo object, the factory needs to match the factory for the attribute. If the factories do not match, activerecord-postgis-adapter will attempt to cast the value to the correct factory.
p2 = factory.point(-122, 47) # p2 is a point in a spherical factory record.lonlat = p2 # sets the value to the given point record.shape1 = p2 # shape1 uses a flat geos factory, so it # will cast p2 into that coordinate system # before setting the value record.save
If, however, you attempt to set the value to the wrong type-- for example, setting a linestring attribute to a point value, you will get an exception from Postgres when you attempt to save the record.
record.path = p2 # This will appear to work, but... record.save # This will raise an exception from the database
You can create simple queries based on representational equality in the same way you would on a scalar column:
record2 = MySpatialTable.where(:lonlat => factory.point(-122, 47)).first
You can also use WKT:
record3 = MySpatialTable.where(:lonlat => 'POINT(-122 47)').first
Note that these queries use representational equality, meaning they return records where the lonlat value matches the given value exactly. A 0.00001 degree difference would not match, nor would a different representation of the same geometry (like a multipoint with a single element). Equality queries aren't generally all that useful in real world applications. Typically, if you want to perform a spatial query, you'll look for, say, all the points within a given area. For those queries, you'll need to use the standard spatial SQL functions provided by PostGIS.
A spatial database is one that includes a set of data types, functions, tables, and other objects related to geospatial data. When these objects are present in your database, you can use them to store and query spatial objects such as points, lines, and polygons.
PostGIS is an extension for PostgreSQL that provides definitions for the objects you need to add to a database to enable geospatial capabilities.
When you create your Rails database as described above in the section on installation and configuration, activerecord-postgis-adapter automatically invokes PostGIS to add the appropriate definitions to your database. You can determine whether your database includes the correct definitions by attempting to invoke the POSTGIS_VERSION function:
SELECT POSTGIS_VERSION(); # succeeds if PostGIS objects are present.
Standard spatial databases also include a table called
table includes a set of "spatial reference systems", or coordinate systems---
for example, WGS84 latitude and longitude, or Mercator Projection. Spatial
databases also usually include a table called
includes information on each database column that includes geometric data. In
recent versions of PostGIS,
geometry_columns is actually not a table but a
view into the system catalogs.
Development and Support
RDoc Documentation is available at http://rdoc.info/gems/activerecord-postgis-adapter
Contributions are welcome. See CONTRIBUTING.md for instructions.
Report issues at http://github.com/rgeo/activerecord-postgis-adapter/issues
Support is also available on the rgeo-users google group at http://groups.google.com/group/rgeo-users
Development is supported by:
This adapter implementation owes some debt to the spatial_adapter plugin (http://github.com/fragility/spatial_adapter). Although we made some different design decisions for this adapter, studying the spatial_adapter source gave us a head start on the implementation.
Copyright 2015 Daniel Azuma, Tee Parham