Implement crs and geometrycolumn fallbacks via DataAPI (#161)

* Implement crs and geometrycolumn fallbacks via DataAPI

* fix function name

* use type not object to query DataAPI

* Move core functionality to a new `metadata.jl`

* add aftercare for geometry columns

* add docs about geometrycolumns and crs for feature collections

* minor bugfixes + add tests

* avoid extra entry

* add constants that define what the keys are, for portability

Project.toml

@@ -4,10 +4,12 @@ authors = ["JuliaGeo and contributors"]
 version = "1.3.8"
 
 [deps]
+DataAPI = "9a962f9c-6df0-11e9-0e5d-c546b8b5ee8a"
 Extents = "411431e0-e8b7-467b-b5e0-f676ba4f2910"
 GeoFormatTypes = "68eda718-8dee-11e9-39e7-89f7f65f511f"
 
 [compat]
+DataAPI = "1"
 Extents = "0.1.1"
 GeoFormatTypes = "0.4"
 julia = "1"

docs/src/guides/developer.md

@@ -86,6 +86,22 @@ GeoInterface.geometrycolumns(::customcollection) = (:geometry,)  # can be multip
 
 The `geometrycolumns` enables other packages to know which field in a row, or column in a table, contains the geometry or geometries.
 
+It's important to note that the `geometrycolumns` should always return a `Tuple` of `Symbol`s.  However, it does have a fallback method
+that uses [DataAPI.jl](https://github.com/JuliaData/DataAPI.jl) metadata, if it exists, to retrieve the geometry columns.  This relies on
+the `"GEOINTERFACE:geometrycolumns"` metadata key.  GeoInterface.jl compatible writers may set this metadata key if writing to a format
+that does not have its own mechanism to store known geometry columns, like Arrow.
+
+Optionally, the `crs` method can also be implemented:
+```julia
+GeoInterface.crs(::customcollection)
+```
+
+This should return a `GeoFormatTypes.CoordinateReferenceSystem` type, such as `EPSG(code::Int)`, `WellKnownText(GeoFormatTypes.CRS(), wkt::String)`, 
+or `ProjString(p4::String)`.  See [GeoFormatTypes.jl](https://github.com/JuliaGeo/GeoFormatTypes.jl) for more information.
+
+The `crs` method also has a fallback that uses [DataAPI.jl](https://github.com/JuliaData/DataAPI.jl) metadata, if it exists, to retrieve the CRS.
+GeoInterface searches for the `"GEOINTERFACE:crs"` metadata key to retrieve the CRS.
+
 ## Geospatial Operations
 ```julia
 distance(geomtrait(a), geomtrait(b), a, b)

src/GeoInterface.jl

@@ -3,6 +3,7 @@ module GeoInterface
 using Extents: Extents, Extent
 using GeoFormatTypes: CoordinateReferenceSystemFormat
 using Base.Iterators: flatten
+import DataAPI
 
 export testgeometry, isgeometry, trait, geomtrait, ncoord, getcoord, ngeom, getgeom
 
@@ -55,6 +56,7 @@ include("fallbacks.jl")
 include("utils.jl")
 include("base.jl")
 include("wrappers.jl")
+include("metadata.jl")
 
 using .Wrappers
 using .Wrappers: geointerface_geomtype

src/fallbacks.jl

@@ -95,8 +95,11 @@ issimple(t::AbstractMultiPointTrait, geom) = allunique((getgeom(t, geom)))
 issimple(t::AbstractMultiCurveTrait, geom) = all(issimple.(getgeom(t, geom)))
 isclosed(t::AbstractMultiCurveTrait, geom) = all(isclosed.(getgeom(t, geom)))
 
-crs(::Nothing, geom) = nothing
-crs(::AbstractTrait, geom) = nothing
+"The key used to retrieve and store the CRS from DataAPI.jl metadata, if no other solution exists in that format."
+const GEOINTERFACE_CRS_KEY = "GEOINTERFACE:crs"
+
+crs(::Nothing, geom) = _get_dataapi_metadata(geom, GEOINTERFACE_CRS_KEY, nothing) # see `metadata.jl`
+crs(::AbstractTrait, geom) = _get_dataapi_metadata(geom, GEOINTERFACE_CRS_KEY, nothing) # see `metadata.jl`
 
 # FeatureCollection
 getfeature(t::AbstractFeatureCollectionTrait, fc) = (getfeature(t, fc, i) for i in 1:nfeature(t, fc))

src/interface.jl

@@ -38,13 +38,28 @@ automatically delegate to this method.
 isfeaturecollection(x::T) where {T} = isfeaturecollection(T)
 isfeaturecollection(::Type{T}) where {T} = false
 
+"""
+The key used to retrieve and store the geometrycolumns from DataAPI.jl metadata, if no other solution exists in that format.
+"""
+const GEOINTERFACE_GEOMETRYCOLUMNS_KEY = "GEOINTERFACE:geometrycolumns"
+
 """
     GeoInterface.geometrycolumns(featurecollection) => (:geometry,)
 
 Retrieve the geometrycolumn(s) of `featurecollection`; the fields (or columns in a table)
 which contain geometries that support GeoInterface.
+
+This is always a `Tuple` of `Symbol`s.
 """
-geometrycolumns(featurecollection) = (:geometry,)
+function geometrycolumns(featurecollection)
+    gcs = _get_dataapi_metadata(featurecollection, GEOINTERFACE_GEOMETRYCOLUMNS_KEY, (:geometry,)) # see `metadata.jl`
+    return _aftercare_geometrycolumns(gcs)
+end
+
+_aftercare_geometrycolumns(gcs::Tuple{Vararg{Symbol}}) = gcs
+_aftercare_geometrycolumns(gcs::Tuple{Vararg{String}}) = Symbol.(gcs)
+_aftercare_geometrycolumns(gcs::String) = (Symbol(gcs),)
+_aftercare_geometrycolumns(gcs::Symbol) = (gcs,)
 
 """
     GeoInterface.geometry(feat) => geom

src/metadata.jl

@@ -0,0 +1,23 @@
+# This file contains internal helper functions to get metadata from DataAPI objects.
+# At some point, it may also contain methods to set metadata.
+
+"""
+    Internal function.
+
+## Extended help
+
+    _get_dataapi_metadata(geom, key, default)
+
+Get metadata associated with key `key` from some object, `geom`, that has DataAPI.jl metadata support.
+
+If the object does not have metadata support, or the key does not exist, return `default`.
+"""
+function _get_dataapi_metadata(geom::GeomType, key, default) where GeomType
+    if DataAPI.metadatasupport(GeomType).read # check that the type has metadata, and supports reading it
+        if key in DataAPI.metadatakeys(geom) # check that the key exists
+            return DataAPI.metadata(geom, key; style = false) # read the metadata
+        end
+    end
+    return default
+end
+

test/test_dataapi.jl

@@ -0,0 +1,40 @@
+using Test
+using GeoInterface
+using GeoFormatTypes: EPSG
+using GeoInterface.DataAPI
+
+struct TestMetadata
+    geometrycolumns
+    crs
+end
+
+DataAPI.metadatasupport(::Type{TestMetadata}) = (; read = true, write = false)
+DataAPI.metadatakeys(::TestMetadata) = ("GEOINTERFACE:geometrycolumns", "GEOINTERFACE:crs")
+function DataAPI.metadata(x::TestMetadata, key::String; style::Bool=false)
+    if key === "GEOINTERFACE:geometrycolumns"
+        style ? (x.geometrycolumns, :note) : x.geometrycolumns
+    elseif key === "GEOINTERFACE:crs"
+        style ? (x.crs, :note) : x.crs
+    else
+        nothing
+    end
+end
+DataAPI.metadata(x::TestMetadata, key::String, default; style::Bool=false) = something(DataAPI.metadata(x, key; style), style ? (default, :note) : default)
+
+
+
+
+
+@testset "DataAPI" begin
+    td = TestMetadata((:g,), nothing)
+    @test GeoInterface.geometrycolumns(td) == (:g,)
+    @test GeoInterface.crs(td) === nothing
+
+    td = TestMetadata((:g,), EPSG(4326))
+    @test GeoInterface.geometrycolumns(td) == (:g,)
+    @test GeoInterface.crs(td) == EPSG(4326)
+
+    td = TestMetadata("geometry1", EPSG(4326))
+    @test GeoInterface.geometrycolumns(td) == (:geometry1,)
+    @test GeoInterface.crs(td) == EPSG(4326)
+end