Implement all applicable GeoInterface methods for DataFrames

src/io.jl

@@ -82,8 +82,14 @@ function read(ds, layer)
         rename!(df, Symbol("") => :geometry)
         replace!(gnames, Symbol("") => :geometry)
     end
-    metadata!(df, "crs", sr.ptr == C_NULL ? nothing : GFT.WellKnownText(GFT.CRS(), AG.toWKT(sr)), style=:default)
-    metadata!(df, "geometrycolumns", Tuple(gnames), style=:default)
+    println("HELLO")
+    crs = sr.ptr == C_NULL ? nothing : GFT.WellKnownText(GFT.CRS(), AG.toWKT(sr))
+    geometrycolumns = Tuple(gnames)
+    metadata!(df, "crs", crs, style=:default)
+    metadata!(df, "geometrycolumns", geometrycolumns, style=:default)    
+    # Also add the GEOINTERFACE:property as a namespaced thing
+    metadata!(df, "GEOINTERFACE:crs", crs, style=:default)
+    metadata!(df, "GEOINTERFACE:geometrycolumns", geometrycolumns, style=:default)
     return df
 end
 

src/utils.jl

@@ -10,7 +10,11 @@ function getgeometrycolumns(table)
     if GeoInterface.isfeaturecollection(table)
         return GeoInterface.geometrycolumns(table)
     elseif first(DataAPI.metadatasupport(typeof(table)))
-        return metadata(table, "geometrycolumns", (:geometry,))
+        gc = DataAPI.metadata(table, "GEOINTERFACE:geometrycolumns", nothing)
+        if isnothing(gc) # fall back to searching for "geometrycolumns" as a string
+            gc = DataAPI.metadata(table, "geometrycolumns", (:geometry,))
+        end
+        return gc
     else
         return (:geometry,)
     end
@@ -20,8 +24,62 @@ function getcrs(table)
     if GeoInterface.isfeaturecollection(table)
         return GeoInterface.crs(table)
     elseif first(DataAPI.metadatasupport(typeof(table)))
-        return metadata(table, "crs", nothing)
+        crs = DataAPI.metadata(table, "GEOINTERFACE:crs", nothing)
+        if isnothing(crs) # fall back to searching for "crs" as a string
+            crs = DataAPI.metadata(table, "crs", nothing)
+        end
+        return crs
     else
         return nothing
     end
 end
+
+# Override some GeoInterface functions specifically for DataFrames
+
+# These are the basic metadata definitions from which all else follows.
+
+function GeoInterface.crs(table::DataFrame)
+    crs = DataAPI.metadata(table, "GEOINTERFACE:crs", nothing)
+    if isnothing(crs) # fall back to searching for "crs" as a string
+        crs = DataAPI.metadata(table, "crs", nothing)
+    end
+    return crs
+end
+
+function GeoInterface.geometrycolumns(table::DataFrame)
+    gc = DataAPI.metadata(table, "GEOINTERFACE:geometrycolumns", nothing)
+    if isnothing(gc) # fall back to searching for "geometrycolumns" as a string
+        gc = DataAPI.metadata(table, "geometrycolumns", (:geometry,))
+        # TODO: we could search for columns named e.g. `:geometry`, `:geom`. `:shape`, and caps/lowercase versions.
+        # But should we?
+    end
+    return gc
+end
+
+# We don't define DataFrames as feature collections explicitly, since
+# that would complicate handling.  But, we can still implement the 
+# feature interface, for use in generic code.  And dispatch can always
+# handle a DataFrame by fixing the trait in a specialized method.
+
+# Here, we define a feature as a DataFrame row.
+function GeoInterface.getfeature(df::DataFrame, i::Integer)
+    return view(df, i, :)
+end
+# This is simply an optimized method, since we know what we have to do already.
+GeoInterface.getfeature(df::DataFrame) = eachrow(df)
+
+# The geometry is defined as the first of the geometry columns.
+# TODO: how should we choose between the geometry columns?
+function GeoInterface.geometry(row::DataFrameRow)
+    return row[first(GeoInterface.geometrycolumns(row))]
+end
+
+# The properties are all other columns.
+function GeoInterface.properties(row::DataFrameRow)
+    return row[DataFrames.Not(first(GeoInterface.geometrycolumns(row)))]
+end
+
+# Since `DataFrameRow` is simply a view of a DataFrame, we can reach back 
+# to the original DataFrame to get the metadata.
+GeoInterface.geometrycolumns(row::DataFrameRow) = GeoInterface.geometrycolumns(getfield(row, :df)) # get the parent of the row view
+GeoInterface.crs(row::DataFrameRow) = GeoInterface.crs(getfield(row, :df)) # get the parent of the row view

test/runtests.jl

@@ -199,7 +199,12 @@ unknown_crs = GFT.WellKnownText(GFT.CRS(), "GEOGCS[\"Undefined geographic SRS\",
         @test isfile(GDF.write(tfn, table))
         t = GDF.read(tfn)
         meta["crs"] = unknown_crs
-        @test metadata(t) == meta
+        meta["GEOINTERFACE:crs"] = unknown_crs
+        meta["GEOINTERFACE:geometrycolumns"] = meta["geometrycolumns"]
+        @test isempty(setdiff(keys(meta), metadatakeys(t)))
+        for pair in meta
+            @test metadata(t, pair.first, nothing) == pair.second
+        end
     end
 
 end