Fix Tables schema for empty layers (#441)

* Fix empty layer handling.

* Add tests. Fix macro escaping.

Author: evetion

src/tables.jl

@@ -1,5 +1,25 @@
-function Tables.schema(layer::AbstractFeatureLayer)::Nothing
-    return nothing
+function Tables.schema(
+    layer::AbstractFeatureLayer,
+)::Union{Nothing,Tables.Schema}
+    # If the layer has no features, calculate the schema from the layer
+    # otherwise let the features build the schema on the fly
+    # If we always build the schema, all isnullable (by default true) fields
+    # will result in columns with Union{Missing}.
+    nfeature(layer) == 0 || return nothing
+    ld = layerdefn(layer)
+    geom_names, field_names, _, fielddefns = schema_names(ld)
+    names = (geom_names..., field_names...)
+    types = Type[_datatype(getgeomdefn(ld, i - 1)) for i in 1:ngeom(ld)]
+    append!(types, map(_datatype, fielddefns))
+    return Tables.Schema(names, types)
+end
+
+function _datatype(fielddefn::IFieldDefnView)
+    return T = convert(DataType, getfieldtype(fielddefn))
+end
+
+function _datatype(fielddefn::IGeomFieldDefnView)
+    return IGeometry{gettype(fielddefn)}
 end
 
 Tables.istable(::Type{<:AbstractFeatureLayer})::Bool = true

src/utils.jl

@@ -207,23 +207,23 @@ macro ogrerr(code, message)
                 "Unknown error."
             end
 
-            error($message * " ($detailmsg)")
+            error($(esc(message)) * " ($detailmsg)")
         end
     end
 end
 
 macro cplerr(code, message)
     return quote
         if $(esc(code)) != GDAL.CE_None
-            error($message)
+            error($(esc(message)))
         end
     end
 end
 
 macro cplwarn(code, message)
     return quote
         if $(esc(code)) != GDAL.CE_None
-            @warn $message
+            @warn $(esc(message))
         end
     end
 end

test/test_tables.jl

@@ -786,6 +786,45 @@ using Tables
                     )
                 end
             end
+            @testset "Handle empty layers" begin
+                AG.read(
+                    joinpath(@__DIR__, "data/multi_geom.csv"),
+                    options = [
+                        "GEOM_POSSIBLE_NAMES=point,linestring",
+                        "KEEP_GEOM_COLUMNS=NO",
+                    ],
+                ) do ds
+                    layer = AG.copy(AG.getlayer(ds, 0))
+
+                    # With features, no schema is defined
+                    @test isnothing(Tables.schema(layer))
+                    # And tables are built up from features
+                    table = Tables.columntable(layer)
+                    @test Tables.columnnames(table) ==
+                          (:point, :linestring, :id, :zoom, :location)
+                    @test Tables.rowcount(table) == 2
+
+                    for i in 1:AG.nfeature(layer)
+                        AG.deletefeature!(layer, i)
+                    end
+
+                    # Without features, schema is still defined and table is empty
+                    @test Tables.schema(layer) == Tables.Schema(
+                        (:point, :linestring, :id, :zoom, :location),
+                        (
+                            AG.IGeometry{AG.wkbUnknown},
+                            AG.IGeometry{AG.wkbUnknown},
+                            String,
+                            String,
+                            String,
+                        ),
+                    )
+                    table = Tables.columntable(layer)
+                    @test Tables.rowcount(table) == 0
+                    @test Tables.columnnames(table) ==
+                          (:point, :linestring, :id, :zoom, :location)
+                end
+            end
 
             clean_test_dataset_files()
         end

test/test_utils.jl

@@ -3,9 +3,9 @@ import GDAL
 import ArchGDAL as AG
 
 "Test both that an ErrorException is thrown and that the message is as expected"
-function eval_ogrerr(err, expected_message)
-    @test (@test_throws ErrorException AG.@ogrerr err "e:").value.msg ==
-          "e: ($expected_message)"
+function eval_ogrerr(err, expected_message, placeholder = "")
+    @test (@test_throws ErrorException AG.@ogrerr err "e $(placeholder):").value.msg ==
+          "e $(placeholder): ($expected_message)"
 end
 
 @testset "test_utils.jl" begin
@@ -18,7 +18,7 @@ end
 
     @testset "OGR Errors" begin
         @test isnothing(AG.@ogrerr GDAL.OGRERR_NONE "not an error")
-        eval_ogrerr(GDAL.OGRERR_NOT_ENOUGH_DATA, "Not enough data.")
+        eval_ogrerr(GDAL.OGRERR_NOT_ENOUGH_DATA, "Not enough data.", "foo")
         eval_ogrerr(GDAL.OGRERR_NOT_ENOUGH_MEMORY, "Not enough memory.")
         eval_ogrerr(
             GDAL.OGRERR_UNSUPPORTED_GEOMETRY_TYPE,