add colmetadata support (#311)

src/StructArrays.jl

@@ -25,6 +25,19 @@ function refvalue(s::StructArray{T}, v::Tup) where {T}
     createinstance(T, map(refvalue, components(s), v)...)
 end
 
+# implement colmetadata for StructArray based on metadata of individual columns
+import DataAPI: metadata, metadatasupport, colmetadata, colmetadatasupport
+
+colmetadatasupport(::Type{T}) where {T<:StructArray} = (
+    read=any(col -> metadatasupport(col).read, array_types(T).parameters),
+    write=false,  # not implemented
+)
+colmetadata(sa::StructArray, col::Symbol) = metadata(getproperty(sa, col))
+colmetadata(sa::StructArray) =
+    map(Tables.columns(sa)) do col
+        metadatasupport(typeof(col)).read ? metadata(col) : nothing
+    end
+
 @static if !isdefined(Base, :get_extension)
     include("../ext/StructArraysAdaptExt.jl")
     include("../ext/StructArraysGPUArraysCoreExt.jl")

test/runtests.jl

@@ -1190,6 +1190,24 @@ end
     end
 end
 
+@testset "metadata" begin
+    using DataAPI: colmetadatasupport, colmetadata, metadata, DataAPI
+
+    struct MyArray <: AbstractVector{Int} end
+    Base.size(::MyArray) = (2,)
+    DataAPI.metadatasupport(::Type{<:MyArray}) = (read=true, write=false)
+    DataAPI.metadata(::MyArray) = (x=1, y="2")
+
+    sa = StructArray(a=[1,2], b=[1,2])
+    @test colmetadatasupport(typeof(sa)) == (read=false, write=false)
+
+    sa = StructArray(a=MyArray(), b=[1,2])
+    @test metadata(sa.a) == (x=1, y="2")
+    @test colmetadatasupport(typeof(sa)) == (read=true, write=false)
+    @test colmetadata(sa, :a) == (x=1, y="2")
+    @test colmetadata(sa) == (a=(x=1, y="2"), b=nothing)
+end
+
 struct ArrayConverter end
 
 Adapt.adapt_storage(::ArrayConverter, xs::AbstractArray) = convert(Array, xs)