Add reading metadata from Arrow.Table (#481)

Partially solves #337 for
`Arrow.Table`.

src/table.jl

@@ -363,6 +363,53 @@ See [the official Arrow documentation for more details on custom application met
 getmetadata(t::Table) = getfield(t, :metadata)[]
 getmetadata(::Any) = nothing
 
+DataAPI.metadatasupport(::Type{Table}) = (read=true, write=false)
+DataAPI.colmetadatasupport(::Type{Table}) = (read=true, write=false)
+
+function DataAPI.metadata(t::Table, key::AbstractString; style::Bool=false)
+    meta = getmetadata(t)[key]
+    return style ? (meta, :default) : meta
+end
+
+function DataAPI.metadata(t::Table, key::AbstractString, default; style::Bool=false)
+    meta = getmetadata(t)
+    if meta !== nothing
+        haskey(meta, key) && return style ? meta[key] : (meta[key], :default)
+    end
+    return style ? (default, :default) : default
+end
+
+function DataAPI.metadatakeys(t::Table)
+    meta = getmetadata(t)
+    meta === nothing && return ()
+    return keys(meta)
+end
+
+function DataAPI.colmetadata(t::Table, col, key::AbstractString; style::Bool=false)
+    meta = getmetadata(t[col])[key]
+    return style ? (meta, :default) : meta
+end
+
+function DataAPI.colmetadata(t::Table, col, key::AbstractString, default; style::Bool=false)
+    meta = getmetadata(t[col])
+    if meta !== nothing
+        haskey(meta, key) && return style ? (meta[key], :default) : meta[key]
+    end
+    return style ? (default, :default) : default
+end
+
+function DataAPI.colmetadatakeys(t::Table, col)
+    meta = getmetadata(t[col])
+    meta === nothing && return ()
+    return keys(meta)
+end
+
+function DataAPI.colmetadatakeys(t::Table)
+    return (col => DataAPI.colmetadatakeys(t, col) for
+            col in Tables.columnnames(t) if
+            getmetadata(t[col]) !== nothing)
+end
+
 Tables.istable(::Table) = true
 Tables.columnaccess(::Table) = true
 Tables.columns(t::Table) = Tables.CopiedColumns(t)

test/runtests.jl

@@ -1070,4 +1070,44 @@ end
             @test tbl.col16[1] == Dates.Time(0, 0, 0)
         end
     end # @testset "misc"
+
+    @testset "DataAPI.metadata" begin
+        df = DataFrame(a=1, b=2, c=3)
+        for i in 1:2
+            io = IOBuffer()
+            if i == 1 # skip writing metadata in the first iteration
+                Arrow.write(io, df)
+            else
+                Arrow.write(io, df, metadata=metadata(df), colmetadata=colmetadata(df))
+            end
+            seekstart(io)
+            tbl = Arrow.Table(io)
+
+            @test DataAPI.metadatasupport(typeof(tbl)) == (read=true, write=false)
+            @test metadata(tbl) == metadata(df)
+            @test metadata(tbl; style=true) == metadata(df; style=true)
+            @test_throws Exception metadata(tbl, "xyz")
+            @test metadata(tbl, "xyz", "something") == "something"
+            @test metadata(tbl, "xyz", "something"; style=true) == ("something", :default)
+            @test metadatakeys(tbl) == metadatakeys(df)
+
+            @test DataAPI.colmetadatasupport(typeof(tbl)) == (read=true, write=false)
+            @test colmetadata(tbl) == colmetadata(df)
+            @test colmetadata(tbl; style=true) == colmetadata(df; style=true)
+            @test_throws MethodError colmetadata(tbl, "xyz")
+            @test_throws KeyError colmetadata(tbl, :xyz)
+            @test colmetadata(tbl, :b) == colmetadata(df, :b)
+            @test_throws MethodError colmetadata(tbl, :b, "xyz")
+            @test colmetadata(tbl, :b, "xyz", "something") == "something"
+            @test colmetadata(tbl, :b, "xyz", "something"; style=true) == ("something", :default)
+            @test Set(colmetadatakeys(tbl)) == Set(colmetadatakeys(df))
+
+            # add metadata for the second iteration
+            metadata!(df, "tkey", "tvalue")
+            metadata!(df, "tkey2", "tvalue2")
+            colmetadata!(df, :a, "ackey", "acvalue")
+            colmetadata!(df, :a, "ackey2", "acvalue2")
+            colmetadata!(df, :c, "cckey", "ccvalue")
+        end
+    end # @testset "DataAPI.metadata"
 end